Groundwater salinity and theeffects of produced water disposalin the Lost HillsndashBelridge oilfields Kern County CaliforniaJanice M Gillespie Tracy A Davis Michael J StephensLyndsay B Ball and Matthew K Landon

ABSTRACT

Increased oil and gas production in many areas has led to concerns overthe effects these activities may be having on nearby groundwaterquality In this study we determine the lateral and vertical extentof groundwater with less than 10000 mgL total dissolved solidsnear the Lost HillsndashBelridge oil fields in northwestern Kern CountyCalifornia and document evidence of impacts by produced waterdisposal within the Tulare aquifer and overlying alluvium the primaryprotected aquifers in the area

The depth at which groundwater salinity surpasses 10000 mgLranges from150m (500 ft) in the northwestern part of the study area to490ndash550 m (1600ndash1800 ft) in the south and east respectively asdetermined by geophysical log analysis and lab analysis of producedwater samples Comparison of logs from replacement wells with logsfrom their older counterparts shows relatively higher-resistivity intervalsrepresenting the vadose zone or fresher groundwater being replaced byintervals with much lower resistivity because of infiltration of brinesfrom surface disposal ponds and injection of brines into disposal wellsThe effect of the surface ponds is confined to the alluvial aquifermdashtheunderlying Tulare aquifer is largely protected by a regional clay layer atthe base of the alluvium Sand layers affected by injection of producedwaters in nearby disposal wells commonly exhibit log resistivity profilesthat change from high resistivity in their upper parts to low resistivitynear the base because of stratification by gravity segregation of thedenser brines within each affected sand The effects of produced waterinjection are mainly evident within the Tulare Formation and can benoted as far as 550 m (1800 ft) from the main group of disposalwells located along the east flank of South Belridge

AUTHORS

Janice M Gillespie ~ California WaterScience Center US Geological Survey(USGS) Sacramento Californiajmgillespieusgsgov

Jan Gillespie received her BS degree ingeology from Bemidji State UniversityMinnesota her MS from South Dakota Schoolof Mines and Technology and her PhD fromthe University of Wyoming Formerly apetroleum and hydrogeology professor in theDepartment of Geosciences at California StateUniversity Bakersfield and a petroleumgeologist in the San Joaquin Valley of Californiashe is now a research scientist for the USGSrsquosregional aquifer monitoring project forCalifornia SB4 (the Well Stimulation Bill)delineating protected aquifers near oilproducing areas

Tracy A Davis ~ California Water ScienceCenter USGS San Diego Californiatadavisusgsgov

Tracy Davis received her BS degree in earthsciences with emphasis on geochemistry fromthe University of California San Diego Shebegan her career in hydrology at the USGS in2007 studying groundwater quality of aquifersused for public supply Her current researchfocuses on areas of oil and gas developmentand characterizing risks to Californiarsquosgroundwater resources

Michael J Stephens ~ California WaterScience Center USGS SacramentoCalifornia mjstephensusgsgov

Michael Stephens received his BS degreein geology from the University of SouthernIndiana and his MS degree in geology fromCalifornia State University Sacramento Hecurrently works on the California Oil Gas andGroundwater Project at the USGS His researchis focused on modeling groundwater salinityaquifer architecture and the controls on salinitydistributions

Lyndsay B Ball ~ Geology Geophysicsand Geochemistry Science Center USGSDenver Colorado lbballusgsgov

Lyndsay Ball received her BS degree inenvironmental science from Virginia Tech and

Copyright copy2019 The American Association of Petroleum GeologistsDivision of EnvironmentalGeosciences All rights reserved Gold Open Access This paper is published under the terms ofthe CC-BY license

Manuscript received October 30 2018 provisional acceptance March 20 2019 revised manuscript receivedMarch 27 2019 final acceptance June 20 2019

DOI101306eg02271918009

Environmental Geosciences v 26 no 3 (September 2019) pp 73ndash96 73

INTRODUCTION

Increased oil and gas production in many areas has led to concernsover the effects these activities may be having on groundwaterquality Historically both the California State Water ResourcesControl Board (State Water Board) and the California Departmentof Conservation Division of Oil Gas and Geothermal Resources(DOGGR) have defined groundwater resources needing specificprotection from oil and gas activities as those containing less than 3000mgL total dissolved solids (TDS) Several recent developments haveled the state to reconsider the 3000mgL TDS target First there hasbeen an increased use of brackish groundwater resources having TDSof 1000 to 10000 mgL because these resources can be treated fordomestic and industrial use for a lower cost than desalination ofseawater (Leitz and Boegli 2011 Mickley 2012 Eastern MunicipalWater District 2019 McCann et al 2018) Second an audit by theUS Environmental Protection Agency (EPA) of California oil and gasunderground injection practices noted that the state has not consis-tently used federal standards to delineate protected groundwaterresources (US Environmental Protection Agency 2012) Thirdpublic concerns about hydraulic fracturing and waste disposal prac-tices of the oil and gas industry in general led to new legislation

California Senate Bill 4 (SB 4 statutes of 2013) authorized theStateWater Board to implement a program to monitor water qualityin areas of oil and gas production beginning in 2015 The newprogram the regional monitoring program of groundwater qualityin areas of oil and gas development conducted in cooperation withthe US Geological Survey includes assessing potential impacts togroundwater associated with well stimulation (hydraulic fracturing)enhancedoil recovery (EOR) (water and steamflooding) anddisposal ofproduced water by underground injection or surface sumps (CaliforniaState Water Resources Control Board 2018b)

The Lost HillsndashBelridge study area lies in northwestern KernCounty California (Figure 1) and contains three large oil fieldsNorth Belridge South Belridge (collectively termed the Bel-ridge oil fields) and Lost Hills These oil fields are among thelargest in California South Belridge and Lost Hills were the third(226millionbblyr) andsixth (103millionbblyr) largestoil-producingfields respectively in 2016 (California Division of Oil Gas andGeothermalResources 2017)Thearea adjacent to theoilfields is alsoextensively farmedWater quality in the aquifers is generally too poorto support large-scale farmingwithout being blendedwithwater fromsurface sources such as the California Aqueduct However some ofthe aquifers contain water with less than 10000 mgL TDS and areclassified as underground sources of drinking water (USDW) underthe Safe Drinking Water Act (1974) These aquifers must be pro-tected from contamination when not exempted UndergroundInjection Control regulations allow the EPA to exempt aquifersthat do not currently or are not expected to serve as a sourceof drinking water allowing these underground waters to be used

her PhD in geology from the University ofColorado at Boulder She began her career atthe USGS in 2003 In her current position as aresearch geophysicist her work focuses onhydrogeologic systems and processes usingintegrated hydrological and geophysicalapproaches

Matthew K Landon ~ California WaterScience Center USGS San Diego Californialandonusgsgov

Matthew Landon has been a hydrologistwith the USGS since 1990 He received aMSin geology from the University of Minnesotain 1993 and a BS in geology from theUniversity of Kansas in 1987 He hasconducted studies of groundwaterhydrology and geochemistry in MinnesotaNebraska California and Southeast Asia Hehas been with the USGS California WaterScience Center in San Diego since 2005currently serving as Program Chief for theUSGS California Oil Gas and GroundwaterProgram in support of the California StateWater Boardrsquos regional monitoring programin areas of oil and gas production

ACKNOWLEDGMENTS

The authors would like to acknowledge thefollowing individuals for their supportCalifornia State University Bakersfieldstudents Maryanne Bobbitt Phillip HerreraVictor Medrano and Zach Wolpe for theirhelp in compiling data on hydrocarbonshows and wet zones and also for theirassistance with log evaluation DavidShimabukuro (California State UniversitySacramento and USGS) Will Chang (TuleAnalytics) Kim Taylor and Rhett Everett(USGS) for providing information and adviceJordan Martin of the California State WaterResources Control Board alerted us to thepresence of numerous online pressure testsin the South Belridge field Finally we wouldlike to thank Jane McNaboe for providing atimely and helpful peer review This work wasfunded by the California State Water ResourcesControl Boardrsquos Regional GroundwaterMonitoring in Areas of Oil and Gas ProductionProgram and the USGS Cooperative WaterProgram

74 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 1 Location of the study area Lost Hills and North and South Belridge oil fields shown in red Study area outlined in black

Gillespieetal

75

for oil or mineral extraction or disposal purposes (USEnvironmental Protection Agency 2018)

A primary goal of this study is to determine thedepth to the base of protected groundwater USDWtypically defined as waters containing less than 10000mgL TDS that are not mineral hydrocarbon or geo-thermal energy producing (US Environmental Protec-tion Agency 2018) Because water analyses in deeperbrackish aquifers are sparse this study also uses bore-hole geophysical log analysis to delineate protectedaquifers in the area Borehole geophysical logs fromdifferent time periods are used to map changes insubsurface salinity over time caused by produced waterdisposal in ponds and injection wells These approachescan be used to improve the understanding of the oc-currence of high salinity produced waters in proximityto oil fields at site-specific scales movement of thosefluids over time and to supplement more areally exten-sive but commonly shallower and less spatially densemonitoring well networks used to monitor shallowgroundwater plumes from sites such as sump ponds

GEOLOGIC SETTING

The Belridge and Lost Hills fields are located in thesouthwestern part of the San Joaquin Valley (SJV)which forms the southern part of the larger CentralValley of California (Figure 1) The SJV is bounded tothe east and southby intrusive igneous andmetamorphicrocks of the Sierra Nevada and to the south and westby igneousmetamorphic andmarine sedimentary rocksof the Coast Ranges The northern geologic boundaryis placed at the Stockton Arch a broad low amplitudeuplift which lies east of San Francisco

The Central Valley initially formed as a forearcbasin landward of a subduction zone in which theeastern Pacific plate was subducted beneath the NorthAmerican continent Circa 25 mya the East Pacific Riseencountered the trench offshore present-day southernCalifornia and the platemargin changed from convergentto transform creating the San Andreas fault (Atwater1970) Movement along the San Andreas fault duringthe Miocene commonly cut off the southernmost SJVfrom open ocean circulation forming a bottom layerof anoxic seawater in which organic-rich shales of theMonterey Formation (Figure 2) formed in a deep oceansetting (Graham1987)These shales ultimately becamethe source of much of the oil in the associated turbiditesands and overlying formations (Magoon et al 2007)

Over time the rate of sedimentation in the southernSan Joaquin basin outpaced the rate of subsidence andshallow marine deposits of the Etchegoin and SanJoaquin Formations were deposited above the deeperwater deposits of the Monterey and Reef Ridge For-mations (Lettis 1982) Eventually nonmarine conditionsprevailed resulting in deposition of the Pleistocene- ageTulare Formation and overlying alluvium across thearea (Figure 2)

The southern part of the SJV is the most tectonicallyactive area of the Central Valley Transpressional forcesalong the San Andreas fault created an east-northeastvergent fold and thrust belt subparallel to the fault(Bartow 1991) along the west side of the southern SJVThese folds and faults created numerous opportunitiesfor the entrapment of vast pools of oil The North andSouth Belridge oil fields and the Lost Hills oil field arelocated on south-plunging anticlines within the westside foldndashthrust belt

Hydrogeology and Aquifer Stratigraphy

The west side of the southern SJV lies in the rain shadowof the Coast Ranges to the west The average rainfall isonly 12ndash23 cm (5ndash9 in) in the study area (Wood andDavis 1959) What rain occurs falls mainly in winterTemperatures commonly exceed 100degF during thesummer resulting in high rates of evaporation Con-sequently streams draining the east side of the CoastRanges are generally ephemeral and flow mainly inwinter and spring The water quality of these streamsnear the Belridge oil fields ranges from 1900 to 3200mgL TDS (Wood and Davis 1959) This is partially theresult of high evaporation rates butmay also be attributedto the relatively soluble rock types found in the meta-morphic and marine sedimentary rocks in the CoastRanges (Laudon and Belitz 1991) These streams are themain source of natural recharge to the groundwater sys-tem in the study area In the last century recharge fromanthropogenic land use includes return flow from agri-cultural activity and leakage from unlined canals (Woodand Davis 1959)

The Pleistocene Tulare Formation and overlyingalluvium form the main aquifer in this area (Figure 2)Maps of the surface geology show that the Tulare For-mation crops out in the northern Lost Hills and NorthBelridge fields where most of the formation has beenremoved by uplift and erosion along the anticlines(Wood and Davis 1959) The maximum thickness of

76 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 2 Type log (API 0402974434 shown by red star on inset map) of the alluvium and Tulare Formation in the study area Yellow shading on spontaneous potential (SP) curveindicates sand bodies in the aquifer Yellow shading on resistivity curve indicates resistivity greater than 3 ohm m Major confining clays are shown in gray Depth values in feetStratigraphic column of the southern San Joaquin Valley is shown on the right with formations addressed in this study highlighted in red Modified from Scheirer and Magoon (2007)Fm = Formation Mtn = Mountain No = North Sh = Shale So = South

Gillespieetal

77

the Tulare Formation on the western basin margin is1147 m (3500 ft) (Woodring et al 1940 Loomis1990)

Where it does not crop out the Tulare Formation isoverlain by alluvial deposits Both the Tulare Formationand the overlying alluvium are derived primarily fromthe Coast Ranges and interfinger with Sierran-derivedsediments from the east (Laudon and Belitz 1991) TheTulare Formation and overlying alluvium have similarlithologic compositions and were deposited in fluvial andlacustrine environments therefore they are not readilydistinguishable especially in the subsurface Maps ofthewater-level contours in the Tulare and alluvial aquiferby Wood and Davis (1959) indicate eastward gradientsin the aquifer

At Belridge and Lost Hills the basal Tulare For-mation contact is an angular unconformity belowwhichthe San Joaquin Formation has been removed overmostof the area As a result the Tulare Formation commonlylies directly upon marine deposits of the late Mioceneto Pliocene Etchegoin Formation along the crests ofthe anticlines

In the study area the lower Tulare Formationconsists of lacustrine and deltaic sands and the upperTulare contains alluvial fan meandering channels andfloodplain facies (Kiser et al 1988 Miller et al 1990)The Tulare Formation and overlying alluvium also in-clude lacustrine clays which form confining bedsSeveral distinct regional clay units within the TulareFormation are present in the study area Some of theseclays are not present throughout the study area butwhen present act as local confining layers The threemain clays mapped using borehole geophysical logsin this study are the Amnicola the Middle Tulare andthe Corcoran Clay Equivalent (CCE) (Figure 2) The D-EClay is only present in the southeastern part of the areaand was not mapped in this study

The Amnicola is the oldest of the three mapped claylayers It is described as an olive gray partly calcareousdolomitic claystone that contains the gastropodAmnicola(Woodring et al 1932 Berryman 1973 Maher et al1975) In the southeast part of the study area where itis best developed the Amnicola Clay lies atop a largecoarsening upward sand The Amnicola Clay is absentacross the anticlinal crests and in the northern part ofthe syncline between Lost Hills and North BelridgeFossil evidence from recent drilling in the area indi-cates that the Amnicola Clay is likely present alongthe east flank of northern Lost Hills

Overlying the Amnicola Clay is a coarsening up-ward zone of sands silts and clay that ranges from ap-proximately 120ndash200 m (~400ndash700 ft) thick The zonethickens to the south and thins to the north and along thecrest of the anticlines A clay layer with a pronouncedhigh gamma-ray signature on geophysical logs liesabove this zone and is termed the Middle Tulare Clayin this report The Middle Tulare Clay marks a changein log character from vertically stacked funnel shapedspontaneous potential (SP) and resistivity patterns withrelatively high gamma-ray response (80ndash100 API in thelowerTulare Formation to a blocky SP and resistivity logpattern with lower gamma-ray response (40ndash80API) inthe upper Tulare Formation This change in log patternmay represent a change from a series of prograding la-custrine delta environments in the lower Tulare For-mation to a fluvial meander belt setting in the upperTulare Formation as noted by Miller et al (1990)

The Corcoran Clay described by Frink and Kues(1954) also known as the E clay (Croft 1972) or blueclay by water well drillers is the youngest of the threemapped clay layers and is an important marker bedwithin the SJV aquifer system It divides the ground-water system of the western SJV into an upper semi-confined zone and a lower confined zone (Williamsonet al 1989 Belitz and Heimes 1990) Several studieshave mapped the upper clays of the Tulare Formationbeginning with Croft (1972) who mapped numerousclay layers in the southern SJV including the study areaMore recent studies have focused specifically on theCorcoran Clay and have used both surface and sub-surface data to map the extent and thickness of the clay(Page 1986 Burow et al 2004 Faunt 2009)

Petroleum companies operating in the area refer toa basal alluvial clay at the contact between the TulareFormation and overlying alluvium as the CCE althoughits relationship to the Corcoran Clay as defined by Frinkand Kues (1954) is not clear because Frink and Kues(1954) place the Corcoran Clay within the TulareFormation and Kiser et al (1988) place it within thealluvium In the northeastern part of the study area theCCE correlates to the E clay of Croft (1972) South ofthe study area in the SJV the identification of theCorcoran Clay becomes more problematic and Page(1986) refers to the Corcoran Clay in this area as themodified E clay Because the exact relationship betweenthe CCE and the Corcoran Clay is not known the claywill be termed the CCE in this paper

The CCE is up to 60 m (200 ft) thick within thestudy area It is absent in the northwestern part of the

78 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

study area and above the Lost Hills and Belridge anti-clines Drillers in the area observe a color change in cut-tings from orange-brown above the CCE to graybelow it A thick (18ndash21 m [60ndash70 ft]) sand the 22Ksand lies above the CCE near South Belridge (Figure2) Where the 22K sand is present above the CCE itforms a separate aquifer from the underlying TulareFormation

OIL FIELD ACTIVITY

The Lost Hills and Belridge fields were discoveredduring 1910ndash1912 Initial production was from heavyoil sands in the Tulare and Etchegoin Formations andfrom fractured shales of the Monterey Formation In1930 light oil was discovered in the deeper TemblorFormation at North Belridge but it is the shallowerheavy oil along with fractured diatomaceous depositsand shales in the Etchegoin Reef Ridge and MontereyFormations that have been the mainstay of the fieldsto the present day because of the development of EORtechniques such as steamflooding waterflooding andhydraulic fracturing (Bailey 1939 Ritzius 1950 Land1984 California Division of Oil Gas and GeothermalResources 1998)

The waterflood technique was first initiated in thelate 1940s in the Monterey shales in Lost Hills and themid-1950s in the Temblor Formation sandstones inNorth Belridge (Figure 3) In the 1980s waterfloodscommenced in the Tulare Formation and the diatomitezones of the Monterey Reef Ridge and Etchegoin For-mations In the 1950s and 1960s thermal EOR tech-niques (mainly steamcycling andflooding and to a lesserextent firefloods) were initiated to maximize produc-tion of heavy oil in the Tulare and Etchegoin Formationsand Monterey diatomite

In the diatomite zones large-scale waterfloodingprimarily to enhance oil recovery after hydraulic frac-turing began in the late 1960s and 1970s but becameincreasingly common after 1980 Although the water-floods increased production they also served to mitigateland subsidence and wellbore casing collapse caused byfluid removal from the high-porosity diatomite zones(Dale et al 1996)

As oil is removed water commonly comes in to fillthe void and it is common for waterndashoil ratios (WORs)to increase over the life of an oil field In the Belridge oilfields the WOR increased from 009 in 1931 to ap-proximately 14 in 2016 and in Lost Hills the WOR

increased from15 in 1931 to approximately 12 in 2016Some of the produced water is reused for EOR prac-tices such as waterflooding but the remainder is dis-posed of by a variety ofmeans Initially water disposalwas accomplished by spreading the produced wateron the ground commonly in surface ponds or drystream beds and the water was allowed to evaporateor percolate into the underlying alluvium (Mitchell1989)

A series of legislative bills passed at both the stateand federal level starting in 1969 with the Porter Co-logne Water Quality Act the 1972 Clean Water Actand the Safe Drinking Water Act (1974) provided au-thority to establish regulations regarding surface waterand groundwater degradation As a result of theseregulations produced water disposal began to movefrom surface ponds to injection into nonoil-producingzones via water disposal wells The federal Under-ground Injection Control program established nation-wide requirements for the protection of all aquiferscontaining water with less than 10000 mgL TDS

At Lost Hills oil field water disposal by injectionhas increased from approximately 160000 m3yr(~1 million bblyr) in 1973 to 48 million m3yr(30 million bblyr) in 2009 Disposal initially occurredinto both the Tulare and Etchegoin Formations butby 1990 disposal was almost exclusively into theEtchegoin Formation At North and South Belridgefields disposal began at 636000 m3yr (4 millionbblsyr) in 1970 and increased to more than 12million m3yr (gt78 million bblyr) in 2009 (Figure 3)Belridge disposal is almost entirely into the TulareFormation

METHODS

The main data used in this study are borehole geo-physical logs and water quality laboratory analysesThese data were used to pick marker horizons forgeologic mapping and to determine water salinityand aquifer pressures Data compiled and analyzedfor this study are available from Gillespie et al(2019)

Mapping

Approximately 900 borehole geophysical logs fromwells within the study area were used to determinethe lateral continuity and stratigraphic relationships

Gillespie et al 79

between the late Miocene to Holocene formationsElectric logs (resistivity SP) and gamma-ray logs wereused to identify correlatable layers (Figure 2) Well data(geophysical logs mud logs core data and well his-tories) were obtained primarily from oil and gas wellrecords available from theDOGGR(CaliforniaDivisionof Oil Gas and Geothermal Resources 2018) and fromwater well records obtained from well owners and theCalifornia Department of Water Resources

Because the Tulare Formation and overlying allu-vium form the major nonexempt aquifers in the area

these layers were mapped in more detail particularlywith regard to clay layers Many of the clay markers losetheir coherence when mapped over large distances andbetween distantly spaced wells outside the oil fieldboundaries because of the rapidly changing characterof the Tulare fluvialndashlacustrine delta system In manyplaces it was not possible to correlate the clay layersthroughout the region The correlations were used toconstruct thickness (isochore) maps of the combinedalluvium andTulare Formation (Figure 4) andCCE andAmnicola clays (Figure 5) The correlated logs were

Figure3 Production (prod) and injection (inj) curves for the Lost Hills and North and South Belridge fields (combined) EOR= enhancedoil recovery HF = initiation of hydrofracking Inj = initiation of water injection for enhanced oil recovery WD = initiation of disposal ofproduced water in injection wells Data prior to 1973 are from California Division of Oil Gas and Geothermal Resources (2015) post-1973data are from California Division of Oil Gas and Geothermal Resources (2019)

80 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

compared to salinity data to determine the extent towhich salinity is controlled by aquifer stratigraphy

Wireline formation testers measure pressure buildup within a small interval of the sands (Schlumberger2006) Within a single homogeneous aquifer the pres-sure values should increase with depth along a straightline representing the hydrostatic gradient Shifts fromthe hydrostatic pressure gradient line indicate the pres-ence of pressures in individual aquifer layers that implyrestricted hydraulic connections with other aquiferlayers caused by confining clay layers (Coburn andGillespie 2002) Wireline formation tests in the TulareFormation have been conducted in some test holesdrilled in the study area by oil companies prior to wellinstallation (Gillespie et al 2019) These recordscompiled for 59 wells were analyzed to help deter-mine which clays are significant confining layers

Salinity Determination

For this study two different methods were used to de-termine water salinity within the aquifers in and aroundthe BelridgendashLost Hills oil fields The first is indirect es-timation of salinity using geophysical logs collected priorto installation of oil wells The second is direct mea-surement by lab analysis of producedwater samples fromoil wells in the area which were compared to the in-direct estimates

Log AnalysisAlthough direct sampling and chemical analyses arecommonly considered the best method for determiningTDS in any aquifer in many cases (particularly in deepaquifers containing brackish water) these analyses areuncommon In addition water samples provide only

Figure 4 Map showing depth to base of Tulare Formation The values are equivalent to the thickness of the Tulare Formation andoverlying alluvium Wells shown are those used to determine thickness Black lines are oil field administrative boundaries Gray lines aretownship boundaries Red lines are faults (Young 1968 California Division of Oil Gas and Geothermal Resources 1998) Abd= abandoned

Gillespie et al 81

Figure 5 Isochore maps of the (A) Amnicola Clay and (B) Corcoran Clay Equivalent layers in the study area Wells shown are those used to determine thickness Gray lines aretownship boundaries Red lines are faults (Young 1968 California Division of Oil Gas and Geothermal Resources 1998) Abd = abandoned Approx = Approximate

82Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

point-source data from a single depth interval and do nottell us at what depth the salinity changes However inactive petroleum-producing basins such as the SJV open-hole geophysical logs are abundant Geophysical logsprovide continuous vertical records of the properties ofthe formation matrix fluids adjacent to the boreholeand fluids within the borehole Using log analysis it ispossible to calculate the depths at which salinity changesoccur These depthsmay be verified by chemical analyseswhere available Of particular interest for determiningsalinity are the electrical logs (SP and resistivity) becausethe ability of water to conduct an electrical current isstrongly influenced by both the temperature and salinity ofthe formation water

Use of electrical logs to determine water salinity ismost common in the petroleum literature In oil-bearingreservoirs it is important to know the resistivity of thewater which occurs with the oil to determine the oilsaturation using the Archie (1942) equation or one ofits many variants

Fewer studies have evaluated the use of electricallogs to determine salinity in fresh to brackish aquifersHowells (1990) andHowells et al (1987) used well-loginterpretation to determine the location of the base ofmoderately saline (lt10000 mgL TDS) water in UtahLindner-Lunsford and Bruce (1995) used the ArchieEquation to determine salinity in aquifers with TDS lessthan 1000 mgL in southwest Wyoming Schnoebelenet al (1995) used geophysical logs to map the depth to10000 mgL TDS water in carbonate aquifers in Indianausing the SP log mud filtrate resistivity and the resis-tivity porosity method of Archie They compared theresults to sampled water analyses and found the resis-tivity porosity method to be most accurate More re-cently Hamlin and de la Rocha (2015) used the ArchieEquation to map fresh (lt1000 mgL TDS) slightlysaline (1000ndash3000mgL TDS) moderately saline (3000ndash10000mgL TDS) and very saline (gt10000mgL TDS)water in the CarrizondashWilcox aquifer of south TexasGillespie et al (2017) used log analysis to determinedepth to 10000 mgL TDS on a regional basis through-out much of the southern San Joaquin Basin but thewestern margin fields were not considered in theirstudy because the presence of shallow oil reservoirs requiremore extensive mapping to locate wet sands for loganalysis

In this study a variation of the Archie Equationwas used to estimate salinity (as NaCl equivalent)from the geophysical logs Archie (1942) related thein situ electrical resistivity of a fully water saturated

sedimentary rock (Ro) to its porosity (f) nondimen-sional factors related to matrix properties (the cemen-tation factor [m]and the tortuosity factor [a]) and theresistivity of the formation water (Rw) as shown inequation 1

Rw = Ropethfm=aTHORN (1)

Ro is the resistivity of a clean (minimal clay content) oil-free wet sand and can be obtained from the deepresistivity curve Sands were chosen based on an evalu-ation of nearby core mud log and drillers log data in-dicating that the sands analyzed did not contain oil orgas The porosity value is obtained from porosity logssuch as density neutron sonic or nuclear magneticresonance logs and core analysis For this study sonicporositieswere not used the poorly consolidated natureof the aquifer sediments causes additional slowing ofthe sonic wave from compression of the material re-sulting in artificially high porosity values

Because of the poorly sorted nature of the sedimentsand the presence of large amounts of clay and silt in somezones the average porosity used for salinity calcula-tions weighted density measurements twice as much asneutron values Neutron values for porosity are basedupon the abundance of hydrogen atoms in a formationBecause clays contain hydroxide groups within theircrystal lattice the neutron curve tends to overestimateporosity when clays are present The largest separationbetween the density and neutron curves on each logwasconsidered to be a zone with 100 clay The separationof the densityndashneutron curves for each evaluated in-terval was compared to the 100 clay separation valueto estimate clay content The Rw values were not cal-culated for intervals containing more than 25 clay

The exponentm (unitless) is related to the degree ofcementation in clastic rocks It typically ranges from 13to 26 (Wylie and Rose 1950) The value a (unitless) iscalled the tortuosity factor and is related to the lengththe current must travel through the formation Thevalue for a typically ranges from 05 to 15 For thisstudy special core analyses from five intervals in theTulare Formation were available in the CaliforniaDOGGRonline files from awater disposal well south ofthe Lost Hills field (American Petroleum Institute[API] well number [no] 0402974433) The value fora was kept constant at a value of 1 and m was allowedto vary The measured values for m ranged from 15 to18 with a mean value of 17 (standard deviation of015) and a median value of 18 For this study salinity

Gillespie et al 83

calculations used a = 1 andm = 17 based on the specialcore analysis results

Upon obtaining a value for Rw and calculating thetemperature of the zone of interest from a linear in-terpolation between bottom borehole temperature andair temperatureof 75degF the formationwater salinity canbeestimated from empirical charts such as those provided bySchlumberger (1997) or from equations such as thosein Bateman and Konen (1977) relating fluid resistivity toNaCl equivalent TDS concentration

Geochemical AnalysesThe California Division of Oil Gas and GeothermalResources (2016) maintains a website containing scan-ned copies of chemical analyses of oil field waters Thesedata are useful in determining the salinity of the deeperaquifers not normally used for drinking water or irriga-tion Data taken from these reports included dates oftesting source of water for test (when available) majorion concentrations and TDS concentrations The compileddata are available from Gillespie et al (2019) Chemicalanalysis of groundwater samples compiled from varioussources were also available (Metzger et al 2018) How-ever these samples were generally available from rela-tively shallow depths and had TDS ranging fromapproximately 1000 to 3000 mgL and therefore did notdirectly contribute to efforts to map the depth to USDW

Scanned copies of well completion reports are alsoavailable for the wells with chemical analysis from theCalifornia Division of Oil Gas and Geothermal Re-sources (2018) Well Finder online search engine Thecompletion reports for the wells provide informationincluding perforated intervals date perforated geolog-ical formation sampled (in some cases) scanned copiesof borehole geophysical logs pressure tests and bottom-hole temperatures These records were used to de-termine the depth interval and formation representedby the produced water chemical analysis data (Gillespieet al 2019)

Produced water chemical analyses from approxi-mately 150 oil production injection and observationwells in the study area were available from DOGGRrsquosonline chemical analysis database (California Divisionof Oil Gas and Geothermal Resources 2016) Addi-tionally 12 samples from the Tulare Formation and al-luvium were available from groundwater monitoringwells in the Lost Hills oil field on the Geotracker website(California State Water Resources Control Board2018a) The vintage of samples in the database rangesfrom 1932 to 2017

The sample analyses may not always representnatural aquifer conditions For example water samplesare commonly collected when a well has productionproblems or during the initial development of a well(Blondes et al 2016) Production problems may in-clude holes in well casings or annular cement that mayhave caused water from a different zone to enter theperforations In most cases few data exist regarding thesampling procedures and analysis techniques particu-larly for the older samples However older samples wereconsidered to be important for this study because theypredate the use of many EOR techniques such as waterflooding waste disposal and steam injection the large-scale use of which commenced in the early to mid-1960s These EOR processes may significantly alterthe salinity of the samples and cause the more recentsamples to be an inaccurate reflection of the originalaquifer conditions Samples taken at different timeintervals are helpful in determining temporal changes insalinity that may have occurred because of oil fieldactivities

One quality control method used in this study wasto perform a charge balance on the analyses Becausewater is electrically neutral the negative (anions) andpositive (cations) charged ions should sum to zero Thedifference between the total milliequivalents of thecations and anions is divided by the sum to give a chargebalance error We used a cutoff of ndash5 to determinewhich samples to discard in this analysis

For wells used for injection purposes the initiationof water injection relative to the date of sampling maybe important to consider Samples taken after the startof injection in the sampled well may not accuratelyrepresent the true formation water composition In mostcases it was not feasible to catalog the time of initiationof injection in nearby wells because of the large numberof injectionwells that could potentially affect a sampledwell Consequently the effect of subsurface injectionon sample TDS data remains a source of uncertainty inthe analysis

RESULTS

The methods and data discussed in the previous sec-tion were used to construct geologic maps showing thethickness and continuity of aquifers and confining unitsas well as to determine aquifer salinity and temporalchanges in salinity These results are discussed below

84 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 6 Graphs of shut-in pressure (from open-hole sequential formation testers) versus depth for three wells across the study area Red lines are used to emphasize the pressuregradient Tan areas showmajor clay layers the 22K sand is shown in yellow Base of Tulare and top of thewater table are also notedWells are identified by their API number and the yeardrilled and logged at the top of each graph

Gillespieetal

85

Geologic Mapping

The thickness of the Tulare Formation and overlyingalluvium in the study area (Figure 4) ranges from1200m (4000 ft) in the southeast part of the study areato 75 m (~250 ft) along the anticline crests in thenorthern parts of Lost Hills and North Belridge oilfields Because chemical analyses indicate that theprotected aquifers occur largely within the Tulare For-mation and overlying alluvium throughout the studyarea the map of the depth to the base of the TulareFormation defines the extent of potential protectedaquifers outside the oil field limits

The Amnicola Clay is thickest (67 m [~220 ft])along the northeastern and southeastern flank of theLost Hills anticline and east of the South Belridge an-ticline (Figure 5A) It thins to zero in the northwest partof the study area The clay is replaced by sandier sedi-ments east of the central part of the Lost Hills anticlineThe CCE thickens to 61m (~200 ft) in the northcentralpart of the study area in the Tulare lakebed but iscommonly less than30m(lt~100 ft) thick (Figure5B) Itis absent over the Lost Hills anticline crest and in thenorthwestern part of the study area These clay maps(Figure 5A B) are used to determine the presence ofpossible confining beds that in some cases may controlthe distribution of protected water resources and restrictthe migration of potential contaminants

Graphs of pressure versus depth collected in 1985from three wells along a northeast-trending transectfrom South Belridge to south of Lost Hills are shown inFigure 6 Marked changes in the hydrostatic pressuregradient in well API no 02974434 occur across theMiddle Tulare Clay and Amnicola Clay layers An addi-tional deeper clay here termed the D-E Clay forms alocal confining layer isolating a relatively low perme-ability section at the base of the Tulare Formation fromthe rest of the aquifer These characteristic deviationsin hydrostatic gradients indicates that the clays hydrau-lically separate individual aquifer layers in the area ofthis well

Farther northeast in wells API no 02974436 andAPI no 02974433 these deviations from hydrostaticgradient are much less noticeable especially across theAmnicola Clay This suggests that either (1) the claylayers are less confining south of Lost Hills or (2) thehydrologic regime near South Belridge has been alteredby production and injection into the Tulare Formationand the pressure responses in aquifer layers near the fieldin 1985 had not had sufficient time for their pressures

to reach equilibrium Recent drilling and multiple wellmonitoring site data (April 2018) off the eastern flank ofthe northern Lost Hills oil field indicates a 45 m (~15 ft)head difference across the Amnicola Clay with an upwardgradient of 0038 (measured May 31 2018) suggestingthat it is a confining layer in the northeastern part of thestudy area (US Geological Survey 2018a b)

Salinity Analysis

Log analysis from 30 wells in areas outside the oil fieldboundaries or within the oil fields but below the oilndashwater contact were used to create a map of the depthto the base of USDW The depth map to the base ofUSDW shows two trends (Figure 7) In the area be-tween Lost Hills and the Belridge oil fields depthto USDW increases toward the south from 150 m(~500 ft) between North Belridge and Lost Hills to490 m (~1600 ft) southeast of South Belridge In the

Figure 7 Map showing depth to base of underground sourcesof drinking water (USDW) (total dissolved solids = 10000 mgL)based on log analysis Red lines indicate faults (Young 1968California Division of Oil Gas and Geothermal Resources 1998)Black lines are oil field administrative boundaries Wells shown asblue dots are those used to determine the base of USDW Depthvalues in feet CI = contour interval No = North So = South

86 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

eastern part of the study area depth to base of USDWincreases sharply eastward from approximately 215 m(~700 ft) on the west flank of the Lost Hills anticline toas great as 550 m (~1800 ft) within 3 km (~2 mi)

Salinityndashdepth profiles based on chemical analysesof producedwater samples were compared to the depthto base USDW map estimated from borehole geo-physical log analysis However this comparison waslimited because most of the lab analyses come from oil-producing wells within the field boundaries rather thanin areas outside the fields The presence of oil in thesampled intervals within the fields precludes the useof log analysis to calculate the salinity in these samewells Plots of TDS versus depth of the samples in eachfield are shown in Figures 8 and 9 along with log-calculated TDS

Wood and Davis (1959) reported the groundwaterchemistry in this area as being an NaSO4 type howeveronly one Tulare Formation monitoring well Mackesseyno 1 (API no 02987363) was an NaSO4 type the restare NaCl type Because the calculated salinity fromArchie log analysis assumes NaCl water types the log-calculated and lab sample TDS values should be com-parable The two monitoring wells completed in thealluvium have higher SO4 concentrations relative tothe Tulare Formation wells however Cl is the domi-nant anion

Only four Tulare Formation lab samples (blue dots)in Lost Hills are in the California DOGGR databaseData from 14 monitoring wells in the central part ofthe Lost Hills field were used to enlarge the data set(Figure 8) With the exception of one sample TDSconcentrations in the Tulare Formation and alluviumare less than 10000 mgL indicating protected (USDW)aquifers These wells have top perforations ranging from40 to 290 m (133ndash950 ft) The well with a lab analysisindicating a salinity of 12295 mgL TDS in the TulareFormation had a top perforation at 300 m (984 ft) Thissuggests that the base of USDW lies somewhere be-tween 290 and 300 m (950 and 985 ft) within the fieldLog analysis suggests that the base may occur as deepas 580 m (~1900 ft) but many of the wells used for loganalysis come from areas east of the field boundarywhere the Tulare Formation is much thicker (200 ftwithin the field vs 2000 ft east of the field [Figure 4])Within the field interpolated contour lines from log datashow the depth to base USDWat 213m (~700 ft) alongthe western field boundary and 365 m (~1200 ft) alongthe eastern field boundary a trend that is consistentwith the more limited water sample data

Typical TDS concentrations in the underlyingEtchegoin Formation (red dots) range from 20000 to35000 mgL probably because of the presence oftrapped connate seawater within these marine depositsIn northern Lost Hills (Figure 8) two samples fromEtchegoin sands have TDS concentrations below 10000mgL However in this area an active steamflood isongoing within the Tulare and Etchegoin FormationsHere the Etchegoin Formation sands lie at shallowdepths (lt150 m [~500 ft]) immediately below pro-ducing intervals in the Tulare Formation The EtchegoinFormation samples in this area are much more dilute(5600ndash8000 mgL) than in other areas This may be be-cause the sands are in hydraulic communication with thefresher water sands of the overlying Tulare Formation orbecause the Etchegoin sands in this area are steamfloodedand the more saline formation water is diluted by theinjected steam Since the twoEtchegoinUSDWsamplesare post-steamflood initiation the latter explanation islikely

Only one chemical analysis sample from the TulareFormation is available in the North Belridge field (Fig-ure 9) The TDS concentration of the sample is 6055mgL from a depth interval of 157ndash211 m (515ndash692ft) It confirms that waters above approximately 211 m(~700 ft) are USDW consistent with the log-deriveddepth to USDW map which shows depth to USDWnear the well at 275 m (900 ft)

Many lab analyses are available for the Tulare For-mation at South Belridge (Figure 9) but they do notshow salinity values that are correlated with depth Thelog-generated depth to USDW map shows that salinitytrends along the northern part of the oil field are vari-able but the southern part of the field shows depth toUSDW increasing toward the southeast Thereforesamples from northern and southern South Belridgewere considered separately The boundary betweenthe two subareas is shown by the dashed line in Figure 9Because of the variability in sample TDS values withdepth comparisons of sample data to calculated salin-ities fromwell-log analyses can only bemade in a generalfashion

In the northern part of the South Belridge field nosamples below 365 m (1200 ft) are USDW suggestingthat the base of USDW lies above this depth The log-derived map shows depth to base of USDW at 335 m(1100 ft) in the north To the south the base of USDWincreases to 427 m (~1400 ft) Near the dividing linebetween the north and south parts of the field thedepth to base USDW decreases to 365 m (1200 ft)

Gillespie et al 87

Figure 8 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare and alluvium (blue) and Etchegoin (red) formations in and near the Lost Hills oilfield Gray dots represent TDS calculated from geophysical logs in the field The horizontal red linemarks 10000mgL and the vertical dashed red linemarks the approximate depth of thedeepest underground sources of drinking water (USDW) sample The adjacent map shows the location of the Tulare and alluviumwells sampled as blue dots and shallow (lt150m [500ft] deep) Etchegoin samples as red dots (deeper Etchegoin sample sites are not shown onmap) and the contours to base USDW from log analysis Lost Hills is the only area that containsbrackish water in the Etchegoin Formation However the brackish waters in the Etchegoin occur in an oil-producing zone that is steam flooded so it is considered to be an exempt aquiferand is not protected CI = contour interval

88Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

Figure 9 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare Formation in and near the North and South Belridge oil fields The northern andsouthern parts of the South Belridge field (marked by the red line on the map) are shown in separate graphs Blue dots show Tulare Formation (Fm) Measurements of TDS from labanalyses Gray dots show Tulare Fm The TDS calculated from log analysis Horizontal red lines on the graphs mark 10000 mgL and the dashed vertical red lines mark the approximatedepth of USDW from lab samples The adjacent map shows the location of the wells sampled (blue dots) and the contours to base underground sources of drinking water (USDW) (infeet) from log analysis Black lines are oil field administrative boundaries and red lines are faults CI = contour interval No = North So = South

Gillespieetal

89

Figure 10 Cross section showing three wells drilled and logged in different years near a produced water disposal pond (light blue polygon on inset map) Yellow shading onspontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivity curve (right track at scale of 0ndash20 ohmm) indicate resistivity of more than 3 ohmm Pinkbars indicate low resistivity sands affected by infiltration of saline produced water from the pond The API numbers at the top of the logs are used for well identification Year drilled andlogged in parentheses Depth scale in feet

90Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

This may be caused by the presence of an active waterdisposal field in the Tulare Formation near this areaOverall the log-derived map shows base of USDWapproximately 61m (~200 ft) deeper than lab analyseswould suggest As at Lost Hills this may be caused bythe geographic distribution of the two data sets the labsamples coming mainly from within the oil fieldboundaries whereas the log data are confined to thearea outside the oil field

In the southern part of South Belridge field a wellnear the southern tip of the field was sampled at mul-tiple intervals Samples as deep as 455 m (~1495 ft) areUSDW and the deepest sample from 480 m (1575 ft)has a TDS value of 13074 mgL suggesting that thebase of USDW in the southern part of South Belridge liesbetween 457 and 480 m (1495ndash1575 ft) (Figure 9) Thedepth to USDW map from log analysis in the southernpart of South Belridge shows that the base of USDW

ranges from 365 m (1200 ft) in the north to approxi-mately 487 m (~1600 ft) in the south

Effects of Oil Field Activities on Water Salinity

The effects of water disposal in surface ponds on ground-water salinity is evident by comparing geophysical logsfrom old wells near the pond locations to logs fromnewerwells drilled nearbyOlder geophysical logs (pre-1970s) indicate that the upper sands in the alluviumare desaturated (vadose zone) as indicated by areas ofcrossover on densityndashneutron logs and have high re-sistivity (gt20ohmmFigure 10)However geophysicallogs collected from wells drilled later in the area donot show densityndashneutron crossover characteristic of avadose zone andhave extremely low resistivity (lt1ohmm)in the upper alluvial sands because of the presence ofhigh salinityproducedwater thathasfilled thevadosezoneInvasion of low resistivity water is especially apparent

Figure 11 Two adjacent wells drilled and logged in different years (1990 and 2012) in an area containing numerous disposal wells on theeast flank of South Belridge Yellow shading on spontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivitycurve indicates resistivity greater than 3 ohm m Pink bars indicate low resistivity sands in the 2012 well (0403046566) affected by injectedsaline produced water The API numbers at the top of the logs are used for well identification Year drilled and logged in parentheses

Gillespie et al 91

where saline waters have infiltrated the 22K sandwhich lies directly above the CCE (well 03047481 inFigure 10) in the lower part of the alluvium The salinewater lies atop the CCE which appears to be pro-tecting the underlying Tulare aquifer from contami-nation Log-calculated andmeasuredTDS concentrationscan exceed 20000mgL in affected parts of the alluvium

The effects of surface disposal ponds are less evidentin Lost Hills and North Belridge compared to the areaadjacent to South Belridge Lost Hills in particular tends tohave highly saline water in the shallow parts of the aquifereven where disposal ponds are not present Swain andDuell (1989) noted TDS concentrations as high as 21700mgL from very shallowwells (6m [~20 ft]) along the eastflank of Lost Hills but did not note the cause of the highsalinity in the area

The effects of water disposal injection wells aremore subtle than that of the ponds and are especiallydifficult to identify in sands closer to the base of USDWwhere resistivity is naturally fairly low Geophysicallogs in sands affected by saline injection tend to have aconcave resistivity profile and resistivities are as small ashalf that in logs fromolder unaffectedwells (Figure 11)Injected saline water tends to follow individual sand

layers leaving some layers relatively unaffected In somecases only the lower part of an individual sand containssaline water whereas the waters in the upper part re-mainbrackishThis suggests that thewaters are stratifiedand mixing has not had time to occur Log-calculatedTDS concentrations show a relatively smooth increasewith depth inwells logged prior to the start of large-scaledisposal activities whereas recent logs in these areasshow a much more variable TDS concentration profilewith depth Additionally densityndashneutron curves inwells affected by disposal of produced waters show thinintervals (~1 m [~3 ft] thick) of crossover This may becaused by bacterial metabolism of trace amounts oforganic matter in the disposal water resulting in thegeneration of small amounts of biogenic methane(McMahon et al 2018)

The cross section in Figure 12 shows wells affectedby saline produced water disposal by injection (belowthe CCE) and percolation (above the CCE) along theeast flank of the South Belridge oil field Wells affectedby disposal of produced waters in saline ponds (n = 139)and by disposal in injection wells (n = 125) are alsoshown in the map in Figure 13 The effects of disposalponds are evident from changes in borehole resistivity

Figure 12 Cross section showing intervals affected by disposal of saline produced water (pink boxes) at South Belridge oil field Wellsidentified by API number at top of each well Year drilled and logged shown at bottom of eachwell Insetmap shows location of cross sectionProduced water disposal wells are highlighted in pink and disposal ponds are shown in blue in the inset map

92 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

for oil or mineral extraction or disposal purposes (USEnvironmental Protection Agency 2018)

A primary goal of this study is to determine thedepth to the base of protected groundwater USDWtypically defined as waters containing less than 10000mgL TDS that are not mineral hydrocarbon or geo-thermal energy producing (US Environmental Protec-tion Agency 2018) Because water analyses in deeperbrackish aquifers are sparse this study also uses bore-hole geophysical log analysis to delineate protectedaquifers in the area Borehole geophysical logs fromdifferent time periods are used to map changes insubsurface salinity over time caused by produced waterdisposal in ponds and injection wells These approachescan be used to improve the understanding of the oc-currence of high salinity produced waters in proximityto oil fields at site-specific scales movement of thosefluids over time and to supplement more areally exten-sive but commonly shallower and less spatially densemonitoring well networks used to monitor shallowgroundwater plumes from sites such as sump ponds

GEOLOGIC SETTING

The Belridge and Lost Hills fields are located in thesouthwestern part of the San Joaquin Valley (SJV)which forms the southern part of the larger CentralValley of California (Figure 1) The SJV is bounded tothe east and southby intrusive igneous andmetamorphicrocks of the Sierra Nevada and to the south and westby igneousmetamorphic andmarine sedimentary rocksof the Coast Ranges The northern geologic boundaryis placed at the Stockton Arch a broad low amplitudeuplift which lies east of San Francisco

The Central Valley initially formed as a forearcbasin landward of a subduction zone in which theeastern Pacific plate was subducted beneath the NorthAmerican continent Circa 25 mya the East Pacific Riseencountered the trench offshore present-day southernCalifornia and the platemargin changed from convergentto transform creating the San Andreas fault (Atwater1970) Movement along the San Andreas fault duringthe Miocene commonly cut off the southernmost SJVfrom open ocean circulation forming a bottom layerof anoxic seawater in which organic-rich shales of theMonterey Formation (Figure 2) formed in a deep oceansetting (Graham1987)These shales ultimately becamethe source of much of the oil in the associated turbiditesands and overlying formations (Magoon et al 2007)

Over time the rate of sedimentation in the southernSan Joaquin basin outpaced the rate of subsidence andshallow marine deposits of the Etchegoin and SanJoaquin Formations were deposited above the deeperwater deposits of the Monterey and Reef Ridge For-mations (Lettis 1982) Eventually nonmarine conditionsprevailed resulting in deposition of the Pleistocene- ageTulare Formation and overlying alluvium across thearea (Figure 2)

The southern part of the SJV is the most tectonicallyactive area of the Central Valley Transpressional forcesalong the San Andreas fault created an east-northeastvergent fold and thrust belt subparallel to the fault(Bartow 1991) along the west side of the southern SJVThese folds and faults created numerous opportunitiesfor the entrapment of vast pools of oil The North andSouth Belridge oil fields and the Lost Hills oil field arelocated on south-plunging anticlines within the westside foldndashthrust belt

Hydrogeology and Aquifer Stratigraphy

The west side of the southern SJV lies in the rain shadowof the Coast Ranges to the west The average rainfall isonly 12ndash23 cm (5ndash9 in) in the study area (Wood andDavis 1959) What rain occurs falls mainly in winterTemperatures commonly exceed 100degF during thesummer resulting in high rates of evaporation Con-sequently streams draining the east side of the CoastRanges are generally ephemeral and flow mainly inwinter and spring The water quality of these streamsnear the Belridge oil fields ranges from 1900 to 3200mgL TDS (Wood and Davis 1959) This is partially theresult of high evaporation rates butmay also be attributedto the relatively soluble rock types found in the meta-morphic and marine sedimentary rocks in the CoastRanges (Laudon and Belitz 1991) These streams are themain source of natural recharge to the groundwater sys-tem in the study area In the last century recharge fromanthropogenic land use includes return flow from agri-cultural activity and leakage from unlined canals (Woodand Davis 1959)

The Pleistocene Tulare Formation and overlyingalluvium form the main aquifer in this area (Figure 2)Maps of the surface geology show that the Tulare For-mation crops out in the northern Lost Hills and NorthBelridge fields where most of the formation has beenremoved by uplift and erosion along the anticlines(Wood and Davis 1959) The maximum thickness of

76 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 2 Type log (API 0402974434 shown by red star on inset map) of the alluvium and Tulare Formation in the study area Yellow shading on spontaneous potential (SP) curveindicates sand bodies in the aquifer Yellow shading on resistivity curve indicates resistivity greater than 3 ohm m Major confining clays are shown in gray Depth values in feetStratigraphic column of the southern San Joaquin Valley is shown on the right with formations addressed in this study highlighted in red Modified from Scheirer and Magoon (2007)Fm = Formation Mtn = Mountain No = North Sh = Shale So = South

Gillespieetal

77

the Tulare Formation on the western basin margin is1147 m (3500 ft) (Woodring et al 1940 Loomis1990)

Where it does not crop out the Tulare Formation isoverlain by alluvial deposits Both the Tulare Formationand the overlying alluvium are derived primarily fromthe Coast Ranges and interfinger with Sierran-derivedsediments from the east (Laudon and Belitz 1991) TheTulare Formation and overlying alluvium have similarlithologic compositions and were deposited in fluvial andlacustrine environments therefore they are not readilydistinguishable especially in the subsurface Maps ofthewater-level contours in the Tulare and alluvial aquiferby Wood and Davis (1959) indicate eastward gradientsin the aquifer

At Belridge and Lost Hills the basal Tulare For-mation contact is an angular unconformity belowwhichthe San Joaquin Formation has been removed overmostof the area As a result the Tulare Formation commonlylies directly upon marine deposits of the late Mioceneto Pliocene Etchegoin Formation along the crests ofthe anticlines

In the study area the lower Tulare Formationconsists of lacustrine and deltaic sands and the upperTulare contains alluvial fan meandering channels andfloodplain facies (Kiser et al 1988 Miller et al 1990)The Tulare Formation and overlying alluvium also in-clude lacustrine clays which form confining bedsSeveral distinct regional clay units within the TulareFormation are present in the study area Some of theseclays are not present throughout the study area butwhen present act as local confining layers The threemain clays mapped using borehole geophysical logsin this study are the Amnicola the Middle Tulare andthe Corcoran Clay Equivalent (CCE) (Figure 2) The D-EClay is only present in the southeastern part of the areaand was not mapped in this study

The Amnicola is the oldest of the three mapped claylayers It is described as an olive gray partly calcareousdolomitic claystone that contains the gastropodAmnicola(Woodring et al 1932 Berryman 1973 Maher et al1975) In the southeast part of the study area where itis best developed the Amnicola Clay lies atop a largecoarsening upward sand The Amnicola Clay is absentacross the anticlinal crests and in the northern part ofthe syncline between Lost Hills and North BelridgeFossil evidence from recent drilling in the area indi-cates that the Amnicola Clay is likely present alongthe east flank of northern Lost Hills

Overlying the Amnicola Clay is a coarsening up-ward zone of sands silts and clay that ranges from ap-proximately 120ndash200 m (~400ndash700 ft) thick The zonethickens to the south and thins to the north and along thecrest of the anticlines A clay layer with a pronouncedhigh gamma-ray signature on geophysical logs liesabove this zone and is termed the Middle Tulare Clayin this report The Middle Tulare Clay marks a changein log character from vertically stacked funnel shapedspontaneous potential (SP) and resistivity patterns withrelatively high gamma-ray response (80ndash100 API in thelowerTulare Formation to a blocky SP and resistivity logpattern with lower gamma-ray response (40ndash80API) inthe upper Tulare Formation This change in log patternmay represent a change from a series of prograding la-custrine delta environments in the lower Tulare For-mation to a fluvial meander belt setting in the upperTulare Formation as noted by Miller et al (1990)

The Corcoran Clay described by Frink and Kues(1954) also known as the E clay (Croft 1972) or blueclay by water well drillers is the youngest of the threemapped clay layers and is an important marker bedwithin the SJV aquifer system It divides the ground-water system of the western SJV into an upper semi-confined zone and a lower confined zone (Williamsonet al 1989 Belitz and Heimes 1990) Several studieshave mapped the upper clays of the Tulare Formationbeginning with Croft (1972) who mapped numerousclay layers in the southern SJV including the study areaMore recent studies have focused specifically on theCorcoran Clay and have used both surface and sub-surface data to map the extent and thickness of the clay(Page 1986 Burow et al 2004 Faunt 2009)

Petroleum companies operating in the area refer toa basal alluvial clay at the contact between the TulareFormation and overlying alluvium as the CCE althoughits relationship to the Corcoran Clay as defined by Frinkand Kues (1954) is not clear because Frink and Kues(1954) place the Corcoran Clay within the TulareFormation and Kiser et al (1988) place it within thealluvium In the northeastern part of the study area theCCE correlates to the E clay of Croft (1972) South ofthe study area in the SJV the identification of theCorcoran Clay becomes more problematic and Page(1986) refers to the Corcoran Clay in this area as themodified E clay Because the exact relationship betweenthe CCE and the Corcoran Clay is not known the claywill be termed the CCE in this paper

The CCE is up to 60 m (200 ft) thick within thestudy area It is absent in the northwestern part of the

78 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

study area and above the Lost Hills and Belridge anti-clines Drillers in the area observe a color change in cut-tings from orange-brown above the CCE to graybelow it A thick (18ndash21 m [60ndash70 ft]) sand the 22Ksand lies above the CCE near South Belridge (Figure2) Where the 22K sand is present above the CCE itforms a separate aquifer from the underlying TulareFormation

OIL FIELD ACTIVITY

The Lost Hills and Belridge fields were discoveredduring 1910ndash1912 Initial production was from heavyoil sands in the Tulare and Etchegoin Formations andfrom fractured shales of the Monterey Formation In1930 light oil was discovered in the deeper TemblorFormation at North Belridge but it is the shallowerheavy oil along with fractured diatomaceous depositsand shales in the Etchegoin Reef Ridge and MontereyFormations that have been the mainstay of the fieldsto the present day because of the development of EORtechniques such as steamflooding waterflooding andhydraulic fracturing (Bailey 1939 Ritzius 1950 Land1984 California Division of Oil Gas and GeothermalResources 1998)

The waterflood technique was first initiated in thelate 1940s in the Monterey shales in Lost Hills and themid-1950s in the Temblor Formation sandstones inNorth Belridge (Figure 3) In the 1980s waterfloodscommenced in the Tulare Formation and the diatomitezones of the Monterey Reef Ridge and Etchegoin For-mations In the 1950s and 1960s thermal EOR tech-niques (mainly steamcycling andflooding and to a lesserextent firefloods) were initiated to maximize produc-tion of heavy oil in the Tulare and Etchegoin Formationsand Monterey diatomite

In the diatomite zones large-scale waterfloodingprimarily to enhance oil recovery after hydraulic frac-turing began in the late 1960s and 1970s but becameincreasingly common after 1980 Although the water-floods increased production they also served to mitigateland subsidence and wellbore casing collapse caused byfluid removal from the high-porosity diatomite zones(Dale et al 1996)

As oil is removed water commonly comes in to fillthe void and it is common for waterndashoil ratios (WORs)to increase over the life of an oil field In the Belridge oilfields the WOR increased from 009 in 1931 to ap-proximately 14 in 2016 and in Lost Hills the WOR

increased from15 in 1931 to approximately 12 in 2016Some of the produced water is reused for EOR prac-tices such as waterflooding but the remainder is dis-posed of by a variety ofmeans Initially water disposalwas accomplished by spreading the produced wateron the ground commonly in surface ponds or drystream beds and the water was allowed to evaporateor percolate into the underlying alluvium (Mitchell1989)

A series of legislative bills passed at both the stateand federal level starting in 1969 with the Porter Co-logne Water Quality Act the 1972 Clean Water Actand the Safe Drinking Water Act (1974) provided au-thority to establish regulations regarding surface waterand groundwater degradation As a result of theseregulations produced water disposal began to movefrom surface ponds to injection into nonoil-producingzones via water disposal wells The federal Under-ground Injection Control program established nation-wide requirements for the protection of all aquiferscontaining water with less than 10000 mgL TDS

At Lost Hills oil field water disposal by injectionhas increased from approximately 160000 m3yr(~1 million bblyr) in 1973 to 48 million m3yr(30 million bblyr) in 2009 Disposal initially occurredinto both the Tulare and Etchegoin Formations butby 1990 disposal was almost exclusively into theEtchegoin Formation At North and South Belridgefields disposal began at 636000 m3yr (4 millionbblsyr) in 1970 and increased to more than 12million m3yr (gt78 million bblyr) in 2009 (Figure 3)Belridge disposal is almost entirely into the TulareFormation

METHODS

The main data used in this study are borehole geo-physical logs and water quality laboratory analysesThese data were used to pick marker horizons forgeologic mapping and to determine water salinityand aquifer pressures Data compiled and analyzedfor this study are available from Gillespie et al(2019)

Mapping

Approximately 900 borehole geophysical logs fromwells within the study area were used to determinethe lateral continuity and stratigraphic relationships

Gillespie et al 79

between the late Miocene to Holocene formationsElectric logs (resistivity SP) and gamma-ray logs wereused to identify correlatable layers (Figure 2) Well data(geophysical logs mud logs core data and well his-tories) were obtained primarily from oil and gas wellrecords available from theDOGGR(CaliforniaDivisionof Oil Gas and Geothermal Resources 2018) and fromwater well records obtained from well owners and theCalifornia Department of Water Resources

Because the Tulare Formation and overlying allu-vium form the major nonexempt aquifers in the area

these layers were mapped in more detail particularlywith regard to clay layers Many of the clay markers losetheir coherence when mapped over large distances andbetween distantly spaced wells outside the oil fieldboundaries because of the rapidly changing characterof the Tulare fluvialndashlacustrine delta system In manyplaces it was not possible to correlate the clay layersthroughout the region The correlations were used toconstruct thickness (isochore) maps of the combinedalluvium andTulare Formation (Figure 4) andCCE andAmnicola clays (Figure 5) The correlated logs were

Figure3 Production (prod) and injection (inj) curves for the Lost Hills and North and South Belridge fields (combined) EOR= enhancedoil recovery HF = initiation of hydrofracking Inj = initiation of water injection for enhanced oil recovery WD = initiation of disposal ofproduced water in injection wells Data prior to 1973 are from California Division of Oil Gas and Geothermal Resources (2015) post-1973data are from California Division of Oil Gas and Geothermal Resources (2019)

80 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

compared to salinity data to determine the extent towhich salinity is controlled by aquifer stratigraphy

Wireline formation testers measure pressure buildup within a small interval of the sands (Schlumberger2006) Within a single homogeneous aquifer the pres-sure values should increase with depth along a straightline representing the hydrostatic gradient Shifts fromthe hydrostatic pressure gradient line indicate the pres-ence of pressures in individual aquifer layers that implyrestricted hydraulic connections with other aquiferlayers caused by confining clay layers (Coburn andGillespie 2002) Wireline formation tests in the TulareFormation have been conducted in some test holesdrilled in the study area by oil companies prior to wellinstallation (Gillespie et al 2019) These recordscompiled for 59 wells were analyzed to help deter-mine which clays are significant confining layers

Salinity Determination

For this study two different methods were used to de-termine water salinity within the aquifers in and aroundthe BelridgendashLost Hills oil fields The first is indirect es-timation of salinity using geophysical logs collected priorto installation of oil wells The second is direct mea-surement by lab analysis of producedwater samples fromoil wells in the area which were compared to the in-direct estimates

Log AnalysisAlthough direct sampling and chemical analyses arecommonly considered the best method for determiningTDS in any aquifer in many cases (particularly in deepaquifers containing brackish water) these analyses areuncommon In addition water samples provide only

Figure 4 Map showing depth to base of Tulare Formation The values are equivalent to the thickness of the Tulare Formation andoverlying alluvium Wells shown are those used to determine thickness Black lines are oil field administrative boundaries Gray lines aretownship boundaries Red lines are faults (Young 1968 California Division of Oil Gas and Geothermal Resources 1998) Abd= abandoned

Gillespie et al 81

Figure 5 Isochore maps of the (A) Amnicola Clay and (B) Corcoran Clay Equivalent layers in the study area Wells shown are those used to determine thickness Gray lines aretownship boundaries Red lines are faults (Young 1968 California Division of Oil Gas and Geothermal Resources 1998) Abd = abandoned Approx = Approximate

82Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

point-source data from a single depth interval and do nottell us at what depth the salinity changes However inactive petroleum-producing basins such as the SJV open-hole geophysical logs are abundant Geophysical logsprovide continuous vertical records of the properties ofthe formation matrix fluids adjacent to the boreholeand fluids within the borehole Using log analysis it ispossible to calculate the depths at which salinity changesoccur These depthsmay be verified by chemical analyseswhere available Of particular interest for determiningsalinity are the electrical logs (SP and resistivity) becausethe ability of water to conduct an electrical current isstrongly influenced by both the temperature and salinity ofthe formation water

Use of electrical logs to determine water salinity ismost common in the petroleum literature In oil-bearingreservoirs it is important to know the resistivity of thewater which occurs with the oil to determine the oilsaturation using the Archie (1942) equation or one ofits many variants

Fewer studies have evaluated the use of electricallogs to determine salinity in fresh to brackish aquifersHowells (1990) andHowells et al (1987) used well-loginterpretation to determine the location of the base ofmoderately saline (lt10000 mgL TDS) water in UtahLindner-Lunsford and Bruce (1995) used the ArchieEquation to determine salinity in aquifers with TDS lessthan 1000 mgL in southwest Wyoming Schnoebelenet al (1995) used geophysical logs to map the depth to10000 mgL TDS water in carbonate aquifers in Indianausing the SP log mud filtrate resistivity and the resis-tivity porosity method of Archie They compared theresults to sampled water analyses and found the resis-tivity porosity method to be most accurate More re-cently Hamlin and de la Rocha (2015) used the ArchieEquation to map fresh (lt1000 mgL TDS) slightlysaline (1000ndash3000mgL TDS) moderately saline (3000ndash10000mgL TDS) and very saline (gt10000mgL TDS)water in the CarrizondashWilcox aquifer of south TexasGillespie et al (2017) used log analysis to determinedepth to 10000 mgL TDS on a regional basis through-out much of the southern San Joaquin Basin but thewestern margin fields were not considered in theirstudy because the presence of shallow oil reservoirs requiremore extensive mapping to locate wet sands for loganalysis

In this study a variation of the Archie Equationwas used to estimate salinity (as NaCl equivalent)from the geophysical logs Archie (1942) related thein situ electrical resistivity of a fully water saturated

sedimentary rock (Ro) to its porosity (f) nondimen-sional factors related to matrix properties (the cemen-tation factor [m]and the tortuosity factor [a]) and theresistivity of the formation water (Rw) as shown inequation 1

Rw = Ropethfm=aTHORN (1)

Ro is the resistivity of a clean (minimal clay content) oil-free wet sand and can be obtained from the deepresistivity curve Sands were chosen based on an evalu-ation of nearby core mud log and drillers log data in-dicating that the sands analyzed did not contain oil orgas The porosity value is obtained from porosity logssuch as density neutron sonic or nuclear magneticresonance logs and core analysis For this study sonicporositieswere not used the poorly consolidated natureof the aquifer sediments causes additional slowing ofthe sonic wave from compression of the material re-sulting in artificially high porosity values

Because of the poorly sorted nature of the sedimentsand the presence of large amounts of clay and silt in somezones the average porosity used for salinity calcula-tions weighted density measurements twice as much asneutron values Neutron values for porosity are basedupon the abundance of hydrogen atoms in a formationBecause clays contain hydroxide groups within theircrystal lattice the neutron curve tends to overestimateporosity when clays are present The largest separationbetween the density and neutron curves on each logwasconsidered to be a zone with 100 clay The separationof the densityndashneutron curves for each evaluated in-terval was compared to the 100 clay separation valueto estimate clay content The Rw values were not cal-culated for intervals containing more than 25 clay

The exponentm (unitless) is related to the degree ofcementation in clastic rocks It typically ranges from 13to 26 (Wylie and Rose 1950) The value a (unitless) iscalled the tortuosity factor and is related to the lengththe current must travel through the formation Thevalue for a typically ranges from 05 to 15 For thisstudy special core analyses from five intervals in theTulare Formation were available in the CaliforniaDOGGRonline files from awater disposal well south ofthe Lost Hills field (American Petroleum Institute[API] well number [no] 0402974433) The value fora was kept constant at a value of 1 and m was allowedto vary The measured values for m ranged from 15 to18 with a mean value of 17 (standard deviation of015) and a median value of 18 For this study salinity

Gillespie et al 83

calculations used a = 1 andm = 17 based on the specialcore analysis results

Upon obtaining a value for Rw and calculating thetemperature of the zone of interest from a linear in-terpolation between bottom borehole temperature andair temperatureof 75degF the formationwater salinity canbeestimated from empirical charts such as those provided bySchlumberger (1997) or from equations such as thosein Bateman and Konen (1977) relating fluid resistivity toNaCl equivalent TDS concentration

Geochemical AnalysesThe California Division of Oil Gas and GeothermalResources (2016) maintains a website containing scan-ned copies of chemical analyses of oil field waters Thesedata are useful in determining the salinity of the deeperaquifers not normally used for drinking water or irriga-tion Data taken from these reports included dates oftesting source of water for test (when available) majorion concentrations and TDS concentrations The compileddata are available from Gillespie et al (2019) Chemicalanalysis of groundwater samples compiled from varioussources were also available (Metzger et al 2018) How-ever these samples were generally available from rela-tively shallow depths and had TDS ranging fromapproximately 1000 to 3000 mgL and therefore did notdirectly contribute to efforts to map the depth to USDW

Scanned copies of well completion reports are alsoavailable for the wells with chemical analysis from theCalifornia Division of Oil Gas and Geothermal Re-sources (2018) Well Finder online search engine Thecompletion reports for the wells provide informationincluding perforated intervals date perforated geolog-ical formation sampled (in some cases) scanned copiesof borehole geophysical logs pressure tests and bottom-hole temperatures These records were used to de-termine the depth interval and formation representedby the produced water chemical analysis data (Gillespieet al 2019)

Produced water chemical analyses from approxi-mately 150 oil production injection and observationwells in the study area were available from DOGGRrsquosonline chemical analysis database (California Divisionof Oil Gas and Geothermal Resources 2016) Addi-tionally 12 samples from the Tulare Formation and al-luvium were available from groundwater monitoringwells in the Lost Hills oil field on the Geotracker website(California State Water Resources Control Board2018a) The vintage of samples in the database rangesfrom 1932 to 2017

The sample analyses may not always representnatural aquifer conditions For example water samplesare commonly collected when a well has productionproblems or during the initial development of a well(Blondes et al 2016) Production problems may in-clude holes in well casings or annular cement that mayhave caused water from a different zone to enter theperforations In most cases few data exist regarding thesampling procedures and analysis techniques particu-larly for the older samples However older samples wereconsidered to be important for this study because theypredate the use of many EOR techniques such as waterflooding waste disposal and steam injection the large-scale use of which commenced in the early to mid-1960s These EOR processes may significantly alterthe salinity of the samples and cause the more recentsamples to be an inaccurate reflection of the originalaquifer conditions Samples taken at different timeintervals are helpful in determining temporal changes insalinity that may have occurred because of oil fieldactivities

One quality control method used in this study wasto perform a charge balance on the analyses Becausewater is electrically neutral the negative (anions) andpositive (cations) charged ions should sum to zero Thedifference between the total milliequivalents of thecations and anions is divided by the sum to give a chargebalance error We used a cutoff of ndash5 to determinewhich samples to discard in this analysis

For wells used for injection purposes the initiationof water injection relative to the date of sampling maybe important to consider Samples taken after the startof injection in the sampled well may not accuratelyrepresent the true formation water composition In mostcases it was not feasible to catalog the time of initiationof injection in nearby wells because of the large numberof injectionwells that could potentially affect a sampledwell Consequently the effect of subsurface injectionon sample TDS data remains a source of uncertainty inthe analysis

RESULTS

The methods and data discussed in the previous sec-tion were used to construct geologic maps showing thethickness and continuity of aquifers and confining unitsas well as to determine aquifer salinity and temporalchanges in salinity These results are discussed below

84 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 6 Graphs of shut-in pressure (from open-hole sequential formation testers) versus depth for three wells across the study area Red lines are used to emphasize the pressuregradient Tan areas showmajor clay layers the 22K sand is shown in yellow Base of Tulare and top of thewater table are also notedWells are identified by their API number and the yeardrilled and logged at the top of each graph

Gillespieetal

85

Geologic Mapping

The thickness of the Tulare Formation and overlyingalluvium in the study area (Figure 4) ranges from1200m (4000 ft) in the southeast part of the study areato 75 m (~250 ft) along the anticline crests in thenorthern parts of Lost Hills and North Belridge oilfields Because chemical analyses indicate that theprotected aquifers occur largely within the Tulare For-mation and overlying alluvium throughout the studyarea the map of the depth to the base of the TulareFormation defines the extent of potential protectedaquifers outside the oil field limits

The Amnicola Clay is thickest (67 m [~220 ft])along the northeastern and southeastern flank of theLost Hills anticline and east of the South Belridge an-ticline (Figure 5A) It thins to zero in the northwest partof the study area The clay is replaced by sandier sedi-ments east of the central part of the Lost Hills anticlineThe CCE thickens to 61m (~200 ft) in the northcentralpart of the study area in the Tulare lakebed but iscommonly less than30m(lt~100 ft) thick (Figure5B) Itis absent over the Lost Hills anticline crest and in thenorthwestern part of the study area These clay maps(Figure 5A B) are used to determine the presence ofpossible confining beds that in some cases may controlthe distribution of protected water resources and restrictthe migration of potential contaminants

Graphs of pressure versus depth collected in 1985from three wells along a northeast-trending transectfrom South Belridge to south of Lost Hills are shown inFigure 6 Marked changes in the hydrostatic pressuregradient in well API no 02974434 occur across theMiddle Tulare Clay and Amnicola Clay layers An addi-tional deeper clay here termed the D-E Clay forms alocal confining layer isolating a relatively low perme-ability section at the base of the Tulare Formation fromthe rest of the aquifer These characteristic deviationsin hydrostatic gradients indicates that the clays hydrau-lically separate individual aquifer layers in the area ofthis well

Farther northeast in wells API no 02974436 andAPI no 02974433 these deviations from hydrostaticgradient are much less noticeable especially across theAmnicola Clay This suggests that either (1) the claylayers are less confining south of Lost Hills or (2) thehydrologic regime near South Belridge has been alteredby production and injection into the Tulare Formationand the pressure responses in aquifer layers near the fieldin 1985 had not had sufficient time for their pressures

to reach equilibrium Recent drilling and multiple wellmonitoring site data (April 2018) off the eastern flank ofthe northern Lost Hills oil field indicates a 45 m (~15 ft)head difference across the Amnicola Clay with an upwardgradient of 0038 (measured May 31 2018) suggestingthat it is a confining layer in the northeastern part of thestudy area (US Geological Survey 2018a b)

Salinity Analysis

Log analysis from 30 wells in areas outside the oil fieldboundaries or within the oil fields but below the oilndashwater contact were used to create a map of the depthto the base of USDW The depth map to the base ofUSDW shows two trends (Figure 7) In the area be-tween Lost Hills and the Belridge oil fields depthto USDW increases toward the south from 150 m(~500 ft) between North Belridge and Lost Hills to490 m (~1600 ft) southeast of South Belridge In the

Figure 7 Map showing depth to base of underground sourcesof drinking water (USDW) (total dissolved solids = 10000 mgL)based on log analysis Red lines indicate faults (Young 1968California Division of Oil Gas and Geothermal Resources 1998)Black lines are oil field administrative boundaries Wells shown asblue dots are those used to determine the base of USDW Depthvalues in feet CI = contour interval No = North So = South

86 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

eastern part of the study area depth to base of USDWincreases sharply eastward from approximately 215 m(~700 ft) on the west flank of the Lost Hills anticline toas great as 550 m (~1800 ft) within 3 km (~2 mi)

Salinityndashdepth profiles based on chemical analysesof producedwater samples were compared to the depthto base USDW map estimated from borehole geo-physical log analysis However this comparison waslimited because most of the lab analyses come from oil-producing wells within the field boundaries rather thanin areas outside the fields The presence of oil in thesampled intervals within the fields precludes the useof log analysis to calculate the salinity in these samewells Plots of TDS versus depth of the samples in eachfield are shown in Figures 8 and 9 along with log-calculated TDS

Wood and Davis (1959) reported the groundwaterchemistry in this area as being an NaSO4 type howeveronly one Tulare Formation monitoring well Mackesseyno 1 (API no 02987363) was an NaSO4 type the restare NaCl type Because the calculated salinity fromArchie log analysis assumes NaCl water types the log-calculated and lab sample TDS values should be com-parable The two monitoring wells completed in thealluvium have higher SO4 concentrations relative tothe Tulare Formation wells however Cl is the domi-nant anion

Only four Tulare Formation lab samples (blue dots)in Lost Hills are in the California DOGGR databaseData from 14 monitoring wells in the central part ofthe Lost Hills field were used to enlarge the data set(Figure 8) With the exception of one sample TDSconcentrations in the Tulare Formation and alluviumare less than 10000 mgL indicating protected (USDW)aquifers These wells have top perforations ranging from40 to 290 m (133ndash950 ft) The well with a lab analysisindicating a salinity of 12295 mgL TDS in the TulareFormation had a top perforation at 300 m (984 ft) Thissuggests that the base of USDW lies somewhere be-tween 290 and 300 m (950 and 985 ft) within the fieldLog analysis suggests that the base may occur as deepas 580 m (~1900 ft) but many of the wells used for loganalysis come from areas east of the field boundarywhere the Tulare Formation is much thicker (200 ftwithin the field vs 2000 ft east of the field [Figure 4])Within the field interpolated contour lines from log datashow the depth to base USDWat 213m (~700 ft) alongthe western field boundary and 365 m (~1200 ft) alongthe eastern field boundary a trend that is consistentwith the more limited water sample data

Typical TDS concentrations in the underlyingEtchegoin Formation (red dots) range from 20000 to35000 mgL probably because of the presence oftrapped connate seawater within these marine depositsIn northern Lost Hills (Figure 8) two samples fromEtchegoin sands have TDS concentrations below 10000mgL However in this area an active steamflood isongoing within the Tulare and Etchegoin FormationsHere the Etchegoin Formation sands lie at shallowdepths (lt150 m [~500 ft]) immediately below pro-ducing intervals in the Tulare Formation The EtchegoinFormation samples in this area are much more dilute(5600ndash8000 mgL) than in other areas This may be be-cause the sands are in hydraulic communication with thefresher water sands of the overlying Tulare Formation orbecause the Etchegoin sands in this area are steamfloodedand the more saline formation water is diluted by theinjected steam Since the twoEtchegoinUSDWsamplesare post-steamflood initiation the latter explanation islikely

Only one chemical analysis sample from the TulareFormation is available in the North Belridge field (Fig-ure 9) The TDS concentration of the sample is 6055mgL from a depth interval of 157ndash211 m (515ndash692ft) It confirms that waters above approximately 211 m(~700 ft) are USDW consistent with the log-deriveddepth to USDW map which shows depth to USDWnear the well at 275 m (900 ft)

Many lab analyses are available for the Tulare For-mation at South Belridge (Figure 9) but they do notshow salinity values that are correlated with depth Thelog-generated depth to USDW map shows that salinitytrends along the northern part of the oil field are vari-able but the southern part of the field shows depth toUSDW increasing toward the southeast Thereforesamples from northern and southern South Belridgewere considered separately The boundary betweenthe two subareas is shown by the dashed line in Figure 9Because of the variability in sample TDS values withdepth comparisons of sample data to calculated salin-ities fromwell-log analyses can only bemade in a generalfashion

In the northern part of the South Belridge field nosamples below 365 m (1200 ft) are USDW suggestingthat the base of USDW lies above this depth The log-derived map shows depth to base of USDW at 335 m(1100 ft) in the north To the south the base of USDWincreases to 427 m (~1400 ft) Near the dividing linebetween the north and south parts of the field thedepth to base USDW decreases to 365 m (1200 ft)

Gillespie et al 87

Figure 8 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare and alluvium (blue) and Etchegoin (red) formations in and near the Lost Hills oilfield Gray dots represent TDS calculated from geophysical logs in the field The horizontal red linemarks 10000mgL and the vertical dashed red linemarks the approximate depth of thedeepest underground sources of drinking water (USDW) sample The adjacent map shows the location of the Tulare and alluviumwells sampled as blue dots and shallow (lt150m [500ft] deep) Etchegoin samples as red dots (deeper Etchegoin sample sites are not shown onmap) and the contours to base USDW from log analysis Lost Hills is the only area that containsbrackish water in the Etchegoin Formation However the brackish waters in the Etchegoin occur in an oil-producing zone that is steam flooded so it is considered to be an exempt aquiferand is not protected CI = contour interval

88Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

Figure 9 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare Formation in and near the North and South Belridge oil fields The northern andsouthern parts of the South Belridge field (marked by the red line on the map) are shown in separate graphs Blue dots show Tulare Formation (Fm) Measurements of TDS from labanalyses Gray dots show Tulare Fm The TDS calculated from log analysis Horizontal red lines on the graphs mark 10000 mgL and the dashed vertical red lines mark the approximatedepth of USDW from lab samples The adjacent map shows the location of the wells sampled (blue dots) and the contours to base underground sources of drinking water (USDW) (infeet) from log analysis Black lines are oil field administrative boundaries and red lines are faults CI = contour interval No = North So = South

Gillespieetal

89

Figure 10 Cross section showing three wells drilled and logged in different years near a produced water disposal pond (light blue polygon on inset map) Yellow shading onspontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivity curve (right track at scale of 0ndash20 ohmm) indicate resistivity of more than 3 ohmm Pinkbars indicate low resistivity sands affected by infiltration of saline produced water from the pond The API numbers at the top of the logs are used for well identification Year drilled andlogged in parentheses Depth scale in feet

90Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

This may be caused by the presence of an active waterdisposal field in the Tulare Formation near this areaOverall the log-derived map shows base of USDWapproximately 61m (~200 ft) deeper than lab analyseswould suggest As at Lost Hills this may be caused bythe geographic distribution of the two data sets the labsamples coming mainly from within the oil fieldboundaries whereas the log data are confined to thearea outside the oil field

In the southern part of South Belridge field a wellnear the southern tip of the field was sampled at mul-tiple intervals Samples as deep as 455 m (~1495 ft) areUSDW and the deepest sample from 480 m (1575 ft)has a TDS value of 13074 mgL suggesting that thebase of USDW in the southern part of South Belridge liesbetween 457 and 480 m (1495ndash1575 ft) (Figure 9) Thedepth to USDW map from log analysis in the southernpart of South Belridge shows that the base of USDW

ranges from 365 m (1200 ft) in the north to approxi-mately 487 m (~1600 ft) in the south

Effects of Oil Field Activities on Water Salinity

The effects of water disposal in surface ponds on ground-water salinity is evident by comparing geophysical logsfrom old wells near the pond locations to logs fromnewerwells drilled nearbyOlder geophysical logs (pre-1970s) indicate that the upper sands in the alluviumare desaturated (vadose zone) as indicated by areas ofcrossover on densityndashneutron logs and have high re-sistivity (gt20ohmmFigure 10)However geophysicallogs collected from wells drilled later in the area donot show densityndashneutron crossover characteristic of avadose zone andhave extremely low resistivity (lt1ohmm)in the upper alluvial sands because of the presence ofhigh salinityproducedwater thathasfilled thevadosezoneInvasion of low resistivity water is especially apparent

Figure 11 Two adjacent wells drilled and logged in different years (1990 and 2012) in an area containing numerous disposal wells on theeast flank of South Belridge Yellow shading on spontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivitycurve indicates resistivity greater than 3 ohm m Pink bars indicate low resistivity sands in the 2012 well (0403046566) affected by injectedsaline produced water The API numbers at the top of the logs are used for well identification Year drilled and logged in parentheses

Gillespie et al 91

where saline waters have infiltrated the 22K sandwhich lies directly above the CCE (well 03047481 inFigure 10) in the lower part of the alluvium The salinewater lies atop the CCE which appears to be pro-tecting the underlying Tulare aquifer from contami-nation Log-calculated andmeasuredTDS concentrationscan exceed 20000mgL in affected parts of the alluvium

The effects of surface disposal ponds are less evidentin Lost Hills and North Belridge compared to the areaadjacent to South Belridge Lost Hills in particular tends tohave highly saline water in the shallow parts of the aquifereven where disposal ponds are not present Swain andDuell (1989) noted TDS concentrations as high as 21700mgL from very shallowwells (6m [~20 ft]) along the eastflank of Lost Hills but did not note the cause of the highsalinity in the area

The effects of water disposal injection wells aremore subtle than that of the ponds and are especiallydifficult to identify in sands closer to the base of USDWwhere resistivity is naturally fairly low Geophysicallogs in sands affected by saline injection tend to have aconcave resistivity profile and resistivities are as small ashalf that in logs fromolder unaffectedwells (Figure 11)Injected saline water tends to follow individual sand

layers leaving some layers relatively unaffected In somecases only the lower part of an individual sand containssaline water whereas the waters in the upper part re-mainbrackishThis suggests that thewaters are stratifiedand mixing has not had time to occur Log-calculatedTDS concentrations show a relatively smooth increasewith depth inwells logged prior to the start of large-scaledisposal activities whereas recent logs in these areasshow a much more variable TDS concentration profilewith depth Additionally densityndashneutron curves inwells affected by disposal of produced waters show thinintervals (~1 m [~3 ft] thick) of crossover This may becaused by bacterial metabolism of trace amounts oforganic matter in the disposal water resulting in thegeneration of small amounts of biogenic methane(McMahon et al 2018)

The cross section in Figure 12 shows wells affectedby saline produced water disposal by injection (belowthe CCE) and percolation (above the CCE) along theeast flank of the South Belridge oil field Wells affectedby disposal of produced waters in saline ponds (n = 139)and by disposal in injection wells (n = 125) are alsoshown in the map in Figure 13 The effects of disposalponds are evident from changes in borehole resistivity

Figure 12 Cross section showing intervals affected by disposal of saline produced water (pink boxes) at South Belridge oil field Wellsidentified by API number at top of each well Year drilled and logged shown at bottom of eachwell Insetmap shows location of cross sectionProduced water disposal wells are highlighted in pink and disposal ponds are shown in blue in the inset map

92 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 2 Type log (API 0402974434 shown by red star on inset map) of the alluvium and Tulare Formation in the study area Yellow shading on spontaneous potential (SP) curveindicates sand bodies in the aquifer Yellow shading on resistivity curve indicates resistivity greater than 3 ohm m Major confining clays are shown in gray Depth values in feetStratigraphic column of the southern San Joaquin Valley is shown on the right with formations addressed in this study highlighted in red Modified from Scheirer and Magoon (2007)Fm = Formation Mtn = Mountain No = North Sh = Shale So = South

Gillespieetal

77

the Tulare Formation on the western basin margin is1147 m (3500 ft) (Woodring et al 1940 Loomis1990)

Where it does not crop out the Tulare Formation isoverlain by alluvial deposits Both the Tulare Formationand the overlying alluvium are derived primarily fromthe Coast Ranges and interfinger with Sierran-derivedsediments from the east (Laudon and Belitz 1991) TheTulare Formation and overlying alluvium have similarlithologic compositions and were deposited in fluvial andlacustrine environments therefore they are not readilydistinguishable especially in the subsurface Maps ofthewater-level contours in the Tulare and alluvial aquiferby Wood and Davis (1959) indicate eastward gradientsin the aquifer

At Belridge and Lost Hills the basal Tulare For-mation contact is an angular unconformity belowwhichthe San Joaquin Formation has been removed overmostof the area As a result the Tulare Formation commonlylies directly upon marine deposits of the late Mioceneto Pliocene Etchegoin Formation along the crests ofthe anticlines

In the study area the lower Tulare Formationconsists of lacustrine and deltaic sands and the upperTulare contains alluvial fan meandering channels andfloodplain facies (Kiser et al 1988 Miller et al 1990)The Tulare Formation and overlying alluvium also in-clude lacustrine clays which form confining bedsSeveral distinct regional clay units within the TulareFormation are present in the study area Some of theseclays are not present throughout the study area butwhen present act as local confining layers The threemain clays mapped using borehole geophysical logsin this study are the Amnicola the Middle Tulare andthe Corcoran Clay Equivalent (CCE) (Figure 2) The D-EClay is only present in the southeastern part of the areaand was not mapped in this study

The Amnicola is the oldest of the three mapped claylayers It is described as an olive gray partly calcareousdolomitic claystone that contains the gastropodAmnicola(Woodring et al 1932 Berryman 1973 Maher et al1975) In the southeast part of the study area where itis best developed the Amnicola Clay lies atop a largecoarsening upward sand The Amnicola Clay is absentacross the anticlinal crests and in the northern part ofthe syncline between Lost Hills and North BelridgeFossil evidence from recent drilling in the area indi-cates that the Amnicola Clay is likely present alongthe east flank of northern Lost Hills

Overlying the Amnicola Clay is a coarsening up-ward zone of sands silts and clay that ranges from ap-proximately 120ndash200 m (~400ndash700 ft) thick The zonethickens to the south and thins to the north and along thecrest of the anticlines A clay layer with a pronouncedhigh gamma-ray signature on geophysical logs liesabove this zone and is termed the Middle Tulare Clayin this report The Middle Tulare Clay marks a changein log character from vertically stacked funnel shapedspontaneous potential (SP) and resistivity patterns withrelatively high gamma-ray response (80ndash100 API in thelowerTulare Formation to a blocky SP and resistivity logpattern with lower gamma-ray response (40ndash80API) inthe upper Tulare Formation This change in log patternmay represent a change from a series of prograding la-custrine delta environments in the lower Tulare For-mation to a fluvial meander belt setting in the upperTulare Formation as noted by Miller et al (1990)

The Corcoran Clay described by Frink and Kues(1954) also known as the E clay (Croft 1972) or blueclay by water well drillers is the youngest of the threemapped clay layers and is an important marker bedwithin the SJV aquifer system It divides the ground-water system of the western SJV into an upper semi-confined zone and a lower confined zone (Williamsonet al 1989 Belitz and Heimes 1990) Several studieshave mapped the upper clays of the Tulare Formationbeginning with Croft (1972) who mapped numerousclay layers in the southern SJV including the study areaMore recent studies have focused specifically on theCorcoran Clay and have used both surface and sub-surface data to map the extent and thickness of the clay(Page 1986 Burow et al 2004 Faunt 2009)

Petroleum companies operating in the area refer toa basal alluvial clay at the contact between the TulareFormation and overlying alluvium as the CCE althoughits relationship to the Corcoran Clay as defined by Frinkand Kues (1954) is not clear because Frink and Kues(1954) place the Corcoran Clay within the TulareFormation and Kiser et al (1988) place it within thealluvium In the northeastern part of the study area theCCE correlates to the E clay of Croft (1972) South ofthe study area in the SJV the identification of theCorcoran Clay becomes more problematic and Page(1986) refers to the Corcoran Clay in this area as themodified E clay Because the exact relationship betweenthe CCE and the Corcoran Clay is not known the claywill be termed the CCE in this paper

The CCE is up to 60 m (200 ft) thick within thestudy area It is absent in the northwestern part of the

78 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

study area and above the Lost Hills and Belridge anti-clines Drillers in the area observe a color change in cut-tings from orange-brown above the CCE to graybelow it A thick (18ndash21 m [60ndash70 ft]) sand the 22Ksand lies above the CCE near South Belridge (Figure2) Where the 22K sand is present above the CCE itforms a separate aquifer from the underlying TulareFormation

OIL FIELD ACTIVITY

The Lost Hills and Belridge fields were discoveredduring 1910ndash1912 Initial production was from heavyoil sands in the Tulare and Etchegoin Formations andfrom fractured shales of the Monterey Formation In1930 light oil was discovered in the deeper TemblorFormation at North Belridge but it is the shallowerheavy oil along with fractured diatomaceous depositsand shales in the Etchegoin Reef Ridge and MontereyFormations that have been the mainstay of the fieldsto the present day because of the development of EORtechniques such as steamflooding waterflooding andhydraulic fracturing (Bailey 1939 Ritzius 1950 Land1984 California Division of Oil Gas and GeothermalResources 1998)

The waterflood technique was first initiated in thelate 1940s in the Monterey shales in Lost Hills and themid-1950s in the Temblor Formation sandstones inNorth Belridge (Figure 3) In the 1980s waterfloodscommenced in the Tulare Formation and the diatomitezones of the Monterey Reef Ridge and Etchegoin For-mations In the 1950s and 1960s thermal EOR tech-niques (mainly steamcycling andflooding and to a lesserextent firefloods) were initiated to maximize produc-tion of heavy oil in the Tulare and Etchegoin Formationsand Monterey diatomite

In the diatomite zones large-scale waterfloodingprimarily to enhance oil recovery after hydraulic frac-turing began in the late 1960s and 1970s but becameincreasingly common after 1980 Although the water-floods increased production they also served to mitigateland subsidence and wellbore casing collapse caused byfluid removal from the high-porosity diatomite zones(Dale et al 1996)

As oil is removed water commonly comes in to fillthe void and it is common for waterndashoil ratios (WORs)to increase over the life of an oil field In the Belridge oilfields the WOR increased from 009 in 1931 to ap-proximately 14 in 2016 and in Lost Hills the WOR

increased from15 in 1931 to approximately 12 in 2016Some of the produced water is reused for EOR prac-tices such as waterflooding but the remainder is dis-posed of by a variety ofmeans Initially water disposalwas accomplished by spreading the produced wateron the ground commonly in surface ponds or drystream beds and the water was allowed to evaporateor percolate into the underlying alluvium (Mitchell1989)

A series of legislative bills passed at both the stateand federal level starting in 1969 with the Porter Co-logne Water Quality Act the 1972 Clean Water Actand the Safe Drinking Water Act (1974) provided au-thority to establish regulations regarding surface waterand groundwater degradation As a result of theseregulations produced water disposal began to movefrom surface ponds to injection into nonoil-producingzones via water disposal wells The federal Under-ground Injection Control program established nation-wide requirements for the protection of all aquiferscontaining water with less than 10000 mgL TDS

At Lost Hills oil field water disposal by injectionhas increased from approximately 160000 m3yr(~1 million bblyr) in 1973 to 48 million m3yr(30 million bblyr) in 2009 Disposal initially occurredinto both the Tulare and Etchegoin Formations butby 1990 disposal was almost exclusively into theEtchegoin Formation At North and South Belridgefields disposal began at 636000 m3yr (4 millionbblsyr) in 1970 and increased to more than 12million m3yr (gt78 million bblyr) in 2009 (Figure 3)Belridge disposal is almost entirely into the TulareFormation

METHODS

The main data used in this study are borehole geo-physical logs and water quality laboratory analysesThese data were used to pick marker horizons forgeologic mapping and to determine water salinityand aquifer pressures Data compiled and analyzedfor this study are available from Gillespie et al(2019)

Mapping

Approximately 900 borehole geophysical logs fromwells within the study area were used to determinethe lateral continuity and stratigraphic relationships

Gillespie et al 79

between the late Miocene to Holocene formationsElectric logs (resistivity SP) and gamma-ray logs wereused to identify correlatable layers (Figure 2) Well data(geophysical logs mud logs core data and well his-tories) were obtained primarily from oil and gas wellrecords available from theDOGGR(CaliforniaDivisionof Oil Gas and Geothermal Resources 2018) and fromwater well records obtained from well owners and theCalifornia Department of Water Resources

Because the Tulare Formation and overlying allu-vium form the major nonexempt aquifers in the area

these layers were mapped in more detail particularlywith regard to clay layers Many of the clay markers losetheir coherence when mapped over large distances andbetween distantly spaced wells outside the oil fieldboundaries because of the rapidly changing characterof the Tulare fluvialndashlacustrine delta system In manyplaces it was not possible to correlate the clay layersthroughout the region The correlations were used toconstruct thickness (isochore) maps of the combinedalluvium andTulare Formation (Figure 4) andCCE andAmnicola clays (Figure 5) The correlated logs were

Figure3 Production (prod) and injection (inj) curves for the Lost Hills and North and South Belridge fields (combined) EOR= enhancedoil recovery HF = initiation of hydrofracking Inj = initiation of water injection for enhanced oil recovery WD = initiation of disposal ofproduced water in injection wells Data prior to 1973 are from California Division of Oil Gas and Geothermal Resources (2015) post-1973data are from California Division of Oil Gas and Geothermal Resources (2019)

80 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

compared to salinity data to determine the extent towhich salinity is controlled by aquifer stratigraphy

Wireline formation testers measure pressure buildup within a small interval of the sands (Schlumberger2006) Within a single homogeneous aquifer the pres-sure values should increase with depth along a straightline representing the hydrostatic gradient Shifts fromthe hydrostatic pressure gradient line indicate the pres-ence of pressures in individual aquifer layers that implyrestricted hydraulic connections with other aquiferlayers caused by confining clay layers (Coburn andGillespie 2002) Wireline formation tests in the TulareFormation have been conducted in some test holesdrilled in the study area by oil companies prior to wellinstallation (Gillespie et al 2019) These recordscompiled for 59 wells were analyzed to help deter-mine which clays are significant confining layers

Salinity Determination

For this study two different methods were used to de-termine water salinity within the aquifers in and aroundthe BelridgendashLost Hills oil fields The first is indirect es-timation of salinity using geophysical logs collected priorto installation of oil wells The second is direct mea-surement by lab analysis of producedwater samples fromoil wells in the area which were compared to the in-direct estimates

Log AnalysisAlthough direct sampling and chemical analyses arecommonly considered the best method for determiningTDS in any aquifer in many cases (particularly in deepaquifers containing brackish water) these analyses areuncommon In addition water samples provide only

Figure 4 Map showing depth to base of Tulare Formation The values are equivalent to the thickness of the Tulare Formation andoverlying alluvium Wells shown are those used to determine thickness Black lines are oil field administrative boundaries Gray lines aretownship boundaries Red lines are faults (Young 1968 California Division of Oil Gas and Geothermal Resources 1998) Abd= abandoned

Gillespie et al 81

Figure 5 Isochore maps of the (A) Amnicola Clay and (B) Corcoran Clay Equivalent layers in the study area Wells shown are those used to determine thickness Gray lines aretownship boundaries Red lines are faults (Young 1968 California Division of Oil Gas and Geothermal Resources 1998) Abd = abandoned Approx = Approximate

82Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

point-source data from a single depth interval and do nottell us at what depth the salinity changes However inactive petroleum-producing basins such as the SJV open-hole geophysical logs are abundant Geophysical logsprovide continuous vertical records of the properties ofthe formation matrix fluids adjacent to the boreholeand fluids within the borehole Using log analysis it ispossible to calculate the depths at which salinity changesoccur These depthsmay be verified by chemical analyseswhere available Of particular interest for determiningsalinity are the electrical logs (SP and resistivity) becausethe ability of water to conduct an electrical current isstrongly influenced by both the temperature and salinity ofthe formation water

Use of electrical logs to determine water salinity ismost common in the petroleum literature In oil-bearingreservoirs it is important to know the resistivity of thewater which occurs with the oil to determine the oilsaturation using the Archie (1942) equation or one ofits many variants

Fewer studies have evaluated the use of electricallogs to determine salinity in fresh to brackish aquifersHowells (1990) andHowells et al (1987) used well-loginterpretation to determine the location of the base ofmoderately saline (lt10000 mgL TDS) water in UtahLindner-Lunsford and Bruce (1995) used the ArchieEquation to determine salinity in aquifers with TDS lessthan 1000 mgL in southwest Wyoming Schnoebelenet al (1995) used geophysical logs to map the depth to10000 mgL TDS water in carbonate aquifers in Indianausing the SP log mud filtrate resistivity and the resis-tivity porosity method of Archie They compared theresults to sampled water analyses and found the resis-tivity porosity method to be most accurate More re-cently Hamlin and de la Rocha (2015) used the ArchieEquation to map fresh (lt1000 mgL TDS) slightlysaline (1000ndash3000mgL TDS) moderately saline (3000ndash10000mgL TDS) and very saline (gt10000mgL TDS)water in the CarrizondashWilcox aquifer of south TexasGillespie et al (2017) used log analysis to determinedepth to 10000 mgL TDS on a regional basis through-out much of the southern San Joaquin Basin but thewestern margin fields were not considered in theirstudy because the presence of shallow oil reservoirs requiremore extensive mapping to locate wet sands for loganalysis

In this study a variation of the Archie Equationwas used to estimate salinity (as NaCl equivalent)from the geophysical logs Archie (1942) related thein situ electrical resistivity of a fully water saturated

sedimentary rock (Ro) to its porosity (f) nondimen-sional factors related to matrix properties (the cemen-tation factor [m]and the tortuosity factor [a]) and theresistivity of the formation water (Rw) as shown inequation 1

Rw = Ropethfm=aTHORN (1)

Ro is the resistivity of a clean (minimal clay content) oil-free wet sand and can be obtained from the deepresistivity curve Sands were chosen based on an evalu-ation of nearby core mud log and drillers log data in-dicating that the sands analyzed did not contain oil orgas The porosity value is obtained from porosity logssuch as density neutron sonic or nuclear magneticresonance logs and core analysis For this study sonicporositieswere not used the poorly consolidated natureof the aquifer sediments causes additional slowing ofthe sonic wave from compression of the material re-sulting in artificially high porosity values

Because of the poorly sorted nature of the sedimentsand the presence of large amounts of clay and silt in somezones the average porosity used for salinity calcula-tions weighted density measurements twice as much asneutron values Neutron values for porosity are basedupon the abundance of hydrogen atoms in a formationBecause clays contain hydroxide groups within theircrystal lattice the neutron curve tends to overestimateporosity when clays are present The largest separationbetween the density and neutron curves on each logwasconsidered to be a zone with 100 clay The separationof the densityndashneutron curves for each evaluated in-terval was compared to the 100 clay separation valueto estimate clay content The Rw values were not cal-culated for intervals containing more than 25 clay

The exponentm (unitless) is related to the degree ofcementation in clastic rocks It typically ranges from 13to 26 (Wylie and Rose 1950) The value a (unitless) iscalled the tortuosity factor and is related to the lengththe current must travel through the formation Thevalue for a typically ranges from 05 to 15 For thisstudy special core analyses from five intervals in theTulare Formation were available in the CaliforniaDOGGRonline files from awater disposal well south ofthe Lost Hills field (American Petroleum Institute[API] well number [no] 0402974433) The value fora was kept constant at a value of 1 and m was allowedto vary The measured values for m ranged from 15 to18 with a mean value of 17 (standard deviation of015) and a median value of 18 For this study salinity

Gillespie et al 83

calculations used a = 1 andm = 17 based on the specialcore analysis results

Upon obtaining a value for Rw and calculating thetemperature of the zone of interest from a linear in-terpolation between bottom borehole temperature andair temperatureof 75degF the formationwater salinity canbeestimated from empirical charts such as those provided bySchlumberger (1997) or from equations such as thosein Bateman and Konen (1977) relating fluid resistivity toNaCl equivalent TDS concentration

Geochemical AnalysesThe California Division of Oil Gas and GeothermalResources (2016) maintains a website containing scan-ned copies of chemical analyses of oil field waters Thesedata are useful in determining the salinity of the deeperaquifers not normally used for drinking water or irriga-tion Data taken from these reports included dates oftesting source of water for test (when available) majorion concentrations and TDS concentrations The compileddata are available from Gillespie et al (2019) Chemicalanalysis of groundwater samples compiled from varioussources were also available (Metzger et al 2018) How-ever these samples were generally available from rela-tively shallow depths and had TDS ranging fromapproximately 1000 to 3000 mgL and therefore did notdirectly contribute to efforts to map the depth to USDW

Scanned copies of well completion reports are alsoavailable for the wells with chemical analysis from theCalifornia Division of Oil Gas and Geothermal Re-sources (2018) Well Finder online search engine Thecompletion reports for the wells provide informationincluding perforated intervals date perforated geolog-ical formation sampled (in some cases) scanned copiesof borehole geophysical logs pressure tests and bottom-hole temperatures These records were used to de-termine the depth interval and formation representedby the produced water chemical analysis data (Gillespieet al 2019)

Produced water chemical analyses from approxi-mately 150 oil production injection and observationwells in the study area were available from DOGGRrsquosonline chemical analysis database (California Divisionof Oil Gas and Geothermal Resources 2016) Addi-tionally 12 samples from the Tulare Formation and al-luvium were available from groundwater monitoringwells in the Lost Hills oil field on the Geotracker website(California State Water Resources Control Board2018a) The vintage of samples in the database rangesfrom 1932 to 2017

The sample analyses may not always representnatural aquifer conditions For example water samplesare commonly collected when a well has productionproblems or during the initial development of a well(Blondes et al 2016) Production problems may in-clude holes in well casings or annular cement that mayhave caused water from a different zone to enter theperforations In most cases few data exist regarding thesampling procedures and analysis techniques particu-larly for the older samples However older samples wereconsidered to be important for this study because theypredate the use of many EOR techniques such as waterflooding waste disposal and steam injection the large-scale use of which commenced in the early to mid-1960s These EOR processes may significantly alterthe salinity of the samples and cause the more recentsamples to be an inaccurate reflection of the originalaquifer conditions Samples taken at different timeintervals are helpful in determining temporal changes insalinity that may have occurred because of oil fieldactivities

One quality control method used in this study wasto perform a charge balance on the analyses Becausewater is electrically neutral the negative (anions) andpositive (cations) charged ions should sum to zero Thedifference between the total milliequivalents of thecations and anions is divided by the sum to give a chargebalance error We used a cutoff of ndash5 to determinewhich samples to discard in this analysis

For wells used for injection purposes the initiationof water injection relative to the date of sampling maybe important to consider Samples taken after the startof injection in the sampled well may not accuratelyrepresent the true formation water composition In mostcases it was not feasible to catalog the time of initiationof injection in nearby wells because of the large numberof injectionwells that could potentially affect a sampledwell Consequently the effect of subsurface injectionon sample TDS data remains a source of uncertainty inthe analysis

RESULTS

The methods and data discussed in the previous sec-tion were used to construct geologic maps showing thethickness and continuity of aquifers and confining unitsas well as to determine aquifer salinity and temporalchanges in salinity These results are discussed below

84 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 6 Graphs of shut-in pressure (from open-hole sequential formation testers) versus depth for three wells across the study area Red lines are used to emphasize the pressuregradient Tan areas showmajor clay layers the 22K sand is shown in yellow Base of Tulare and top of thewater table are also notedWells are identified by their API number and the yeardrilled and logged at the top of each graph

Gillespieetal

85

Geologic Mapping

The thickness of the Tulare Formation and overlyingalluvium in the study area (Figure 4) ranges from1200m (4000 ft) in the southeast part of the study areato 75 m (~250 ft) along the anticline crests in thenorthern parts of Lost Hills and North Belridge oilfields Because chemical analyses indicate that theprotected aquifers occur largely within the Tulare For-mation and overlying alluvium throughout the studyarea the map of the depth to the base of the TulareFormation defines the extent of potential protectedaquifers outside the oil field limits

The Amnicola Clay is thickest (67 m [~220 ft])along the northeastern and southeastern flank of theLost Hills anticline and east of the South Belridge an-ticline (Figure 5A) It thins to zero in the northwest partof the study area The clay is replaced by sandier sedi-ments east of the central part of the Lost Hills anticlineThe CCE thickens to 61m (~200 ft) in the northcentralpart of the study area in the Tulare lakebed but iscommonly less than30m(lt~100 ft) thick (Figure5B) Itis absent over the Lost Hills anticline crest and in thenorthwestern part of the study area These clay maps(Figure 5A B) are used to determine the presence ofpossible confining beds that in some cases may controlthe distribution of protected water resources and restrictthe migration of potential contaminants

Graphs of pressure versus depth collected in 1985from three wells along a northeast-trending transectfrom South Belridge to south of Lost Hills are shown inFigure 6 Marked changes in the hydrostatic pressuregradient in well API no 02974434 occur across theMiddle Tulare Clay and Amnicola Clay layers An addi-tional deeper clay here termed the D-E Clay forms alocal confining layer isolating a relatively low perme-ability section at the base of the Tulare Formation fromthe rest of the aquifer These characteristic deviationsin hydrostatic gradients indicates that the clays hydrau-lically separate individual aquifer layers in the area ofthis well

Farther northeast in wells API no 02974436 andAPI no 02974433 these deviations from hydrostaticgradient are much less noticeable especially across theAmnicola Clay This suggests that either (1) the claylayers are less confining south of Lost Hills or (2) thehydrologic regime near South Belridge has been alteredby production and injection into the Tulare Formationand the pressure responses in aquifer layers near the fieldin 1985 had not had sufficient time for their pressures

to reach equilibrium Recent drilling and multiple wellmonitoring site data (April 2018) off the eastern flank ofthe northern Lost Hills oil field indicates a 45 m (~15 ft)head difference across the Amnicola Clay with an upwardgradient of 0038 (measured May 31 2018) suggestingthat it is a confining layer in the northeastern part of thestudy area (US Geological Survey 2018a b)

Salinity Analysis

Log analysis from 30 wells in areas outside the oil fieldboundaries or within the oil fields but below the oilndashwater contact were used to create a map of the depthto the base of USDW The depth map to the base ofUSDW shows two trends (Figure 7) In the area be-tween Lost Hills and the Belridge oil fields depthto USDW increases toward the south from 150 m(~500 ft) between North Belridge and Lost Hills to490 m (~1600 ft) southeast of South Belridge In the

Figure 7 Map showing depth to base of underground sourcesof drinking water (USDW) (total dissolved solids = 10000 mgL)based on log analysis Red lines indicate faults (Young 1968California Division of Oil Gas and Geothermal Resources 1998)Black lines are oil field administrative boundaries Wells shown asblue dots are those used to determine the base of USDW Depthvalues in feet CI = contour interval No = North So = South

86 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

eastern part of the study area depth to base of USDWincreases sharply eastward from approximately 215 m(~700 ft) on the west flank of the Lost Hills anticline toas great as 550 m (~1800 ft) within 3 km (~2 mi)

Salinityndashdepth profiles based on chemical analysesof producedwater samples were compared to the depthto base USDW map estimated from borehole geo-physical log analysis However this comparison waslimited because most of the lab analyses come from oil-producing wells within the field boundaries rather thanin areas outside the fields The presence of oil in thesampled intervals within the fields precludes the useof log analysis to calculate the salinity in these samewells Plots of TDS versus depth of the samples in eachfield are shown in Figures 8 and 9 along with log-calculated TDS

Wood and Davis (1959) reported the groundwaterchemistry in this area as being an NaSO4 type howeveronly one Tulare Formation monitoring well Mackesseyno 1 (API no 02987363) was an NaSO4 type the restare NaCl type Because the calculated salinity fromArchie log analysis assumes NaCl water types the log-calculated and lab sample TDS values should be com-parable The two monitoring wells completed in thealluvium have higher SO4 concentrations relative tothe Tulare Formation wells however Cl is the domi-nant anion

Only four Tulare Formation lab samples (blue dots)in Lost Hills are in the California DOGGR databaseData from 14 monitoring wells in the central part ofthe Lost Hills field were used to enlarge the data set(Figure 8) With the exception of one sample TDSconcentrations in the Tulare Formation and alluviumare less than 10000 mgL indicating protected (USDW)aquifers These wells have top perforations ranging from40 to 290 m (133ndash950 ft) The well with a lab analysisindicating a salinity of 12295 mgL TDS in the TulareFormation had a top perforation at 300 m (984 ft) Thissuggests that the base of USDW lies somewhere be-tween 290 and 300 m (950 and 985 ft) within the fieldLog analysis suggests that the base may occur as deepas 580 m (~1900 ft) but many of the wells used for loganalysis come from areas east of the field boundarywhere the Tulare Formation is much thicker (200 ftwithin the field vs 2000 ft east of the field [Figure 4])Within the field interpolated contour lines from log datashow the depth to base USDWat 213m (~700 ft) alongthe western field boundary and 365 m (~1200 ft) alongthe eastern field boundary a trend that is consistentwith the more limited water sample data

Typical TDS concentrations in the underlyingEtchegoin Formation (red dots) range from 20000 to35000 mgL probably because of the presence oftrapped connate seawater within these marine depositsIn northern Lost Hills (Figure 8) two samples fromEtchegoin sands have TDS concentrations below 10000mgL However in this area an active steamflood isongoing within the Tulare and Etchegoin FormationsHere the Etchegoin Formation sands lie at shallowdepths (lt150 m [~500 ft]) immediately below pro-ducing intervals in the Tulare Formation The EtchegoinFormation samples in this area are much more dilute(5600ndash8000 mgL) than in other areas This may be be-cause the sands are in hydraulic communication with thefresher water sands of the overlying Tulare Formation orbecause the Etchegoin sands in this area are steamfloodedand the more saline formation water is diluted by theinjected steam Since the twoEtchegoinUSDWsamplesare post-steamflood initiation the latter explanation islikely

Only one chemical analysis sample from the TulareFormation is available in the North Belridge field (Fig-ure 9) The TDS concentration of the sample is 6055mgL from a depth interval of 157ndash211 m (515ndash692ft) It confirms that waters above approximately 211 m(~700 ft) are USDW consistent with the log-deriveddepth to USDW map which shows depth to USDWnear the well at 275 m (900 ft)

Many lab analyses are available for the Tulare For-mation at South Belridge (Figure 9) but they do notshow salinity values that are correlated with depth Thelog-generated depth to USDW map shows that salinitytrends along the northern part of the oil field are vari-able but the southern part of the field shows depth toUSDW increasing toward the southeast Thereforesamples from northern and southern South Belridgewere considered separately The boundary betweenthe two subareas is shown by the dashed line in Figure 9Because of the variability in sample TDS values withdepth comparisons of sample data to calculated salin-ities fromwell-log analyses can only bemade in a generalfashion

In the northern part of the South Belridge field nosamples below 365 m (1200 ft) are USDW suggestingthat the base of USDW lies above this depth The log-derived map shows depth to base of USDW at 335 m(1100 ft) in the north To the south the base of USDWincreases to 427 m (~1400 ft) Near the dividing linebetween the north and south parts of the field thedepth to base USDW decreases to 365 m (1200 ft)

Gillespie et al 87

Figure 8 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare and alluvium (blue) and Etchegoin (red) formations in and near the Lost Hills oilfield Gray dots represent TDS calculated from geophysical logs in the field The horizontal red linemarks 10000mgL and the vertical dashed red linemarks the approximate depth of thedeepest underground sources of drinking water (USDW) sample The adjacent map shows the location of the Tulare and alluviumwells sampled as blue dots and shallow (lt150m [500ft] deep) Etchegoin samples as red dots (deeper Etchegoin sample sites are not shown onmap) and the contours to base USDW from log analysis Lost Hills is the only area that containsbrackish water in the Etchegoin Formation However the brackish waters in the Etchegoin occur in an oil-producing zone that is steam flooded so it is considered to be an exempt aquiferand is not protected CI = contour interval

88Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

Figure 9 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare Formation in and near the North and South Belridge oil fields The northern andsouthern parts of the South Belridge field (marked by the red line on the map) are shown in separate graphs Blue dots show Tulare Formation (Fm) Measurements of TDS from labanalyses Gray dots show Tulare Fm The TDS calculated from log analysis Horizontal red lines on the graphs mark 10000 mgL and the dashed vertical red lines mark the approximatedepth of USDW from lab samples The adjacent map shows the location of the wells sampled (blue dots) and the contours to base underground sources of drinking water (USDW) (infeet) from log analysis Black lines are oil field administrative boundaries and red lines are faults CI = contour interval No = North So = South

Gillespieetal

89

Figure 10 Cross section showing three wells drilled and logged in different years near a produced water disposal pond (light blue polygon on inset map) Yellow shading onspontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivity curve (right track at scale of 0ndash20 ohmm) indicate resistivity of more than 3 ohmm Pinkbars indicate low resistivity sands affected by infiltration of saline produced water from the pond The API numbers at the top of the logs are used for well identification Year drilled andlogged in parentheses Depth scale in feet

90Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

This may be caused by the presence of an active waterdisposal field in the Tulare Formation near this areaOverall the log-derived map shows base of USDWapproximately 61m (~200 ft) deeper than lab analyseswould suggest As at Lost Hills this may be caused bythe geographic distribution of the two data sets the labsamples coming mainly from within the oil fieldboundaries whereas the log data are confined to thearea outside the oil field

In the southern part of South Belridge field a wellnear the southern tip of the field was sampled at mul-tiple intervals Samples as deep as 455 m (~1495 ft) areUSDW and the deepest sample from 480 m (1575 ft)has a TDS value of 13074 mgL suggesting that thebase of USDW in the southern part of South Belridge liesbetween 457 and 480 m (1495ndash1575 ft) (Figure 9) Thedepth to USDW map from log analysis in the southernpart of South Belridge shows that the base of USDW

ranges from 365 m (1200 ft) in the north to approxi-mately 487 m (~1600 ft) in the south

Effects of Oil Field Activities on Water Salinity

The effects of water disposal in surface ponds on ground-water salinity is evident by comparing geophysical logsfrom old wells near the pond locations to logs fromnewerwells drilled nearbyOlder geophysical logs (pre-1970s) indicate that the upper sands in the alluviumare desaturated (vadose zone) as indicated by areas ofcrossover on densityndashneutron logs and have high re-sistivity (gt20ohmmFigure 10)However geophysicallogs collected from wells drilled later in the area donot show densityndashneutron crossover characteristic of avadose zone andhave extremely low resistivity (lt1ohmm)in the upper alluvial sands because of the presence ofhigh salinityproducedwater thathasfilled thevadosezoneInvasion of low resistivity water is especially apparent

Figure 11 Two adjacent wells drilled and logged in different years (1990 and 2012) in an area containing numerous disposal wells on theeast flank of South Belridge Yellow shading on spontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivitycurve indicates resistivity greater than 3 ohm m Pink bars indicate low resistivity sands in the 2012 well (0403046566) affected by injectedsaline produced water The API numbers at the top of the logs are used for well identification Year drilled and logged in parentheses

Gillespie et al 91

where saline waters have infiltrated the 22K sandwhich lies directly above the CCE (well 03047481 inFigure 10) in the lower part of the alluvium The salinewater lies atop the CCE which appears to be pro-tecting the underlying Tulare aquifer from contami-nation Log-calculated andmeasuredTDS concentrationscan exceed 20000mgL in affected parts of the alluvium

The effects of surface disposal ponds are less evidentin Lost Hills and North Belridge compared to the areaadjacent to South Belridge Lost Hills in particular tends tohave highly saline water in the shallow parts of the aquifereven where disposal ponds are not present Swain andDuell (1989) noted TDS concentrations as high as 21700mgL from very shallowwells (6m [~20 ft]) along the eastflank of Lost Hills but did not note the cause of the highsalinity in the area

The effects of water disposal injection wells aremore subtle than that of the ponds and are especiallydifficult to identify in sands closer to the base of USDWwhere resistivity is naturally fairly low Geophysicallogs in sands affected by saline injection tend to have aconcave resistivity profile and resistivities are as small ashalf that in logs fromolder unaffectedwells (Figure 11)Injected saline water tends to follow individual sand

layers leaving some layers relatively unaffected In somecases only the lower part of an individual sand containssaline water whereas the waters in the upper part re-mainbrackishThis suggests that thewaters are stratifiedand mixing has not had time to occur Log-calculatedTDS concentrations show a relatively smooth increasewith depth inwells logged prior to the start of large-scaledisposal activities whereas recent logs in these areasshow a much more variable TDS concentration profilewith depth Additionally densityndashneutron curves inwells affected by disposal of produced waters show thinintervals (~1 m [~3 ft] thick) of crossover This may becaused by bacterial metabolism of trace amounts oforganic matter in the disposal water resulting in thegeneration of small amounts of biogenic methane(McMahon et al 2018)

The cross section in Figure 12 shows wells affectedby saline produced water disposal by injection (belowthe CCE) and percolation (above the CCE) along theeast flank of the South Belridge oil field Wells affectedby disposal of produced waters in saline ponds (n = 139)and by disposal in injection wells (n = 125) are alsoshown in the map in Figure 13 The effects of disposalponds are evident from changes in borehole resistivity

Figure 12 Cross section showing intervals affected by disposal of saline produced water (pink boxes) at South Belridge oil field Wellsidentified by API number at top of each well Year drilled and logged shown at bottom of eachwell Insetmap shows location of cross sectionProduced water disposal wells are highlighted in pink and disposal ponds are shown in blue in the inset map

92 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

the Tulare Formation on the western basin margin is1147 m (3500 ft) (Woodring et al 1940 Loomis1990)

Where it does not crop out the Tulare Formation isoverlain by alluvial deposits Both the Tulare Formationand the overlying alluvium are derived primarily fromthe Coast Ranges and interfinger with Sierran-derivedsediments from the east (Laudon and Belitz 1991) TheTulare Formation and overlying alluvium have similarlithologic compositions and were deposited in fluvial andlacustrine environments therefore they are not readilydistinguishable especially in the subsurface Maps ofthewater-level contours in the Tulare and alluvial aquiferby Wood and Davis (1959) indicate eastward gradientsin the aquifer

At Belridge and Lost Hills the basal Tulare For-mation contact is an angular unconformity belowwhichthe San Joaquin Formation has been removed overmostof the area As a result the Tulare Formation commonlylies directly upon marine deposits of the late Mioceneto Pliocene Etchegoin Formation along the crests ofthe anticlines

In the study area the lower Tulare Formationconsists of lacustrine and deltaic sands and the upperTulare contains alluvial fan meandering channels andfloodplain facies (Kiser et al 1988 Miller et al 1990)The Tulare Formation and overlying alluvium also in-clude lacustrine clays which form confining bedsSeveral distinct regional clay units within the TulareFormation are present in the study area Some of theseclays are not present throughout the study area butwhen present act as local confining layers The threemain clays mapped using borehole geophysical logsin this study are the Amnicola the Middle Tulare andthe Corcoran Clay Equivalent (CCE) (Figure 2) The D-EClay is only present in the southeastern part of the areaand was not mapped in this study

The Amnicola is the oldest of the three mapped claylayers It is described as an olive gray partly calcareousdolomitic claystone that contains the gastropodAmnicola(Woodring et al 1932 Berryman 1973 Maher et al1975) In the southeast part of the study area where itis best developed the Amnicola Clay lies atop a largecoarsening upward sand The Amnicola Clay is absentacross the anticlinal crests and in the northern part ofthe syncline between Lost Hills and North BelridgeFossil evidence from recent drilling in the area indi-cates that the Amnicola Clay is likely present alongthe east flank of northern Lost Hills

Overlying the Amnicola Clay is a coarsening up-ward zone of sands silts and clay that ranges from ap-proximately 120ndash200 m (~400ndash700 ft) thick The zonethickens to the south and thins to the north and along thecrest of the anticlines A clay layer with a pronouncedhigh gamma-ray signature on geophysical logs liesabove this zone and is termed the Middle Tulare Clayin this report The Middle Tulare Clay marks a changein log character from vertically stacked funnel shapedspontaneous potential (SP) and resistivity patterns withrelatively high gamma-ray response (80ndash100 API in thelowerTulare Formation to a blocky SP and resistivity logpattern with lower gamma-ray response (40ndash80API) inthe upper Tulare Formation This change in log patternmay represent a change from a series of prograding la-custrine delta environments in the lower Tulare For-mation to a fluvial meander belt setting in the upperTulare Formation as noted by Miller et al (1990)

The Corcoran Clay described by Frink and Kues(1954) also known as the E clay (Croft 1972) or blueclay by water well drillers is the youngest of the threemapped clay layers and is an important marker bedwithin the SJV aquifer system It divides the ground-water system of the western SJV into an upper semi-confined zone and a lower confined zone (Williamsonet al 1989 Belitz and Heimes 1990) Several studieshave mapped the upper clays of the Tulare Formationbeginning with Croft (1972) who mapped numerousclay layers in the southern SJV including the study areaMore recent studies have focused specifically on theCorcoran Clay and have used both surface and sub-surface data to map the extent and thickness of the clay(Page 1986 Burow et al 2004 Faunt 2009)

Petroleum companies operating in the area refer toa basal alluvial clay at the contact between the TulareFormation and overlying alluvium as the CCE althoughits relationship to the Corcoran Clay as defined by Frinkand Kues (1954) is not clear because Frink and Kues(1954) place the Corcoran Clay within the TulareFormation and Kiser et al (1988) place it within thealluvium In the northeastern part of the study area theCCE correlates to the E clay of Croft (1972) South ofthe study area in the SJV the identification of theCorcoran Clay becomes more problematic and Page(1986) refers to the Corcoran Clay in this area as themodified E clay Because the exact relationship betweenthe CCE and the Corcoran Clay is not known the claywill be termed the CCE in this paper

The CCE is up to 60 m (200 ft) thick within thestudy area It is absent in the northwestern part of the

78 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

study area and above the Lost Hills and Belridge anti-clines Drillers in the area observe a color change in cut-tings from orange-brown above the CCE to graybelow it A thick (18ndash21 m [60ndash70 ft]) sand the 22Ksand lies above the CCE near South Belridge (Figure2) Where the 22K sand is present above the CCE itforms a separate aquifer from the underlying TulareFormation

OIL FIELD ACTIVITY

The Lost Hills and Belridge fields were discoveredduring 1910ndash1912 Initial production was from heavyoil sands in the Tulare and Etchegoin Formations andfrom fractured shales of the Monterey Formation In1930 light oil was discovered in the deeper TemblorFormation at North Belridge but it is the shallowerheavy oil along with fractured diatomaceous depositsand shales in the Etchegoin Reef Ridge and MontereyFormations that have been the mainstay of the fieldsto the present day because of the development of EORtechniques such as steamflooding waterflooding andhydraulic fracturing (Bailey 1939 Ritzius 1950 Land1984 California Division of Oil Gas and GeothermalResources 1998)

The waterflood technique was first initiated in thelate 1940s in the Monterey shales in Lost Hills and themid-1950s in the Temblor Formation sandstones inNorth Belridge (Figure 3) In the 1980s waterfloodscommenced in the Tulare Formation and the diatomitezones of the Monterey Reef Ridge and Etchegoin For-mations In the 1950s and 1960s thermal EOR tech-niques (mainly steamcycling andflooding and to a lesserextent firefloods) were initiated to maximize produc-tion of heavy oil in the Tulare and Etchegoin Formationsand Monterey diatomite

In the diatomite zones large-scale waterfloodingprimarily to enhance oil recovery after hydraulic frac-turing began in the late 1960s and 1970s but becameincreasingly common after 1980 Although the water-floods increased production they also served to mitigateland subsidence and wellbore casing collapse caused byfluid removal from the high-porosity diatomite zones(Dale et al 1996)

As oil is removed water commonly comes in to fillthe void and it is common for waterndashoil ratios (WORs)to increase over the life of an oil field In the Belridge oilfields the WOR increased from 009 in 1931 to ap-proximately 14 in 2016 and in Lost Hills the WOR

increased from15 in 1931 to approximately 12 in 2016Some of the produced water is reused for EOR prac-tices such as waterflooding but the remainder is dis-posed of by a variety ofmeans Initially water disposalwas accomplished by spreading the produced wateron the ground commonly in surface ponds or drystream beds and the water was allowed to evaporateor percolate into the underlying alluvium (Mitchell1989)

A series of legislative bills passed at both the stateand federal level starting in 1969 with the Porter Co-logne Water Quality Act the 1972 Clean Water Actand the Safe Drinking Water Act (1974) provided au-thority to establish regulations regarding surface waterand groundwater degradation As a result of theseregulations produced water disposal began to movefrom surface ponds to injection into nonoil-producingzones via water disposal wells The federal Under-ground Injection Control program established nation-wide requirements for the protection of all aquiferscontaining water with less than 10000 mgL TDS

At Lost Hills oil field water disposal by injectionhas increased from approximately 160000 m3yr(~1 million bblyr) in 1973 to 48 million m3yr(30 million bblyr) in 2009 Disposal initially occurredinto both the Tulare and Etchegoin Formations butby 1990 disposal was almost exclusively into theEtchegoin Formation At North and South Belridgefields disposal began at 636000 m3yr (4 millionbblsyr) in 1970 and increased to more than 12million m3yr (gt78 million bblyr) in 2009 (Figure 3)Belridge disposal is almost entirely into the TulareFormation

METHODS

The main data used in this study are borehole geo-physical logs and water quality laboratory analysesThese data were used to pick marker horizons forgeologic mapping and to determine water salinityand aquifer pressures Data compiled and analyzedfor this study are available from Gillespie et al(2019)

Mapping

Approximately 900 borehole geophysical logs fromwells within the study area were used to determinethe lateral continuity and stratigraphic relationships

Gillespie et al 79

between the late Miocene to Holocene formationsElectric logs (resistivity SP) and gamma-ray logs wereused to identify correlatable layers (Figure 2) Well data(geophysical logs mud logs core data and well his-tories) were obtained primarily from oil and gas wellrecords available from theDOGGR(CaliforniaDivisionof Oil Gas and Geothermal Resources 2018) and fromwater well records obtained from well owners and theCalifornia Department of Water Resources

Because the Tulare Formation and overlying allu-vium form the major nonexempt aquifers in the area

these layers were mapped in more detail particularlywith regard to clay layers Many of the clay markers losetheir coherence when mapped over large distances andbetween distantly spaced wells outside the oil fieldboundaries because of the rapidly changing characterof the Tulare fluvialndashlacustrine delta system In manyplaces it was not possible to correlate the clay layersthroughout the region The correlations were used toconstruct thickness (isochore) maps of the combinedalluvium andTulare Formation (Figure 4) andCCE andAmnicola clays (Figure 5) The correlated logs were

Figure3 Production (prod) and injection (inj) curves for the Lost Hills and North and South Belridge fields (combined) EOR= enhancedoil recovery HF = initiation of hydrofracking Inj = initiation of water injection for enhanced oil recovery WD = initiation of disposal ofproduced water in injection wells Data prior to 1973 are from California Division of Oil Gas and Geothermal Resources (2015) post-1973data are from California Division of Oil Gas and Geothermal Resources (2019)

80 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

compared to salinity data to determine the extent towhich salinity is controlled by aquifer stratigraphy

Wireline formation testers measure pressure buildup within a small interval of the sands (Schlumberger2006) Within a single homogeneous aquifer the pres-sure values should increase with depth along a straightline representing the hydrostatic gradient Shifts fromthe hydrostatic pressure gradient line indicate the pres-ence of pressures in individual aquifer layers that implyrestricted hydraulic connections with other aquiferlayers caused by confining clay layers (Coburn andGillespie 2002) Wireline formation tests in the TulareFormation have been conducted in some test holesdrilled in the study area by oil companies prior to wellinstallation (Gillespie et al 2019) These recordscompiled for 59 wells were analyzed to help deter-mine which clays are significant confining layers

Salinity Determination

For this study two different methods were used to de-termine water salinity within the aquifers in and aroundthe BelridgendashLost Hills oil fields The first is indirect es-timation of salinity using geophysical logs collected priorto installation of oil wells The second is direct mea-surement by lab analysis of producedwater samples fromoil wells in the area which were compared to the in-direct estimates

Log AnalysisAlthough direct sampling and chemical analyses arecommonly considered the best method for determiningTDS in any aquifer in many cases (particularly in deepaquifers containing brackish water) these analyses areuncommon In addition water samples provide only

Figure 4 Map showing depth to base of Tulare Formation The values are equivalent to the thickness of the Tulare Formation andoverlying alluvium Wells shown are those used to determine thickness Black lines are oil field administrative boundaries Gray lines aretownship boundaries Red lines are faults (Young 1968 California Division of Oil Gas and Geothermal Resources 1998) Abd= abandoned

Gillespie et al 81

Figure 5 Isochore maps of the (A) Amnicola Clay and (B) Corcoran Clay Equivalent layers in the study area Wells shown are those used to determine thickness Gray lines aretownship boundaries Red lines are faults (Young 1968 California Division of Oil Gas and Geothermal Resources 1998) Abd = abandoned Approx = Approximate

82Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

point-source data from a single depth interval and do nottell us at what depth the salinity changes However inactive petroleum-producing basins such as the SJV open-hole geophysical logs are abundant Geophysical logsprovide continuous vertical records of the properties ofthe formation matrix fluids adjacent to the boreholeand fluids within the borehole Using log analysis it ispossible to calculate the depths at which salinity changesoccur These depthsmay be verified by chemical analyseswhere available Of particular interest for determiningsalinity are the electrical logs (SP and resistivity) becausethe ability of water to conduct an electrical current isstrongly influenced by both the temperature and salinity ofthe formation water

Use of electrical logs to determine water salinity ismost common in the petroleum literature In oil-bearingreservoirs it is important to know the resistivity of thewater which occurs with the oil to determine the oilsaturation using the Archie (1942) equation or one ofits many variants

Fewer studies have evaluated the use of electricallogs to determine salinity in fresh to brackish aquifersHowells (1990) andHowells et al (1987) used well-loginterpretation to determine the location of the base ofmoderately saline (lt10000 mgL TDS) water in UtahLindner-Lunsford and Bruce (1995) used the ArchieEquation to determine salinity in aquifers with TDS lessthan 1000 mgL in southwest Wyoming Schnoebelenet al (1995) used geophysical logs to map the depth to10000 mgL TDS water in carbonate aquifers in Indianausing the SP log mud filtrate resistivity and the resis-tivity porosity method of Archie They compared theresults to sampled water analyses and found the resis-tivity porosity method to be most accurate More re-cently Hamlin and de la Rocha (2015) used the ArchieEquation to map fresh (lt1000 mgL TDS) slightlysaline (1000ndash3000mgL TDS) moderately saline (3000ndash10000mgL TDS) and very saline (gt10000mgL TDS)water in the CarrizondashWilcox aquifer of south TexasGillespie et al (2017) used log analysis to determinedepth to 10000 mgL TDS on a regional basis through-out much of the southern San Joaquin Basin but thewestern margin fields were not considered in theirstudy because the presence of shallow oil reservoirs requiremore extensive mapping to locate wet sands for loganalysis

In this study a variation of the Archie Equationwas used to estimate salinity (as NaCl equivalent)from the geophysical logs Archie (1942) related thein situ electrical resistivity of a fully water saturated

sedimentary rock (Ro) to its porosity (f) nondimen-sional factors related to matrix properties (the cemen-tation factor [m]and the tortuosity factor [a]) and theresistivity of the formation water (Rw) as shown inequation 1

Rw = Ropethfm=aTHORN (1)

Ro is the resistivity of a clean (minimal clay content) oil-free wet sand and can be obtained from the deepresistivity curve Sands were chosen based on an evalu-ation of nearby core mud log and drillers log data in-dicating that the sands analyzed did not contain oil orgas The porosity value is obtained from porosity logssuch as density neutron sonic or nuclear magneticresonance logs and core analysis For this study sonicporositieswere not used the poorly consolidated natureof the aquifer sediments causes additional slowing ofthe sonic wave from compression of the material re-sulting in artificially high porosity values

Because of the poorly sorted nature of the sedimentsand the presence of large amounts of clay and silt in somezones the average porosity used for salinity calcula-tions weighted density measurements twice as much asneutron values Neutron values for porosity are basedupon the abundance of hydrogen atoms in a formationBecause clays contain hydroxide groups within theircrystal lattice the neutron curve tends to overestimateporosity when clays are present The largest separationbetween the density and neutron curves on each logwasconsidered to be a zone with 100 clay The separationof the densityndashneutron curves for each evaluated in-terval was compared to the 100 clay separation valueto estimate clay content The Rw values were not cal-culated for intervals containing more than 25 clay

The exponentm (unitless) is related to the degree ofcementation in clastic rocks It typically ranges from 13to 26 (Wylie and Rose 1950) The value a (unitless) iscalled the tortuosity factor and is related to the lengththe current must travel through the formation Thevalue for a typically ranges from 05 to 15 For thisstudy special core analyses from five intervals in theTulare Formation were available in the CaliforniaDOGGRonline files from awater disposal well south ofthe Lost Hills field (American Petroleum Institute[API] well number [no] 0402974433) The value fora was kept constant at a value of 1 and m was allowedto vary The measured values for m ranged from 15 to18 with a mean value of 17 (standard deviation of015) and a median value of 18 For this study salinity

Gillespie et al 83

calculations used a = 1 andm = 17 based on the specialcore analysis results

Upon obtaining a value for Rw and calculating thetemperature of the zone of interest from a linear in-terpolation between bottom borehole temperature andair temperatureof 75degF the formationwater salinity canbeestimated from empirical charts such as those provided bySchlumberger (1997) or from equations such as thosein Bateman and Konen (1977) relating fluid resistivity toNaCl equivalent TDS concentration

Geochemical AnalysesThe California Division of Oil Gas and GeothermalResources (2016) maintains a website containing scan-ned copies of chemical analyses of oil field waters Thesedata are useful in determining the salinity of the deeperaquifers not normally used for drinking water or irriga-tion Data taken from these reports included dates oftesting source of water for test (when available) majorion concentrations and TDS concentrations The compileddata are available from Gillespie et al (2019) Chemicalanalysis of groundwater samples compiled from varioussources were also available (Metzger et al 2018) How-ever these samples were generally available from rela-tively shallow depths and had TDS ranging fromapproximately 1000 to 3000 mgL and therefore did notdirectly contribute to efforts to map the depth to USDW

Scanned copies of well completion reports are alsoavailable for the wells with chemical analysis from theCalifornia Division of Oil Gas and Geothermal Re-sources (2018) Well Finder online search engine Thecompletion reports for the wells provide informationincluding perforated intervals date perforated geolog-ical formation sampled (in some cases) scanned copiesof borehole geophysical logs pressure tests and bottom-hole temperatures These records were used to de-termine the depth interval and formation representedby the produced water chemical analysis data (Gillespieet al 2019)

Produced water chemical analyses from approxi-mately 150 oil production injection and observationwells in the study area were available from DOGGRrsquosonline chemical analysis database (California Divisionof Oil Gas and Geothermal Resources 2016) Addi-tionally 12 samples from the Tulare Formation and al-luvium were available from groundwater monitoringwells in the Lost Hills oil field on the Geotracker website(California State Water Resources Control Board2018a) The vintage of samples in the database rangesfrom 1932 to 2017

The sample analyses may not always representnatural aquifer conditions For example water samplesare commonly collected when a well has productionproblems or during the initial development of a well(Blondes et al 2016) Production problems may in-clude holes in well casings or annular cement that mayhave caused water from a different zone to enter theperforations In most cases few data exist regarding thesampling procedures and analysis techniques particu-larly for the older samples However older samples wereconsidered to be important for this study because theypredate the use of many EOR techniques such as waterflooding waste disposal and steam injection the large-scale use of which commenced in the early to mid-1960s These EOR processes may significantly alterthe salinity of the samples and cause the more recentsamples to be an inaccurate reflection of the originalaquifer conditions Samples taken at different timeintervals are helpful in determining temporal changes insalinity that may have occurred because of oil fieldactivities

One quality control method used in this study wasto perform a charge balance on the analyses Becausewater is electrically neutral the negative (anions) andpositive (cations) charged ions should sum to zero Thedifference between the total milliequivalents of thecations and anions is divided by the sum to give a chargebalance error We used a cutoff of ndash5 to determinewhich samples to discard in this analysis

For wells used for injection purposes the initiationof water injection relative to the date of sampling maybe important to consider Samples taken after the startof injection in the sampled well may not accuratelyrepresent the true formation water composition In mostcases it was not feasible to catalog the time of initiationof injection in nearby wells because of the large numberof injectionwells that could potentially affect a sampledwell Consequently the effect of subsurface injectionon sample TDS data remains a source of uncertainty inthe analysis

RESULTS

The methods and data discussed in the previous sec-tion were used to construct geologic maps showing thethickness and continuity of aquifers and confining unitsas well as to determine aquifer salinity and temporalchanges in salinity These results are discussed below

84 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 6 Graphs of shut-in pressure (from open-hole sequential formation testers) versus depth for three wells across the study area Red lines are used to emphasize the pressuregradient Tan areas showmajor clay layers the 22K sand is shown in yellow Base of Tulare and top of thewater table are also notedWells are identified by their API number and the yeardrilled and logged at the top of each graph

Gillespieetal

85

Geologic Mapping

The thickness of the Tulare Formation and overlyingalluvium in the study area (Figure 4) ranges from1200m (4000 ft) in the southeast part of the study areato 75 m (~250 ft) along the anticline crests in thenorthern parts of Lost Hills and North Belridge oilfields Because chemical analyses indicate that theprotected aquifers occur largely within the Tulare For-mation and overlying alluvium throughout the studyarea the map of the depth to the base of the TulareFormation defines the extent of potential protectedaquifers outside the oil field limits

The Amnicola Clay is thickest (67 m [~220 ft])along the northeastern and southeastern flank of theLost Hills anticline and east of the South Belridge an-ticline (Figure 5A) It thins to zero in the northwest partof the study area The clay is replaced by sandier sedi-ments east of the central part of the Lost Hills anticlineThe CCE thickens to 61m (~200 ft) in the northcentralpart of the study area in the Tulare lakebed but iscommonly less than30m(lt~100 ft) thick (Figure5B) Itis absent over the Lost Hills anticline crest and in thenorthwestern part of the study area These clay maps(Figure 5A B) are used to determine the presence ofpossible confining beds that in some cases may controlthe distribution of protected water resources and restrictthe migration of potential contaminants

Graphs of pressure versus depth collected in 1985from three wells along a northeast-trending transectfrom South Belridge to south of Lost Hills are shown inFigure 6 Marked changes in the hydrostatic pressuregradient in well API no 02974434 occur across theMiddle Tulare Clay and Amnicola Clay layers An addi-tional deeper clay here termed the D-E Clay forms alocal confining layer isolating a relatively low perme-ability section at the base of the Tulare Formation fromthe rest of the aquifer These characteristic deviationsin hydrostatic gradients indicates that the clays hydrau-lically separate individual aquifer layers in the area ofthis well

Farther northeast in wells API no 02974436 andAPI no 02974433 these deviations from hydrostaticgradient are much less noticeable especially across theAmnicola Clay This suggests that either (1) the claylayers are less confining south of Lost Hills or (2) thehydrologic regime near South Belridge has been alteredby production and injection into the Tulare Formationand the pressure responses in aquifer layers near the fieldin 1985 had not had sufficient time for their pressures

to reach equilibrium Recent drilling and multiple wellmonitoring site data (April 2018) off the eastern flank ofthe northern Lost Hills oil field indicates a 45 m (~15 ft)head difference across the Amnicola Clay with an upwardgradient of 0038 (measured May 31 2018) suggestingthat it is a confining layer in the northeastern part of thestudy area (US Geological Survey 2018a b)

Salinity Analysis

Log analysis from 30 wells in areas outside the oil fieldboundaries or within the oil fields but below the oilndashwater contact were used to create a map of the depthto the base of USDW The depth map to the base ofUSDW shows two trends (Figure 7) In the area be-tween Lost Hills and the Belridge oil fields depthto USDW increases toward the south from 150 m(~500 ft) between North Belridge and Lost Hills to490 m (~1600 ft) southeast of South Belridge In the

Figure 7 Map showing depth to base of underground sourcesof drinking water (USDW) (total dissolved solids = 10000 mgL)based on log analysis Red lines indicate faults (Young 1968California Division of Oil Gas and Geothermal Resources 1998)Black lines are oil field administrative boundaries Wells shown asblue dots are those used to determine the base of USDW Depthvalues in feet CI = contour interval No = North So = South

86 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

eastern part of the study area depth to base of USDWincreases sharply eastward from approximately 215 m(~700 ft) on the west flank of the Lost Hills anticline toas great as 550 m (~1800 ft) within 3 km (~2 mi)

Salinityndashdepth profiles based on chemical analysesof producedwater samples were compared to the depthto base USDW map estimated from borehole geo-physical log analysis However this comparison waslimited because most of the lab analyses come from oil-producing wells within the field boundaries rather thanin areas outside the fields The presence of oil in thesampled intervals within the fields precludes the useof log analysis to calculate the salinity in these samewells Plots of TDS versus depth of the samples in eachfield are shown in Figures 8 and 9 along with log-calculated TDS

Wood and Davis (1959) reported the groundwaterchemistry in this area as being an NaSO4 type howeveronly one Tulare Formation monitoring well Mackesseyno 1 (API no 02987363) was an NaSO4 type the restare NaCl type Because the calculated salinity fromArchie log analysis assumes NaCl water types the log-calculated and lab sample TDS values should be com-parable The two monitoring wells completed in thealluvium have higher SO4 concentrations relative tothe Tulare Formation wells however Cl is the domi-nant anion

Only four Tulare Formation lab samples (blue dots)in Lost Hills are in the California DOGGR databaseData from 14 monitoring wells in the central part ofthe Lost Hills field were used to enlarge the data set(Figure 8) With the exception of one sample TDSconcentrations in the Tulare Formation and alluviumare less than 10000 mgL indicating protected (USDW)aquifers These wells have top perforations ranging from40 to 290 m (133ndash950 ft) The well with a lab analysisindicating a salinity of 12295 mgL TDS in the TulareFormation had a top perforation at 300 m (984 ft) Thissuggests that the base of USDW lies somewhere be-tween 290 and 300 m (950 and 985 ft) within the fieldLog analysis suggests that the base may occur as deepas 580 m (~1900 ft) but many of the wells used for loganalysis come from areas east of the field boundarywhere the Tulare Formation is much thicker (200 ftwithin the field vs 2000 ft east of the field [Figure 4])Within the field interpolated contour lines from log datashow the depth to base USDWat 213m (~700 ft) alongthe western field boundary and 365 m (~1200 ft) alongthe eastern field boundary a trend that is consistentwith the more limited water sample data

Typical TDS concentrations in the underlyingEtchegoin Formation (red dots) range from 20000 to35000 mgL probably because of the presence oftrapped connate seawater within these marine depositsIn northern Lost Hills (Figure 8) two samples fromEtchegoin sands have TDS concentrations below 10000mgL However in this area an active steamflood isongoing within the Tulare and Etchegoin FormationsHere the Etchegoin Formation sands lie at shallowdepths (lt150 m [~500 ft]) immediately below pro-ducing intervals in the Tulare Formation The EtchegoinFormation samples in this area are much more dilute(5600ndash8000 mgL) than in other areas This may be be-cause the sands are in hydraulic communication with thefresher water sands of the overlying Tulare Formation orbecause the Etchegoin sands in this area are steamfloodedand the more saline formation water is diluted by theinjected steam Since the twoEtchegoinUSDWsamplesare post-steamflood initiation the latter explanation islikely

Only one chemical analysis sample from the TulareFormation is available in the North Belridge field (Fig-ure 9) The TDS concentration of the sample is 6055mgL from a depth interval of 157ndash211 m (515ndash692ft) It confirms that waters above approximately 211 m(~700 ft) are USDW consistent with the log-deriveddepth to USDW map which shows depth to USDWnear the well at 275 m (900 ft)

Many lab analyses are available for the Tulare For-mation at South Belridge (Figure 9) but they do notshow salinity values that are correlated with depth Thelog-generated depth to USDW map shows that salinitytrends along the northern part of the oil field are vari-able but the southern part of the field shows depth toUSDW increasing toward the southeast Thereforesamples from northern and southern South Belridgewere considered separately The boundary betweenthe two subareas is shown by the dashed line in Figure 9Because of the variability in sample TDS values withdepth comparisons of sample data to calculated salin-ities fromwell-log analyses can only bemade in a generalfashion

In the northern part of the South Belridge field nosamples below 365 m (1200 ft) are USDW suggestingthat the base of USDW lies above this depth The log-derived map shows depth to base of USDW at 335 m(1100 ft) in the north To the south the base of USDWincreases to 427 m (~1400 ft) Near the dividing linebetween the north and south parts of the field thedepth to base USDW decreases to 365 m (1200 ft)

Gillespie et al 87

Figure 8 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare and alluvium (blue) and Etchegoin (red) formations in and near the Lost Hills oilfield Gray dots represent TDS calculated from geophysical logs in the field The horizontal red linemarks 10000mgL and the vertical dashed red linemarks the approximate depth of thedeepest underground sources of drinking water (USDW) sample The adjacent map shows the location of the Tulare and alluviumwells sampled as blue dots and shallow (lt150m [500ft] deep) Etchegoin samples as red dots (deeper Etchegoin sample sites are not shown onmap) and the contours to base USDW from log analysis Lost Hills is the only area that containsbrackish water in the Etchegoin Formation However the brackish waters in the Etchegoin occur in an oil-producing zone that is steam flooded so it is considered to be an exempt aquiferand is not protected CI = contour interval

88Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

Figure 9 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare Formation in and near the North and South Belridge oil fields The northern andsouthern parts of the South Belridge field (marked by the red line on the map) are shown in separate graphs Blue dots show Tulare Formation (Fm) Measurements of TDS from labanalyses Gray dots show Tulare Fm The TDS calculated from log analysis Horizontal red lines on the graphs mark 10000 mgL and the dashed vertical red lines mark the approximatedepth of USDW from lab samples The adjacent map shows the location of the wells sampled (blue dots) and the contours to base underground sources of drinking water (USDW) (infeet) from log analysis Black lines are oil field administrative boundaries and red lines are faults CI = contour interval No = North So = South

Gillespieetal

89

Figure 10 Cross section showing three wells drilled and logged in different years near a produced water disposal pond (light blue polygon on inset map) Yellow shading onspontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivity curve (right track at scale of 0ndash20 ohmm) indicate resistivity of more than 3 ohmm Pinkbars indicate low resistivity sands affected by infiltration of saline produced water from the pond The API numbers at the top of the logs are used for well identification Year drilled andlogged in parentheses Depth scale in feet

90Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

This may be caused by the presence of an active waterdisposal field in the Tulare Formation near this areaOverall the log-derived map shows base of USDWapproximately 61m (~200 ft) deeper than lab analyseswould suggest As at Lost Hills this may be caused bythe geographic distribution of the two data sets the labsamples coming mainly from within the oil fieldboundaries whereas the log data are confined to thearea outside the oil field

In the southern part of South Belridge field a wellnear the southern tip of the field was sampled at mul-tiple intervals Samples as deep as 455 m (~1495 ft) areUSDW and the deepest sample from 480 m (1575 ft)has a TDS value of 13074 mgL suggesting that thebase of USDW in the southern part of South Belridge liesbetween 457 and 480 m (1495ndash1575 ft) (Figure 9) Thedepth to USDW map from log analysis in the southernpart of South Belridge shows that the base of USDW

ranges from 365 m (1200 ft) in the north to approxi-mately 487 m (~1600 ft) in the south

Effects of Oil Field Activities on Water Salinity

The effects of water disposal in surface ponds on ground-water salinity is evident by comparing geophysical logsfrom old wells near the pond locations to logs fromnewerwells drilled nearbyOlder geophysical logs (pre-1970s) indicate that the upper sands in the alluviumare desaturated (vadose zone) as indicated by areas ofcrossover on densityndashneutron logs and have high re-sistivity (gt20ohmmFigure 10)However geophysicallogs collected from wells drilled later in the area donot show densityndashneutron crossover characteristic of avadose zone andhave extremely low resistivity (lt1ohmm)in the upper alluvial sands because of the presence ofhigh salinityproducedwater thathasfilled thevadosezoneInvasion of low resistivity water is especially apparent

Figure 11 Two adjacent wells drilled and logged in different years (1990 and 2012) in an area containing numerous disposal wells on theeast flank of South Belridge Yellow shading on spontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivitycurve indicates resistivity greater than 3 ohm m Pink bars indicate low resistivity sands in the 2012 well (0403046566) affected by injectedsaline produced water The API numbers at the top of the logs are used for well identification Year drilled and logged in parentheses

Gillespie et al 91

where saline waters have infiltrated the 22K sandwhich lies directly above the CCE (well 03047481 inFigure 10) in the lower part of the alluvium The salinewater lies atop the CCE which appears to be pro-tecting the underlying Tulare aquifer from contami-nation Log-calculated andmeasuredTDS concentrationscan exceed 20000mgL in affected parts of the alluvium

The effects of surface disposal ponds are less evidentin Lost Hills and North Belridge compared to the areaadjacent to South Belridge Lost Hills in particular tends tohave highly saline water in the shallow parts of the aquifereven where disposal ponds are not present Swain andDuell (1989) noted TDS concentrations as high as 21700mgL from very shallowwells (6m [~20 ft]) along the eastflank of Lost Hills but did not note the cause of the highsalinity in the area

The effects of water disposal injection wells aremore subtle than that of the ponds and are especiallydifficult to identify in sands closer to the base of USDWwhere resistivity is naturally fairly low Geophysicallogs in sands affected by saline injection tend to have aconcave resistivity profile and resistivities are as small ashalf that in logs fromolder unaffectedwells (Figure 11)Injected saline water tends to follow individual sand

layers leaving some layers relatively unaffected In somecases only the lower part of an individual sand containssaline water whereas the waters in the upper part re-mainbrackishThis suggests that thewaters are stratifiedand mixing has not had time to occur Log-calculatedTDS concentrations show a relatively smooth increasewith depth inwells logged prior to the start of large-scaledisposal activities whereas recent logs in these areasshow a much more variable TDS concentration profilewith depth Additionally densityndashneutron curves inwells affected by disposal of produced waters show thinintervals (~1 m [~3 ft] thick) of crossover This may becaused by bacterial metabolism of trace amounts oforganic matter in the disposal water resulting in thegeneration of small amounts of biogenic methane(McMahon et al 2018)

The cross section in Figure 12 shows wells affectedby saline produced water disposal by injection (belowthe CCE) and percolation (above the CCE) along theeast flank of the South Belridge oil field Wells affectedby disposal of produced waters in saline ponds (n = 139)and by disposal in injection wells (n = 125) are alsoshown in the map in Figure 13 The effects of disposalponds are evident from changes in borehole resistivity

Figure 12 Cross section showing intervals affected by disposal of saline produced water (pink boxes) at South Belridge oil field Wellsidentified by API number at top of each well Year drilled and logged shown at bottom of eachwell Insetmap shows location of cross sectionProduced water disposal wells are highlighted in pink and disposal ponds are shown in blue in the inset map

92 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

study area and above the Lost Hills and Belridge anti-clines Drillers in the area observe a color change in cut-tings from orange-brown above the CCE to graybelow it A thick (18ndash21 m [60ndash70 ft]) sand the 22Ksand lies above the CCE near South Belridge (Figure2) Where the 22K sand is present above the CCE itforms a separate aquifer from the underlying TulareFormation

OIL FIELD ACTIVITY

The Lost Hills and Belridge fields were discoveredduring 1910ndash1912 Initial production was from heavyoil sands in the Tulare and Etchegoin Formations andfrom fractured shales of the Monterey Formation In1930 light oil was discovered in the deeper TemblorFormation at North Belridge but it is the shallowerheavy oil along with fractured diatomaceous depositsand shales in the Etchegoin Reef Ridge and MontereyFormations that have been the mainstay of the fieldsto the present day because of the development of EORtechniques such as steamflooding waterflooding andhydraulic fracturing (Bailey 1939 Ritzius 1950 Land1984 California Division of Oil Gas and GeothermalResources 1998)

The waterflood technique was first initiated in thelate 1940s in the Monterey shales in Lost Hills and themid-1950s in the Temblor Formation sandstones inNorth Belridge (Figure 3) In the 1980s waterfloodscommenced in the Tulare Formation and the diatomitezones of the Monterey Reef Ridge and Etchegoin For-mations In the 1950s and 1960s thermal EOR tech-niques (mainly steamcycling andflooding and to a lesserextent firefloods) were initiated to maximize produc-tion of heavy oil in the Tulare and Etchegoin Formationsand Monterey diatomite

In the diatomite zones large-scale waterfloodingprimarily to enhance oil recovery after hydraulic frac-turing began in the late 1960s and 1970s but becameincreasingly common after 1980 Although the water-floods increased production they also served to mitigateland subsidence and wellbore casing collapse caused byfluid removal from the high-porosity diatomite zones(Dale et al 1996)

As oil is removed water commonly comes in to fillthe void and it is common for waterndashoil ratios (WORs)to increase over the life of an oil field In the Belridge oilfields the WOR increased from 009 in 1931 to ap-proximately 14 in 2016 and in Lost Hills the WOR

increased from15 in 1931 to approximately 12 in 2016Some of the produced water is reused for EOR prac-tices such as waterflooding but the remainder is dis-posed of by a variety ofmeans Initially water disposalwas accomplished by spreading the produced wateron the ground commonly in surface ponds or drystream beds and the water was allowed to evaporateor percolate into the underlying alluvium (Mitchell1989)

A series of legislative bills passed at both the stateand federal level starting in 1969 with the Porter Co-logne Water Quality Act the 1972 Clean Water Actand the Safe Drinking Water Act (1974) provided au-thority to establish regulations regarding surface waterand groundwater degradation As a result of theseregulations produced water disposal began to movefrom surface ponds to injection into nonoil-producingzones via water disposal wells The federal Under-ground Injection Control program established nation-wide requirements for the protection of all aquiferscontaining water with less than 10000 mgL TDS

At Lost Hills oil field water disposal by injectionhas increased from approximately 160000 m3yr(~1 million bblyr) in 1973 to 48 million m3yr(30 million bblyr) in 2009 Disposal initially occurredinto both the Tulare and Etchegoin Formations butby 1990 disposal was almost exclusively into theEtchegoin Formation At North and South Belridgefields disposal began at 636000 m3yr (4 millionbblsyr) in 1970 and increased to more than 12million m3yr (gt78 million bblyr) in 2009 (Figure 3)Belridge disposal is almost entirely into the TulareFormation

METHODS

The main data used in this study are borehole geo-physical logs and water quality laboratory analysesThese data were used to pick marker horizons forgeologic mapping and to determine water salinityand aquifer pressures Data compiled and analyzedfor this study are available from Gillespie et al(2019)

Mapping

Approximately 900 borehole geophysical logs fromwells within the study area were used to determinethe lateral continuity and stratigraphic relationships

Gillespie et al 79

between the late Miocene to Holocene formationsElectric logs (resistivity SP) and gamma-ray logs wereused to identify correlatable layers (Figure 2) Well data(geophysical logs mud logs core data and well his-tories) were obtained primarily from oil and gas wellrecords available from theDOGGR(CaliforniaDivisionof Oil Gas and Geothermal Resources 2018) and fromwater well records obtained from well owners and theCalifornia Department of Water Resources

Because the Tulare Formation and overlying allu-vium form the major nonexempt aquifers in the area

these layers were mapped in more detail particularlywith regard to clay layers Many of the clay markers losetheir coherence when mapped over large distances andbetween distantly spaced wells outside the oil fieldboundaries because of the rapidly changing characterof the Tulare fluvialndashlacustrine delta system In manyplaces it was not possible to correlate the clay layersthroughout the region The correlations were used toconstruct thickness (isochore) maps of the combinedalluvium andTulare Formation (Figure 4) andCCE andAmnicola clays (Figure 5) The correlated logs were

Figure3 Production (prod) and injection (inj) curves for the Lost Hills and North and South Belridge fields (combined) EOR= enhancedoil recovery HF = initiation of hydrofracking Inj = initiation of water injection for enhanced oil recovery WD = initiation of disposal ofproduced water in injection wells Data prior to 1973 are from California Division of Oil Gas and Geothermal Resources (2015) post-1973data are from California Division of Oil Gas and Geothermal Resources (2019)

80 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

compared to salinity data to determine the extent towhich salinity is controlled by aquifer stratigraphy

Wireline formation testers measure pressure buildup within a small interval of the sands (Schlumberger2006) Within a single homogeneous aquifer the pres-sure values should increase with depth along a straightline representing the hydrostatic gradient Shifts fromthe hydrostatic pressure gradient line indicate the pres-ence of pressures in individual aquifer layers that implyrestricted hydraulic connections with other aquiferlayers caused by confining clay layers (Coburn andGillespie 2002) Wireline formation tests in the TulareFormation have been conducted in some test holesdrilled in the study area by oil companies prior to wellinstallation (Gillespie et al 2019) These recordscompiled for 59 wells were analyzed to help deter-mine which clays are significant confining layers

Salinity Determination

For this study two different methods were used to de-termine water salinity within the aquifers in and aroundthe BelridgendashLost Hills oil fields The first is indirect es-timation of salinity using geophysical logs collected priorto installation of oil wells The second is direct mea-surement by lab analysis of producedwater samples fromoil wells in the area which were compared to the in-direct estimates

Log AnalysisAlthough direct sampling and chemical analyses arecommonly considered the best method for determiningTDS in any aquifer in many cases (particularly in deepaquifers containing brackish water) these analyses areuncommon In addition water samples provide only

Figure 4 Map showing depth to base of Tulare Formation The values are equivalent to the thickness of the Tulare Formation andoverlying alluvium Wells shown are those used to determine thickness Black lines are oil field administrative boundaries Gray lines aretownship boundaries Red lines are faults (Young 1968 California Division of Oil Gas and Geothermal Resources 1998) Abd= abandoned

Gillespie et al 81

Figure 5 Isochore maps of the (A) Amnicola Clay and (B) Corcoran Clay Equivalent layers in the study area Wells shown are those used to determine thickness Gray lines aretownship boundaries Red lines are faults (Young 1968 California Division of Oil Gas and Geothermal Resources 1998) Abd = abandoned Approx = Approximate

82Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

point-source data from a single depth interval and do nottell us at what depth the salinity changes However inactive petroleum-producing basins such as the SJV open-hole geophysical logs are abundant Geophysical logsprovide continuous vertical records of the properties ofthe formation matrix fluids adjacent to the boreholeand fluids within the borehole Using log analysis it ispossible to calculate the depths at which salinity changesoccur These depthsmay be verified by chemical analyseswhere available Of particular interest for determiningsalinity are the electrical logs (SP and resistivity) becausethe ability of water to conduct an electrical current isstrongly influenced by both the temperature and salinity ofthe formation water

Use of electrical logs to determine water salinity ismost common in the petroleum literature In oil-bearingreservoirs it is important to know the resistivity of thewater which occurs with the oil to determine the oilsaturation using the Archie (1942) equation or one ofits many variants

Fewer studies have evaluated the use of electricallogs to determine salinity in fresh to brackish aquifersHowells (1990) andHowells et al (1987) used well-loginterpretation to determine the location of the base ofmoderately saline (lt10000 mgL TDS) water in UtahLindner-Lunsford and Bruce (1995) used the ArchieEquation to determine salinity in aquifers with TDS lessthan 1000 mgL in southwest Wyoming Schnoebelenet al (1995) used geophysical logs to map the depth to10000 mgL TDS water in carbonate aquifers in Indianausing the SP log mud filtrate resistivity and the resis-tivity porosity method of Archie They compared theresults to sampled water analyses and found the resis-tivity porosity method to be most accurate More re-cently Hamlin and de la Rocha (2015) used the ArchieEquation to map fresh (lt1000 mgL TDS) slightlysaline (1000ndash3000mgL TDS) moderately saline (3000ndash10000mgL TDS) and very saline (gt10000mgL TDS)water in the CarrizondashWilcox aquifer of south TexasGillespie et al (2017) used log analysis to determinedepth to 10000 mgL TDS on a regional basis through-out much of the southern San Joaquin Basin but thewestern margin fields were not considered in theirstudy because the presence of shallow oil reservoirs requiremore extensive mapping to locate wet sands for loganalysis

In this study a variation of the Archie Equationwas used to estimate salinity (as NaCl equivalent)from the geophysical logs Archie (1942) related thein situ electrical resistivity of a fully water saturated

sedimentary rock (Ro) to its porosity (f) nondimen-sional factors related to matrix properties (the cemen-tation factor [m]and the tortuosity factor [a]) and theresistivity of the formation water (Rw) as shown inequation 1

Rw = Ropethfm=aTHORN (1)

Ro is the resistivity of a clean (minimal clay content) oil-free wet sand and can be obtained from the deepresistivity curve Sands were chosen based on an evalu-ation of nearby core mud log and drillers log data in-dicating that the sands analyzed did not contain oil orgas The porosity value is obtained from porosity logssuch as density neutron sonic or nuclear magneticresonance logs and core analysis For this study sonicporositieswere not used the poorly consolidated natureof the aquifer sediments causes additional slowing ofthe sonic wave from compression of the material re-sulting in artificially high porosity values

Because of the poorly sorted nature of the sedimentsand the presence of large amounts of clay and silt in somezones the average porosity used for salinity calcula-tions weighted density measurements twice as much asneutron values Neutron values for porosity are basedupon the abundance of hydrogen atoms in a formationBecause clays contain hydroxide groups within theircrystal lattice the neutron curve tends to overestimateporosity when clays are present The largest separationbetween the density and neutron curves on each logwasconsidered to be a zone with 100 clay The separationof the densityndashneutron curves for each evaluated in-terval was compared to the 100 clay separation valueto estimate clay content The Rw values were not cal-culated for intervals containing more than 25 clay

The exponentm (unitless) is related to the degree ofcementation in clastic rocks It typically ranges from 13to 26 (Wylie and Rose 1950) The value a (unitless) iscalled the tortuosity factor and is related to the lengththe current must travel through the formation Thevalue for a typically ranges from 05 to 15 For thisstudy special core analyses from five intervals in theTulare Formation were available in the CaliforniaDOGGRonline files from awater disposal well south ofthe Lost Hills field (American Petroleum Institute[API] well number [no] 0402974433) The value fora was kept constant at a value of 1 and m was allowedto vary The measured values for m ranged from 15 to18 with a mean value of 17 (standard deviation of015) and a median value of 18 For this study salinity

Gillespie et al 83

calculations used a = 1 andm = 17 based on the specialcore analysis results

Upon obtaining a value for Rw and calculating thetemperature of the zone of interest from a linear in-terpolation between bottom borehole temperature andair temperatureof 75degF the formationwater salinity canbeestimated from empirical charts such as those provided bySchlumberger (1997) or from equations such as thosein Bateman and Konen (1977) relating fluid resistivity toNaCl equivalent TDS concentration

Geochemical AnalysesThe California Division of Oil Gas and GeothermalResources (2016) maintains a website containing scan-ned copies of chemical analyses of oil field waters Thesedata are useful in determining the salinity of the deeperaquifers not normally used for drinking water or irriga-tion Data taken from these reports included dates oftesting source of water for test (when available) majorion concentrations and TDS concentrations The compileddata are available from Gillespie et al (2019) Chemicalanalysis of groundwater samples compiled from varioussources were also available (Metzger et al 2018) How-ever these samples were generally available from rela-tively shallow depths and had TDS ranging fromapproximately 1000 to 3000 mgL and therefore did notdirectly contribute to efforts to map the depth to USDW

Scanned copies of well completion reports are alsoavailable for the wells with chemical analysis from theCalifornia Division of Oil Gas and Geothermal Re-sources (2018) Well Finder online search engine Thecompletion reports for the wells provide informationincluding perforated intervals date perforated geolog-ical formation sampled (in some cases) scanned copiesof borehole geophysical logs pressure tests and bottom-hole temperatures These records were used to de-termine the depth interval and formation representedby the produced water chemical analysis data (Gillespieet al 2019)

Produced water chemical analyses from approxi-mately 150 oil production injection and observationwells in the study area were available from DOGGRrsquosonline chemical analysis database (California Divisionof Oil Gas and Geothermal Resources 2016) Addi-tionally 12 samples from the Tulare Formation and al-luvium were available from groundwater monitoringwells in the Lost Hills oil field on the Geotracker website(California State Water Resources Control Board2018a) The vintage of samples in the database rangesfrom 1932 to 2017

The sample analyses may not always representnatural aquifer conditions For example water samplesare commonly collected when a well has productionproblems or during the initial development of a well(Blondes et al 2016) Production problems may in-clude holes in well casings or annular cement that mayhave caused water from a different zone to enter theperforations In most cases few data exist regarding thesampling procedures and analysis techniques particu-larly for the older samples However older samples wereconsidered to be important for this study because theypredate the use of many EOR techniques such as waterflooding waste disposal and steam injection the large-scale use of which commenced in the early to mid-1960s These EOR processes may significantly alterthe salinity of the samples and cause the more recentsamples to be an inaccurate reflection of the originalaquifer conditions Samples taken at different timeintervals are helpful in determining temporal changes insalinity that may have occurred because of oil fieldactivities

One quality control method used in this study wasto perform a charge balance on the analyses Becausewater is electrically neutral the negative (anions) andpositive (cations) charged ions should sum to zero Thedifference between the total milliequivalents of thecations and anions is divided by the sum to give a chargebalance error We used a cutoff of ndash5 to determinewhich samples to discard in this analysis

For wells used for injection purposes the initiationof water injection relative to the date of sampling maybe important to consider Samples taken after the startof injection in the sampled well may not accuratelyrepresent the true formation water composition In mostcases it was not feasible to catalog the time of initiationof injection in nearby wells because of the large numberof injectionwells that could potentially affect a sampledwell Consequently the effect of subsurface injectionon sample TDS data remains a source of uncertainty inthe analysis

RESULTS

The methods and data discussed in the previous sec-tion were used to construct geologic maps showing thethickness and continuity of aquifers and confining unitsas well as to determine aquifer salinity and temporalchanges in salinity These results are discussed below

84 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 6 Graphs of shut-in pressure (from open-hole sequential formation testers) versus depth for three wells across the study area Red lines are used to emphasize the pressuregradient Tan areas showmajor clay layers the 22K sand is shown in yellow Base of Tulare and top of thewater table are also notedWells are identified by their API number and the yeardrilled and logged at the top of each graph

Gillespieetal

85

Geologic Mapping

The thickness of the Tulare Formation and overlyingalluvium in the study area (Figure 4) ranges from1200m (4000 ft) in the southeast part of the study areato 75 m (~250 ft) along the anticline crests in thenorthern parts of Lost Hills and North Belridge oilfields Because chemical analyses indicate that theprotected aquifers occur largely within the Tulare For-mation and overlying alluvium throughout the studyarea the map of the depth to the base of the TulareFormation defines the extent of potential protectedaquifers outside the oil field limits

The Amnicola Clay is thickest (67 m [~220 ft])along the northeastern and southeastern flank of theLost Hills anticline and east of the South Belridge an-ticline (Figure 5A) It thins to zero in the northwest partof the study area The clay is replaced by sandier sedi-ments east of the central part of the Lost Hills anticlineThe CCE thickens to 61m (~200 ft) in the northcentralpart of the study area in the Tulare lakebed but iscommonly less than30m(lt~100 ft) thick (Figure5B) Itis absent over the Lost Hills anticline crest and in thenorthwestern part of the study area These clay maps(Figure 5A B) are used to determine the presence ofpossible confining beds that in some cases may controlthe distribution of protected water resources and restrictthe migration of potential contaminants

Graphs of pressure versus depth collected in 1985from three wells along a northeast-trending transectfrom South Belridge to south of Lost Hills are shown inFigure 6 Marked changes in the hydrostatic pressuregradient in well API no 02974434 occur across theMiddle Tulare Clay and Amnicola Clay layers An addi-tional deeper clay here termed the D-E Clay forms alocal confining layer isolating a relatively low perme-ability section at the base of the Tulare Formation fromthe rest of the aquifer These characteristic deviationsin hydrostatic gradients indicates that the clays hydrau-lically separate individual aquifer layers in the area ofthis well

Farther northeast in wells API no 02974436 andAPI no 02974433 these deviations from hydrostaticgradient are much less noticeable especially across theAmnicola Clay This suggests that either (1) the claylayers are less confining south of Lost Hills or (2) thehydrologic regime near South Belridge has been alteredby production and injection into the Tulare Formationand the pressure responses in aquifer layers near the fieldin 1985 had not had sufficient time for their pressures

to reach equilibrium Recent drilling and multiple wellmonitoring site data (April 2018) off the eastern flank ofthe northern Lost Hills oil field indicates a 45 m (~15 ft)head difference across the Amnicola Clay with an upwardgradient of 0038 (measured May 31 2018) suggestingthat it is a confining layer in the northeastern part of thestudy area (US Geological Survey 2018a b)

Salinity Analysis

Log analysis from 30 wells in areas outside the oil fieldboundaries or within the oil fields but below the oilndashwater contact were used to create a map of the depthto the base of USDW The depth map to the base ofUSDW shows two trends (Figure 7) In the area be-tween Lost Hills and the Belridge oil fields depthto USDW increases toward the south from 150 m(~500 ft) between North Belridge and Lost Hills to490 m (~1600 ft) southeast of South Belridge In the

Figure 7 Map showing depth to base of underground sourcesof drinking water (USDW) (total dissolved solids = 10000 mgL)based on log analysis Red lines indicate faults (Young 1968California Division of Oil Gas and Geothermal Resources 1998)Black lines are oil field administrative boundaries Wells shown asblue dots are those used to determine the base of USDW Depthvalues in feet CI = contour interval No = North So = South

86 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

eastern part of the study area depth to base of USDWincreases sharply eastward from approximately 215 m(~700 ft) on the west flank of the Lost Hills anticline toas great as 550 m (~1800 ft) within 3 km (~2 mi)

Salinityndashdepth profiles based on chemical analysesof producedwater samples were compared to the depthto base USDW map estimated from borehole geo-physical log analysis However this comparison waslimited because most of the lab analyses come from oil-producing wells within the field boundaries rather thanin areas outside the fields The presence of oil in thesampled intervals within the fields precludes the useof log analysis to calculate the salinity in these samewells Plots of TDS versus depth of the samples in eachfield are shown in Figures 8 and 9 along with log-calculated TDS

Wood and Davis (1959) reported the groundwaterchemistry in this area as being an NaSO4 type howeveronly one Tulare Formation monitoring well Mackesseyno 1 (API no 02987363) was an NaSO4 type the restare NaCl type Because the calculated salinity fromArchie log analysis assumes NaCl water types the log-calculated and lab sample TDS values should be com-parable The two monitoring wells completed in thealluvium have higher SO4 concentrations relative tothe Tulare Formation wells however Cl is the domi-nant anion

Only four Tulare Formation lab samples (blue dots)in Lost Hills are in the California DOGGR databaseData from 14 monitoring wells in the central part ofthe Lost Hills field were used to enlarge the data set(Figure 8) With the exception of one sample TDSconcentrations in the Tulare Formation and alluviumare less than 10000 mgL indicating protected (USDW)aquifers These wells have top perforations ranging from40 to 290 m (133ndash950 ft) The well with a lab analysisindicating a salinity of 12295 mgL TDS in the TulareFormation had a top perforation at 300 m (984 ft) Thissuggests that the base of USDW lies somewhere be-tween 290 and 300 m (950 and 985 ft) within the fieldLog analysis suggests that the base may occur as deepas 580 m (~1900 ft) but many of the wells used for loganalysis come from areas east of the field boundarywhere the Tulare Formation is much thicker (200 ftwithin the field vs 2000 ft east of the field [Figure 4])Within the field interpolated contour lines from log datashow the depth to base USDWat 213m (~700 ft) alongthe western field boundary and 365 m (~1200 ft) alongthe eastern field boundary a trend that is consistentwith the more limited water sample data

Typical TDS concentrations in the underlyingEtchegoin Formation (red dots) range from 20000 to35000 mgL probably because of the presence oftrapped connate seawater within these marine depositsIn northern Lost Hills (Figure 8) two samples fromEtchegoin sands have TDS concentrations below 10000mgL However in this area an active steamflood isongoing within the Tulare and Etchegoin FormationsHere the Etchegoin Formation sands lie at shallowdepths (lt150 m [~500 ft]) immediately below pro-ducing intervals in the Tulare Formation The EtchegoinFormation samples in this area are much more dilute(5600ndash8000 mgL) than in other areas This may be be-cause the sands are in hydraulic communication with thefresher water sands of the overlying Tulare Formation orbecause the Etchegoin sands in this area are steamfloodedand the more saline formation water is diluted by theinjected steam Since the twoEtchegoinUSDWsamplesare post-steamflood initiation the latter explanation islikely

Only one chemical analysis sample from the TulareFormation is available in the North Belridge field (Fig-ure 9) The TDS concentration of the sample is 6055mgL from a depth interval of 157ndash211 m (515ndash692ft) It confirms that waters above approximately 211 m(~700 ft) are USDW consistent with the log-deriveddepth to USDW map which shows depth to USDWnear the well at 275 m (900 ft)

Many lab analyses are available for the Tulare For-mation at South Belridge (Figure 9) but they do notshow salinity values that are correlated with depth Thelog-generated depth to USDW map shows that salinitytrends along the northern part of the oil field are vari-able but the southern part of the field shows depth toUSDW increasing toward the southeast Thereforesamples from northern and southern South Belridgewere considered separately The boundary betweenthe two subareas is shown by the dashed line in Figure 9Because of the variability in sample TDS values withdepth comparisons of sample data to calculated salin-ities fromwell-log analyses can only bemade in a generalfashion

In the northern part of the South Belridge field nosamples below 365 m (1200 ft) are USDW suggestingthat the base of USDW lies above this depth The log-derived map shows depth to base of USDW at 335 m(1100 ft) in the north To the south the base of USDWincreases to 427 m (~1400 ft) Near the dividing linebetween the north and south parts of the field thedepth to base USDW decreases to 365 m (1200 ft)

Gillespie et al 87

Figure 8 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare and alluvium (blue) and Etchegoin (red) formations in and near the Lost Hills oilfield Gray dots represent TDS calculated from geophysical logs in the field The horizontal red linemarks 10000mgL and the vertical dashed red linemarks the approximate depth of thedeepest underground sources of drinking water (USDW) sample The adjacent map shows the location of the Tulare and alluviumwells sampled as blue dots and shallow (lt150m [500ft] deep) Etchegoin samples as red dots (deeper Etchegoin sample sites are not shown onmap) and the contours to base USDW from log analysis Lost Hills is the only area that containsbrackish water in the Etchegoin Formation However the brackish waters in the Etchegoin occur in an oil-producing zone that is steam flooded so it is considered to be an exempt aquiferand is not protected CI = contour interval

88Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

Figure 9 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare Formation in and near the North and South Belridge oil fields The northern andsouthern parts of the South Belridge field (marked by the red line on the map) are shown in separate graphs Blue dots show Tulare Formation (Fm) Measurements of TDS from labanalyses Gray dots show Tulare Fm The TDS calculated from log analysis Horizontal red lines on the graphs mark 10000 mgL and the dashed vertical red lines mark the approximatedepth of USDW from lab samples The adjacent map shows the location of the wells sampled (blue dots) and the contours to base underground sources of drinking water (USDW) (infeet) from log analysis Black lines are oil field administrative boundaries and red lines are faults CI = contour interval No = North So = South

Gillespieetal

89

Figure 10 Cross section showing three wells drilled and logged in different years near a produced water disposal pond (light blue polygon on inset map) Yellow shading onspontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivity curve (right track at scale of 0ndash20 ohmm) indicate resistivity of more than 3 ohmm Pinkbars indicate low resistivity sands affected by infiltration of saline produced water from the pond The API numbers at the top of the logs are used for well identification Year drilled andlogged in parentheses Depth scale in feet

90Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

This may be caused by the presence of an active waterdisposal field in the Tulare Formation near this areaOverall the log-derived map shows base of USDWapproximately 61m (~200 ft) deeper than lab analyseswould suggest As at Lost Hills this may be caused bythe geographic distribution of the two data sets the labsamples coming mainly from within the oil fieldboundaries whereas the log data are confined to thearea outside the oil field

In the southern part of South Belridge field a wellnear the southern tip of the field was sampled at mul-tiple intervals Samples as deep as 455 m (~1495 ft) areUSDW and the deepest sample from 480 m (1575 ft)has a TDS value of 13074 mgL suggesting that thebase of USDW in the southern part of South Belridge liesbetween 457 and 480 m (1495ndash1575 ft) (Figure 9) Thedepth to USDW map from log analysis in the southernpart of South Belridge shows that the base of USDW

ranges from 365 m (1200 ft) in the north to approxi-mately 487 m (~1600 ft) in the south

Effects of Oil Field Activities on Water Salinity

The effects of water disposal in surface ponds on ground-water salinity is evident by comparing geophysical logsfrom old wells near the pond locations to logs fromnewerwells drilled nearbyOlder geophysical logs (pre-1970s) indicate that the upper sands in the alluviumare desaturated (vadose zone) as indicated by areas ofcrossover on densityndashneutron logs and have high re-sistivity (gt20ohmmFigure 10)However geophysicallogs collected from wells drilled later in the area donot show densityndashneutron crossover characteristic of avadose zone andhave extremely low resistivity (lt1ohmm)in the upper alluvial sands because of the presence ofhigh salinityproducedwater thathasfilled thevadosezoneInvasion of low resistivity water is especially apparent

Figure 11 Two adjacent wells drilled and logged in different years (1990 and 2012) in an area containing numerous disposal wells on theeast flank of South Belridge Yellow shading on spontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivitycurve indicates resistivity greater than 3 ohm m Pink bars indicate low resistivity sands in the 2012 well (0403046566) affected by injectedsaline produced water The API numbers at the top of the logs are used for well identification Year drilled and logged in parentheses

Gillespie et al 91

where saline waters have infiltrated the 22K sandwhich lies directly above the CCE (well 03047481 inFigure 10) in the lower part of the alluvium The salinewater lies atop the CCE which appears to be pro-tecting the underlying Tulare aquifer from contami-nation Log-calculated andmeasuredTDS concentrationscan exceed 20000mgL in affected parts of the alluvium

The effects of surface disposal ponds are less evidentin Lost Hills and North Belridge compared to the areaadjacent to South Belridge Lost Hills in particular tends tohave highly saline water in the shallow parts of the aquifereven where disposal ponds are not present Swain andDuell (1989) noted TDS concentrations as high as 21700mgL from very shallowwells (6m [~20 ft]) along the eastflank of Lost Hills but did not note the cause of the highsalinity in the area

The effects of water disposal injection wells aremore subtle than that of the ponds and are especiallydifficult to identify in sands closer to the base of USDWwhere resistivity is naturally fairly low Geophysicallogs in sands affected by saline injection tend to have aconcave resistivity profile and resistivities are as small ashalf that in logs fromolder unaffectedwells (Figure 11)Injected saline water tends to follow individual sand

layers leaving some layers relatively unaffected In somecases only the lower part of an individual sand containssaline water whereas the waters in the upper part re-mainbrackishThis suggests that thewaters are stratifiedand mixing has not had time to occur Log-calculatedTDS concentrations show a relatively smooth increasewith depth inwells logged prior to the start of large-scaledisposal activities whereas recent logs in these areasshow a much more variable TDS concentration profilewith depth Additionally densityndashneutron curves inwells affected by disposal of produced waters show thinintervals (~1 m [~3 ft] thick) of crossover This may becaused by bacterial metabolism of trace amounts oforganic matter in the disposal water resulting in thegeneration of small amounts of biogenic methane(McMahon et al 2018)

The cross section in Figure 12 shows wells affectedby saline produced water disposal by injection (belowthe CCE) and percolation (above the CCE) along theeast flank of the South Belridge oil field Wells affectedby disposal of produced waters in saline ponds (n = 139)and by disposal in injection wells (n = 125) are alsoshown in the map in Figure 13 The effects of disposalponds are evident from changes in borehole resistivity

Figure 12 Cross section showing intervals affected by disposal of saline produced water (pink boxes) at South Belridge oil field Wellsidentified by API number at top of each well Year drilled and logged shown at bottom of eachwell Insetmap shows location of cross sectionProduced water disposal wells are highlighted in pink and disposal ponds are shown in blue in the inset map

92 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

between the late Miocene to Holocene formationsElectric logs (resistivity SP) and gamma-ray logs wereused to identify correlatable layers (Figure 2) Well data(geophysical logs mud logs core data and well his-tories) were obtained primarily from oil and gas wellrecords available from theDOGGR(CaliforniaDivisionof Oil Gas and Geothermal Resources 2018) and fromwater well records obtained from well owners and theCalifornia Department of Water Resources

Because the Tulare Formation and overlying allu-vium form the major nonexempt aquifers in the area

these layers were mapped in more detail particularlywith regard to clay layers Many of the clay markers losetheir coherence when mapped over large distances andbetween distantly spaced wells outside the oil fieldboundaries because of the rapidly changing characterof the Tulare fluvialndashlacustrine delta system In manyplaces it was not possible to correlate the clay layersthroughout the region The correlations were used toconstruct thickness (isochore) maps of the combinedalluvium andTulare Formation (Figure 4) andCCE andAmnicola clays (Figure 5) The correlated logs were

Figure3 Production (prod) and injection (inj) curves for the Lost Hills and North and South Belridge fields (combined) EOR= enhancedoil recovery HF = initiation of hydrofracking Inj = initiation of water injection for enhanced oil recovery WD = initiation of disposal ofproduced water in injection wells Data prior to 1973 are from California Division of Oil Gas and Geothermal Resources (2015) post-1973data are from California Division of Oil Gas and Geothermal Resources (2019)

80 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

compared to salinity data to determine the extent towhich salinity is controlled by aquifer stratigraphy

Wireline formation testers measure pressure buildup within a small interval of the sands (Schlumberger2006) Within a single homogeneous aquifer the pres-sure values should increase with depth along a straightline representing the hydrostatic gradient Shifts fromthe hydrostatic pressure gradient line indicate the pres-ence of pressures in individual aquifer layers that implyrestricted hydraulic connections with other aquiferlayers caused by confining clay layers (Coburn andGillespie 2002) Wireline formation tests in the TulareFormation have been conducted in some test holesdrilled in the study area by oil companies prior to wellinstallation (Gillespie et al 2019) These recordscompiled for 59 wells were analyzed to help deter-mine which clays are significant confining layers

Salinity Determination

For this study two different methods were used to de-termine water salinity within the aquifers in and aroundthe BelridgendashLost Hills oil fields The first is indirect es-timation of salinity using geophysical logs collected priorto installation of oil wells The second is direct mea-surement by lab analysis of producedwater samples fromoil wells in the area which were compared to the in-direct estimates

Log AnalysisAlthough direct sampling and chemical analyses arecommonly considered the best method for determiningTDS in any aquifer in many cases (particularly in deepaquifers containing brackish water) these analyses areuncommon In addition water samples provide only

Figure 4 Map showing depth to base of Tulare Formation The values are equivalent to the thickness of the Tulare Formation andoverlying alluvium Wells shown are those used to determine thickness Black lines are oil field administrative boundaries Gray lines aretownship boundaries Red lines are faults (Young 1968 California Division of Oil Gas and Geothermal Resources 1998) Abd= abandoned

Gillespie et al 81

Figure 5 Isochore maps of the (A) Amnicola Clay and (B) Corcoran Clay Equivalent layers in the study area Wells shown are those used to determine thickness Gray lines aretownship boundaries Red lines are faults (Young 1968 California Division of Oil Gas and Geothermal Resources 1998) Abd = abandoned Approx = Approximate

82Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

point-source data from a single depth interval and do nottell us at what depth the salinity changes However inactive petroleum-producing basins such as the SJV open-hole geophysical logs are abundant Geophysical logsprovide continuous vertical records of the properties ofthe formation matrix fluids adjacent to the boreholeand fluids within the borehole Using log analysis it ispossible to calculate the depths at which salinity changesoccur These depthsmay be verified by chemical analyseswhere available Of particular interest for determiningsalinity are the electrical logs (SP and resistivity) becausethe ability of water to conduct an electrical current isstrongly influenced by both the temperature and salinity ofthe formation water

Use of electrical logs to determine water salinity ismost common in the petroleum literature In oil-bearingreservoirs it is important to know the resistivity of thewater which occurs with the oil to determine the oilsaturation using the Archie (1942) equation or one ofits many variants

Fewer studies have evaluated the use of electricallogs to determine salinity in fresh to brackish aquifersHowells (1990) andHowells et al (1987) used well-loginterpretation to determine the location of the base ofmoderately saline (lt10000 mgL TDS) water in UtahLindner-Lunsford and Bruce (1995) used the ArchieEquation to determine salinity in aquifers with TDS lessthan 1000 mgL in southwest Wyoming Schnoebelenet al (1995) used geophysical logs to map the depth to10000 mgL TDS water in carbonate aquifers in Indianausing the SP log mud filtrate resistivity and the resis-tivity porosity method of Archie They compared theresults to sampled water analyses and found the resis-tivity porosity method to be most accurate More re-cently Hamlin and de la Rocha (2015) used the ArchieEquation to map fresh (lt1000 mgL TDS) slightlysaline (1000ndash3000mgL TDS) moderately saline (3000ndash10000mgL TDS) and very saline (gt10000mgL TDS)water in the CarrizondashWilcox aquifer of south TexasGillespie et al (2017) used log analysis to determinedepth to 10000 mgL TDS on a regional basis through-out much of the southern San Joaquin Basin but thewestern margin fields were not considered in theirstudy because the presence of shallow oil reservoirs requiremore extensive mapping to locate wet sands for loganalysis

In this study a variation of the Archie Equationwas used to estimate salinity (as NaCl equivalent)from the geophysical logs Archie (1942) related thein situ electrical resistivity of a fully water saturated

sedimentary rock (Ro) to its porosity (f) nondimen-sional factors related to matrix properties (the cemen-tation factor [m]and the tortuosity factor [a]) and theresistivity of the formation water (Rw) as shown inequation 1

Rw = Ropethfm=aTHORN (1)

Ro is the resistivity of a clean (minimal clay content) oil-free wet sand and can be obtained from the deepresistivity curve Sands were chosen based on an evalu-ation of nearby core mud log and drillers log data in-dicating that the sands analyzed did not contain oil orgas The porosity value is obtained from porosity logssuch as density neutron sonic or nuclear magneticresonance logs and core analysis For this study sonicporositieswere not used the poorly consolidated natureof the aquifer sediments causes additional slowing ofthe sonic wave from compression of the material re-sulting in artificially high porosity values

Because of the poorly sorted nature of the sedimentsand the presence of large amounts of clay and silt in somezones the average porosity used for salinity calcula-tions weighted density measurements twice as much asneutron values Neutron values for porosity are basedupon the abundance of hydrogen atoms in a formationBecause clays contain hydroxide groups within theircrystal lattice the neutron curve tends to overestimateporosity when clays are present The largest separationbetween the density and neutron curves on each logwasconsidered to be a zone with 100 clay The separationof the densityndashneutron curves for each evaluated in-terval was compared to the 100 clay separation valueto estimate clay content The Rw values were not cal-culated for intervals containing more than 25 clay

The exponentm (unitless) is related to the degree ofcementation in clastic rocks It typically ranges from 13to 26 (Wylie and Rose 1950) The value a (unitless) iscalled the tortuosity factor and is related to the lengththe current must travel through the formation Thevalue for a typically ranges from 05 to 15 For thisstudy special core analyses from five intervals in theTulare Formation were available in the CaliforniaDOGGRonline files from awater disposal well south ofthe Lost Hills field (American Petroleum Institute[API] well number [no] 0402974433) The value fora was kept constant at a value of 1 and m was allowedto vary The measured values for m ranged from 15 to18 with a mean value of 17 (standard deviation of015) and a median value of 18 For this study salinity

Gillespie et al 83

calculations used a = 1 andm = 17 based on the specialcore analysis results

Upon obtaining a value for Rw and calculating thetemperature of the zone of interest from a linear in-terpolation between bottom borehole temperature andair temperatureof 75degF the formationwater salinity canbeestimated from empirical charts such as those provided bySchlumberger (1997) or from equations such as thosein Bateman and Konen (1977) relating fluid resistivity toNaCl equivalent TDS concentration

Geochemical AnalysesThe California Division of Oil Gas and GeothermalResources (2016) maintains a website containing scan-ned copies of chemical analyses of oil field waters Thesedata are useful in determining the salinity of the deeperaquifers not normally used for drinking water or irriga-tion Data taken from these reports included dates oftesting source of water for test (when available) majorion concentrations and TDS concentrations The compileddata are available from Gillespie et al (2019) Chemicalanalysis of groundwater samples compiled from varioussources were also available (Metzger et al 2018) How-ever these samples were generally available from rela-tively shallow depths and had TDS ranging fromapproximately 1000 to 3000 mgL and therefore did notdirectly contribute to efforts to map the depth to USDW

Scanned copies of well completion reports are alsoavailable for the wells with chemical analysis from theCalifornia Division of Oil Gas and Geothermal Re-sources (2018) Well Finder online search engine Thecompletion reports for the wells provide informationincluding perforated intervals date perforated geolog-ical formation sampled (in some cases) scanned copiesof borehole geophysical logs pressure tests and bottom-hole temperatures These records were used to de-termine the depth interval and formation representedby the produced water chemical analysis data (Gillespieet al 2019)

Produced water chemical analyses from approxi-mately 150 oil production injection and observationwells in the study area were available from DOGGRrsquosonline chemical analysis database (California Divisionof Oil Gas and Geothermal Resources 2016) Addi-tionally 12 samples from the Tulare Formation and al-luvium were available from groundwater monitoringwells in the Lost Hills oil field on the Geotracker website(California State Water Resources Control Board2018a) The vintage of samples in the database rangesfrom 1932 to 2017

The sample analyses may not always representnatural aquifer conditions For example water samplesare commonly collected when a well has productionproblems or during the initial development of a well(Blondes et al 2016) Production problems may in-clude holes in well casings or annular cement that mayhave caused water from a different zone to enter theperforations In most cases few data exist regarding thesampling procedures and analysis techniques particu-larly for the older samples However older samples wereconsidered to be important for this study because theypredate the use of many EOR techniques such as waterflooding waste disposal and steam injection the large-scale use of which commenced in the early to mid-1960s These EOR processes may significantly alterthe salinity of the samples and cause the more recentsamples to be an inaccurate reflection of the originalaquifer conditions Samples taken at different timeintervals are helpful in determining temporal changes insalinity that may have occurred because of oil fieldactivities

One quality control method used in this study wasto perform a charge balance on the analyses Becausewater is electrically neutral the negative (anions) andpositive (cations) charged ions should sum to zero Thedifference between the total milliequivalents of thecations and anions is divided by the sum to give a chargebalance error We used a cutoff of ndash5 to determinewhich samples to discard in this analysis

For wells used for injection purposes the initiationof water injection relative to the date of sampling maybe important to consider Samples taken after the startof injection in the sampled well may not accuratelyrepresent the true formation water composition In mostcases it was not feasible to catalog the time of initiationof injection in nearby wells because of the large numberof injectionwells that could potentially affect a sampledwell Consequently the effect of subsurface injectionon sample TDS data remains a source of uncertainty inthe analysis

RESULTS

The methods and data discussed in the previous sec-tion were used to construct geologic maps showing thethickness and continuity of aquifers and confining unitsas well as to determine aquifer salinity and temporalchanges in salinity These results are discussed below

84 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 6 Graphs of shut-in pressure (from open-hole sequential formation testers) versus depth for three wells across the study area Red lines are used to emphasize the pressuregradient Tan areas showmajor clay layers the 22K sand is shown in yellow Base of Tulare and top of thewater table are also notedWells are identified by their API number and the yeardrilled and logged at the top of each graph

Gillespieetal

85

Geologic Mapping

The thickness of the Tulare Formation and overlyingalluvium in the study area (Figure 4) ranges from1200m (4000 ft) in the southeast part of the study areato 75 m (~250 ft) along the anticline crests in thenorthern parts of Lost Hills and North Belridge oilfields Because chemical analyses indicate that theprotected aquifers occur largely within the Tulare For-mation and overlying alluvium throughout the studyarea the map of the depth to the base of the TulareFormation defines the extent of potential protectedaquifers outside the oil field limits

The Amnicola Clay is thickest (67 m [~220 ft])along the northeastern and southeastern flank of theLost Hills anticline and east of the South Belridge an-ticline (Figure 5A) It thins to zero in the northwest partof the study area The clay is replaced by sandier sedi-ments east of the central part of the Lost Hills anticlineThe CCE thickens to 61m (~200 ft) in the northcentralpart of the study area in the Tulare lakebed but iscommonly less than30m(lt~100 ft) thick (Figure5B) Itis absent over the Lost Hills anticline crest and in thenorthwestern part of the study area These clay maps(Figure 5A B) are used to determine the presence ofpossible confining beds that in some cases may controlthe distribution of protected water resources and restrictthe migration of potential contaminants

Graphs of pressure versus depth collected in 1985from three wells along a northeast-trending transectfrom South Belridge to south of Lost Hills are shown inFigure 6 Marked changes in the hydrostatic pressuregradient in well API no 02974434 occur across theMiddle Tulare Clay and Amnicola Clay layers An addi-tional deeper clay here termed the D-E Clay forms alocal confining layer isolating a relatively low perme-ability section at the base of the Tulare Formation fromthe rest of the aquifer These characteristic deviationsin hydrostatic gradients indicates that the clays hydrau-lically separate individual aquifer layers in the area ofthis well

Farther northeast in wells API no 02974436 andAPI no 02974433 these deviations from hydrostaticgradient are much less noticeable especially across theAmnicola Clay This suggests that either (1) the claylayers are less confining south of Lost Hills or (2) thehydrologic regime near South Belridge has been alteredby production and injection into the Tulare Formationand the pressure responses in aquifer layers near the fieldin 1985 had not had sufficient time for their pressures

to reach equilibrium Recent drilling and multiple wellmonitoring site data (April 2018) off the eastern flank ofthe northern Lost Hills oil field indicates a 45 m (~15 ft)head difference across the Amnicola Clay with an upwardgradient of 0038 (measured May 31 2018) suggestingthat it is a confining layer in the northeastern part of thestudy area (US Geological Survey 2018a b)

Salinity Analysis

Log analysis from 30 wells in areas outside the oil fieldboundaries or within the oil fields but below the oilndashwater contact were used to create a map of the depthto the base of USDW The depth map to the base ofUSDW shows two trends (Figure 7) In the area be-tween Lost Hills and the Belridge oil fields depthto USDW increases toward the south from 150 m(~500 ft) between North Belridge and Lost Hills to490 m (~1600 ft) southeast of South Belridge In the

Figure 7 Map showing depth to base of underground sourcesof drinking water (USDW) (total dissolved solids = 10000 mgL)based on log analysis Red lines indicate faults (Young 1968California Division of Oil Gas and Geothermal Resources 1998)Black lines are oil field administrative boundaries Wells shown asblue dots are those used to determine the base of USDW Depthvalues in feet CI = contour interval No = North So = South

86 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

eastern part of the study area depth to base of USDWincreases sharply eastward from approximately 215 m(~700 ft) on the west flank of the Lost Hills anticline toas great as 550 m (~1800 ft) within 3 km (~2 mi)

Salinityndashdepth profiles based on chemical analysesof producedwater samples were compared to the depthto base USDW map estimated from borehole geo-physical log analysis However this comparison waslimited because most of the lab analyses come from oil-producing wells within the field boundaries rather thanin areas outside the fields The presence of oil in thesampled intervals within the fields precludes the useof log analysis to calculate the salinity in these samewells Plots of TDS versus depth of the samples in eachfield are shown in Figures 8 and 9 along with log-calculated TDS

Wood and Davis (1959) reported the groundwaterchemistry in this area as being an NaSO4 type howeveronly one Tulare Formation monitoring well Mackesseyno 1 (API no 02987363) was an NaSO4 type the restare NaCl type Because the calculated salinity fromArchie log analysis assumes NaCl water types the log-calculated and lab sample TDS values should be com-parable The two monitoring wells completed in thealluvium have higher SO4 concentrations relative tothe Tulare Formation wells however Cl is the domi-nant anion

Only four Tulare Formation lab samples (blue dots)in Lost Hills are in the California DOGGR databaseData from 14 monitoring wells in the central part ofthe Lost Hills field were used to enlarge the data set(Figure 8) With the exception of one sample TDSconcentrations in the Tulare Formation and alluviumare less than 10000 mgL indicating protected (USDW)aquifers These wells have top perforations ranging from40 to 290 m (133ndash950 ft) The well with a lab analysisindicating a salinity of 12295 mgL TDS in the TulareFormation had a top perforation at 300 m (984 ft) Thissuggests that the base of USDW lies somewhere be-tween 290 and 300 m (950 and 985 ft) within the fieldLog analysis suggests that the base may occur as deepas 580 m (~1900 ft) but many of the wells used for loganalysis come from areas east of the field boundarywhere the Tulare Formation is much thicker (200 ftwithin the field vs 2000 ft east of the field [Figure 4])Within the field interpolated contour lines from log datashow the depth to base USDWat 213m (~700 ft) alongthe western field boundary and 365 m (~1200 ft) alongthe eastern field boundary a trend that is consistentwith the more limited water sample data

Typical TDS concentrations in the underlyingEtchegoin Formation (red dots) range from 20000 to35000 mgL probably because of the presence oftrapped connate seawater within these marine depositsIn northern Lost Hills (Figure 8) two samples fromEtchegoin sands have TDS concentrations below 10000mgL However in this area an active steamflood isongoing within the Tulare and Etchegoin FormationsHere the Etchegoin Formation sands lie at shallowdepths (lt150 m [~500 ft]) immediately below pro-ducing intervals in the Tulare Formation The EtchegoinFormation samples in this area are much more dilute(5600ndash8000 mgL) than in other areas This may be be-cause the sands are in hydraulic communication with thefresher water sands of the overlying Tulare Formation orbecause the Etchegoin sands in this area are steamfloodedand the more saline formation water is diluted by theinjected steam Since the twoEtchegoinUSDWsamplesare post-steamflood initiation the latter explanation islikely

Only one chemical analysis sample from the TulareFormation is available in the North Belridge field (Fig-ure 9) The TDS concentration of the sample is 6055mgL from a depth interval of 157ndash211 m (515ndash692ft) It confirms that waters above approximately 211 m(~700 ft) are USDW consistent with the log-deriveddepth to USDW map which shows depth to USDWnear the well at 275 m (900 ft)

Many lab analyses are available for the Tulare For-mation at South Belridge (Figure 9) but they do notshow salinity values that are correlated with depth Thelog-generated depth to USDW map shows that salinitytrends along the northern part of the oil field are vari-able but the southern part of the field shows depth toUSDW increasing toward the southeast Thereforesamples from northern and southern South Belridgewere considered separately The boundary betweenthe two subareas is shown by the dashed line in Figure 9Because of the variability in sample TDS values withdepth comparisons of sample data to calculated salin-ities fromwell-log analyses can only bemade in a generalfashion

In the northern part of the South Belridge field nosamples below 365 m (1200 ft) are USDW suggestingthat the base of USDW lies above this depth The log-derived map shows depth to base of USDW at 335 m(1100 ft) in the north To the south the base of USDWincreases to 427 m (~1400 ft) Near the dividing linebetween the north and south parts of the field thedepth to base USDW decreases to 365 m (1200 ft)

Gillespie et al 87

Figure 8 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare and alluvium (blue) and Etchegoin (red) formations in and near the Lost Hills oilfield Gray dots represent TDS calculated from geophysical logs in the field The horizontal red linemarks 10000mgL and the vertical dashed red linemarks the approximate depth of thedeepest underground sources of drinking water (USDW) sample The adjacent map shows the location of the Tulare and alluviumwells sampled as blue dots and shallow (lt150m [500ft] deep) Etchegoin samples as red dots (deeper Etchegoin sample sites are not shown onmap) and the contours to base USDW from log analysis Lost Hills is the only area that containsbrackish water in the Etchegoin Formation However the brackish waters in the Etchegoin occur in an oil-producing zone that is steam flooded so it is considered to be an exempt aquiferand is not protected CI = contour interval

88Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

Figure 9 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare Formation in and near the North and South Belridge oil fields The northern andsouthern parts of the South Belridge field (marked by the red line on the map) are shown in separate graphs Blue dots show Tulare Formation (Fm) Measurements of TDS from labanalyses Gray dots show Tulare Fm The TDS calculated from log analysis Horizontal red lines on the graphs mark 10000 mgL and the dashed vertical red lines mark the approximatedepth of USDW from lab samples The adjacent map shows the location of the wells sampled (blue dots) and the contours to base underground sources of drinking water (USDW) (infeet) from log analysis Black lines are oil field administrative boundaries and red lines are faults CI = contour interval No = North So = South

Gillespieetal

89

Figure 10 Cross section showing three wells drilled and logged in different years near a produced water disposal pond (light blue polygon on inset map) Yellow shading onspontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivity curve (right track at scale of 0ndash20 ohmm) indicate resistivity of more than 3 ohmm Pinkbars indicate low resistivity sands affected by infiltration of saline produced water from the pond The API numbers at the top of the logs are used for well identification Year drilled andlogged in parentheses Depth scale in feet

90Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

This may be caused by the presence of an active waterdisposal field in the Tulare Formation near this areaOverall the log-derived map shows base of USDWapproximately 61m (~200 ft) deeper than lab analyseswould suggest As at Lost Hills this may be caused bythe geographic distribution of the two data sets the labsamples coming mainly from within the oil fieldboundaries whereas the log data are confined to thearea outside the oil field

In the southern part of South Belridge field a wellnear the southern tip of the field was sampled at mul-tiple intervals Samples as deep as 455 m (~1495 ft) areUSDW and the deepest sample from 480 m (1575 ft)has a TDS value of 13074 mgL suggesting that thebase of USDW in the southern part of South Belridge liesbetween 457 and 480 m (1495ndash1575 ft) (Figure 9) Thedepth to USDW map from log analysis in the southernpart of South Belridge shows that the base of USDW

ranges from 365 m (1200 ft) in the north to approxi-mately 487 m (~1600 ft) in the south

Effects of Oil Field Activities on Water Salinity

The effects of water disposal in surface ponds on ground-water salinity is evident by comparing geophysical logsfrom old wells near the pond locations to logs fromnewerwells drilled nearbyOlder geophysical logs (pre-1970s) indicate that the upper sands in the alluviumare desaturated (vadose zone) as indicated by areas ofcrossover on densityndashneutron logs and have high re-sistivity (gt20ohmmFigure 10)However geophysicallogs collected from wells drilled later in the area donot show densityndashneutron crossover characteristic of avadose zone andhave extremely low resistivity (lt1ohmm)in the upper alluvial sands because of the presence ofhigh salinityproducedwater thathasfilled thevadosezoneInvasion of low resistivity water is especially apparent

Figure 11 Two adjacent wells drilled and logged in different years (1990 and 2012) in an area containing numerous disposal wells on theeast flank of South Belridge Yellow shading on spontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivitycurve indicates resistivity greater than 3 ohm m Pink bars indicate low resistivity sands in the 2012 well (0403046566) affected by injectedsaline produced water The API numbers at the top of the logs are used for well identification Year drilled and logged in parentheses

Gillespie et al 91

where saline waters have infiltrated the 22K sandwhich lies directly above the CCE (well 03047481 inFigure 10) in the lower part of the alluvium The salinewater lies atop the CCE which appears to be pro-tecting the underlying Tulare aquifer from contami-nation Log-calculated andmeasuredTDS concentrationscan exceed 20000mgL in affected parts of the alluvium

The effects of surface disposal ponds are less evidentin Lost Hills and North Belridge compared to the areaadjacent to South Belridge Lost Hills in particular tends tohave highly saline water in the shallow parts of the aquifereven where disposal ponds are not present Swain andDuell (1989) noted TDS concentrations as high as 21700mgL from very shallowwells (6m [~20 ft]) along the eastflank of Lost Hills but did not note the cause of the highsalinity in the area

The effects of water disposal injection wells aremore subtle than that of the ponds and are especiallydifficult to identify in sands closer to the base of USDWwhere resistivity is naturally fairly low Geophysicallogs in sands affected by saline injection tend to have aconcave resistivity profile and resistivities are as small ashalf that in logs fromolder unaffectedwells (Figure 11)Injected saline water tends to follow individual sand

layers leaving some layers relatively unaffected In somecases only the lower part of an individual sand containssaline water whereas the waters in the upper part re-mainbrackishThis suggests that thewaters are stratifiedand mixing has not had time to occur Log-calculatedTDS concentrations show a relatively smooth increasewith depth inwells logged prior to the start of large-scaledisposal activities whereas recent logs in these areasshow a much more variable TDS concentration profilewith depth Additionally densityndashneutron curves inwells affected by disposal of produced waters show thinintervals (~1 m [~3 ft] thick) of crossover This may becaused by bacterial metabolism of trace amounts oforganic matter in the disposal water resulting in thegeneration of small amounts of biogenic methane(McMahon et al 2018)

The cross section in Figure 12 shows wells affectedby saline produced water disposal by injection (belowthe CCE) and percolation (above the CCE) along theeast flank of the South Belridge oil field Wells affectedby disposal of produced waters in saline ponds (n = 139)and by disposal in injection wells (n = 125) are alsoshown in the map in Figure 13 The effects of disposalponds are evident from changes in borehole resistivity

Figure 12 Cross section showing intervals affected by disposal of saline produced water (pink boxes) at South Belridge oil field Wellsidentified by API number at top of each well Year drilled and logged shown at bottom of eachwell Insetmap shows location of cross sectionProduced water disposal wells are highlighted in pink and disposal ponds are shown in blue in the inset map

92 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

compared to salinity data to determine the extent towhich salinity is controlled by aquifer stratigraphy

Wireline formation testers measure pressure buildup within a small interval of the sands (Schlumberger2006) Within a single homogeneous aquifer the pres-sure values should increase with depth along a straightline representing the hydrostatic gradient Shifts fromthe hydrostatic pressure gradient line indicate the pres-ence of pressures in individual aquifer layers that implyrestricted hydraulic connections with other aquiferlayers caused by confining clay layers (Coburn andGillespie 2002) Wireline formation tests in the TulareFormation have been conducted in some test holesdrilled in the study area by oil companies prior to wellinstallation (Gillespie et al 2019) These recordscompiled for 59 wells were analyzed to help deter-mine which clays are significant confining layers

Salinity Determination

For this study two different methods were used to de-termine water salinity within the aquifers in and aroundthe BelridgendashLost Hills oil fields The first is indirect es-timation of salinity using geophysical logs collected priorto installation of oil wells The second is direct mea-surement by lab analysis of producedwater samples fromoil wells in the area which were compared to the in-direct estimates

Log AnalysisAlthough direct sampling and chemical analyses arecommonly considered the best method for determiningTDS in any aquifer in many cases (particularly in deepaquifers containing brackish water) these analyses areuncommon In addition water samples provide only

Figure 4 Map showing depth to base of Tulare Formation The values are equivalent to the thickness of the Tulare Formation andoverlying alluvium Wells shown are those used to determine thickness Black lines are oil field administrative boundaries Gray lines aretownship boundaries Red lines are faults (Young 1968 California Division of Oil Gas and Geothermal Resources 1998) Abd= abandoned

Gillespie et al 81

Figure 5 Isochore maps of the (A) Amnicola Clay and (B) Corcoran Clay Equivalent layers in the study area Wells shown are those used to determine thickness Gray lines aretownship boundaries Red lines are faults (Young 1968 California Division of Oil Gas and Geothermal Resources 1998) Abd = abandoned Approx = Approximate

82Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

point-source data from a single depth interval and do nottell us at what depth the salinity changes However inactive petroleum-producing basins such as the SJV open-hole geophysical logs are abundant Geophysical logsprovide continuous vertical records of the properties ofthe formation matrix fluids adjacent to the boreholeand fluids within the borehole Using log analysis it ispossible to calculate the depths at which salinity changesoccur These depthsmay be verified by chemical analyseswhere available Of particular interest for determiningsalinity are the electrical logs (SP and resistivity) becausethe ability of water to conduct an electrical current isstrongly influenced by both the temperature and salinity ofthe formation water

Use of electrical logs to determine water salinity ismost common in the petroleum literature In oil-bearingreservoirs it is important to know the resistivity of thewater which occurs with the oil to determine the oilsaturation using the Archie (1942) equation or one ofits many variants

Fewer studies have evaluated the use of electricallogs to determine salinity in fresh to brackish aquifersHowells (1990) andHowells et al (1987) used well-loginterpretation to determine the location of the base ofmoderately saline (lt10000 mgL TDS) water in UtahLindner-Lunsford and Bruce (1995) used the ArchieEquation to determine salinity in aquifers with TDS lessthan 1000 mgL in southwest Wyoming Schnoebelenet al (1995) used geophysical logs to map the depth to10000 mgL TDS water in carbonate aquifers in Indianausing the SP log mud filtrate resistivity and the resis-tivity porosity method of Archie They compared theresults to sampled water analyses and found the resis-tivity porosity method to be most accurate More re-cently Hamlin and de la Rocha (2015) used the ArchieEquation to map fresh (lt1000 mgL TDS) slightlysaline (1000ndash3000mgL TDS) moderately saline (3000ndash10000mgL TDS) and very saline (gt10000mgL TDS)water in the CarrizondashWilcox aquifer of south TexasGillespie et al (2017) used log analysis to determinedepth to 10000 mgL TDS on a regional basis through-out much of the southern San Joaquin Basin but thewestern margin fields were not considered in theirstudy because the presence of shallow oil reservoirs requiremore extensive mapping to locate wet sands for loganalysis

In this study a variation of the Archie Equationwas used to estimate salinity (as NaCl equivalent)from the geophysical logs Archie (1942) related thein situ electrical resistivity of a fully water saturated

sedimentary rock (Ro) to its porosity (f) nondimen-sional factors related to matrix properties (the cemen-tation factor [m]and the tortuosity factor [a]) and theresistivity of the formation water (Rw) as shown inequation 1

Rw = Ropethfm=aTHORN (1)

Ro is the resistivity of a clean (minimal clay content) oil-free wet sand and can be obtained from the deepresistivity curve Sands were chosen based on an evalu-ation of nearby core mud log and drillers log data in-dicating that the sands analyzed did not contain oil orgas The porosity value is obtained from porosity logssuch as density neutron sonic or nuclear magneticresonance logs and core analysis For this study sonicporositieswere not used the poorly consolidated natureof the aquifer sediments causes additional slowing ofthe sonic wave from compression of the material re-sulting in artificially high porosity values

Because of the poorly sorted nature of the sedimentsand the presence of large amounts of clay and silt in somezones the average porosity used for salinity calcula-tions weighted density measurements twice as much asneutron values Neutron values for porosity are basedupon the abundance of hydrogen atoms in a formationBecause clays contain hydroxide groups within theircrystal lattice the neutron curve tends to overestimateporosity when clays are present The largest separationbetween the density and neutron curves on each logwasconsidered to be a zone with 100 clay The separationof the densityndashneutron curves for each evaluated in-terval was compared to the 100 clay separation valueto estimate clay content The Rw values were not cal-culated for intervals containing more than 25 clay

The exponentm (unitless) is related to the degree ofcementation in clastic rocks It typically ranges from 13to 26 (Wylie and Rose 1950) The value a (unitless) iscalled the tortuosity factor and is related to the lengththe current must travel through the formation Thevalue for a typically ranges from 05 to 15 For thisstudy special core analyses from five intervals in theTulare Formation were available in the CaliforniaDOGGRonline files from awater disposal well south ofthe Lost Hills field (American Petroleum Institute[API] well number [no] 0402974433) The value fora was kept constant at a value of 1 and m was allowedto vary The measured values for m ranged from 15 to18 with a mean value of 17 (standard deviation of015) and a median value of 18 For this study salinity

Gillespie et al 83

calculations used a = 1 andm = 17 based on the specialcore analysis results

Upon obtaining a value for Rw and calculating thetemperature of the zone of interest from a linear in-terpolation between bottom borehole temperature andair temperatureof 75degF the formationwater salinity canbeestimated from empirical charts such as those provided bySchlumberger (1997) or from equations such as thosein Bateman and Konen (1977) relating fluid resistivity toNaCl equivalent TDS concentration

Geochemical AnalysesThe California Division of Oil Gas and GeothermalResources (2016) maintains a website containing scan-ned copies of chemical analyses of oil field waters Thesedata are useful in determining the salinity of the deeperaquifers not normally used for drinking water or irriga-tion Data taken from these reports included dates oftesting source of water for test (when available) majorion concentrations and TDS concentrations The compileddata are available from Gillespie et al (2019) Chemicalanalysis of groundwater samples compiled from varioussources were also available (Metzger et al 2018) How-ever these samples were generally available from rela-tively shallow depths and had TDS ranging fromapproximately 1000 to 3000 mgL and therefore did notdirectly contribute to efforts to map the depth to USDW

Scanned copies of well completion reports are alsoavailable for the wells with chemical analysis from theCalifornia Division of Oil Gas and Geothermal Re-sources (2018) Well Finder online search engine Thecompletion reports for the wells provide informationincluding perforated intervals date perforated geolog-ical formation sampled (in some cases) scanned copiesof borehole geophysical logs pressure tests and bottom-hole temperatures These records were used to de-termine the depth interval and formation representedby the produced water chemical analysis data (Gillespieet al 2019)

Produced water chemical analyses from approxi-mately 150 oil production injection and observationwells in the study area were available from DOGGRrsquosonline chemical analysis database (California Divisionof Oil Gas and Geothermal Resources 2016) Addi-tionally 12 samples from the Tulare Formation and al-luvium were available from groundwater monitoringwells in the Lost Hills oil field on the Geotracker website(California State Water Resources Control Board2018a) The vintage of samples in the database rangesfrom 1932 to 2017

The sample analyses may not always representnatural aquifer conditions For example water samplesare commonly collected when a well has productionproblems or during the initial development of a well(Blondes et al 2016) Production problems may in-clude holes in well casings or annular cement that mayhave caused water from a different zone to enter theperforations In most cases few data exist regarding thesampling procedures and analysis techniques particu-larly for the older samples However older samples wereconsidered to be important for this study because theypredate the use of many EOR techniques such as waterflooding waste disposal and steam injection the large-scale use of which commenced in the early to mid-1960s These EOR processes may significantly alterthe salinity of the samples and cause the more recentsamples to be an inaccurate reflection of the originalaquifer conditions Samples taken at different timeintervals are helpful in determining temporal changes insalinity that may have occurred because of oil fieldactivities

One quality control method used in this study wasto perform a charge balance on the analyses Becausewater is electrically neutral the negative (anions) andpositive (cations) charged ions should sum to zero Thedifference between the total milliequivalents of thecations and anions is divided by the sum to give a chargebalance error We used a cutoff of ndash5 to determinewhich samples to discard in this analysis

For wells used for injection purposes the initiationof water injection relative to the date of sampling maybe important to consider Samples taken after the startof injection in the sampled well may not accuratelyrepresent the true formation water composition In mostcases it was not feasible to catalog the time of initiationof injection in nearby wells because of the large numberof injectionwells that could potentially affect a sampledwell Consequently the effect of subsurface injectionon sample TDS data remains a source of uncertainty inthe analysis

RESULTS

The methods and data discussed in the previous sec-tion were used to construct geologic maps showing thethickness and continuity of aquifers and confining unitsas well as to determine aquifer salinity and temporalchanges in salinity These results are discussed below

84 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 6 Graphs of shut-in pressure (from open-hole sequential formation testers) versus depth for three wells across the study area Red lines are used to emphasize the pressuregradient Tan areas showmajor clay layers the 22K sand is shown in yellow Base of Tulare and top of thewater table are also notedWells are identified by their API number and the yeardrilled and logged at the top of each graph

Gillespieetal

85

Geologic Mapping

The thickness of the Tulare Formation and overlyingalluvium in the study area (Figure 4) ranges from1200m (4000 ft) in the southeast part of the study areato 75 m (~250 ft) along the anticline crests in thenorthern parts of Lost Hills and North Belridge oilfields Because chemical analyses indicate that theprotected aquifers occur largely within the Tulare For-mation and overlying alluvium throughout the studyarea the map of the depth to the base of the TulareFormation defines the extent of potential protectedaquifers outside the oil field limits

The Amnicola Clay is thickest (67 m [~220 ft])along the northeastern and southeastern flank of theLost Hills anticline and east of the South Belridge an-ticline (Figure 5A) It thins to zero in the northwest partof the study area The clay is replaced by sandier sedi-ments east of the central part of the Lost Hills anticlineThe CCE thickens to 61m (~200 ft) in the northcentralpart of the study area in the Tulare lakebed but iscommonly less than30m(lt~100 ft) thick (Figure5B) Itis absent over the Lost Hills anticline crest and in thenorthwestern part of the study area These clay maps(Figure 5A B) are used to determine the presence ofpossible confining beds that in some cases may controlthe distribution of protected water resources and restrictthe migration of potential contaminants

Graphs of pressure versus depth collected in 1985from three wells along a northeast-trending transectfrom South Belridge to south of Lost Hills are shown inFigure 6 Marked changes in the hydrostatic pressuregradient in well API no 02974434 occur across theMiddle Tulare Clay and Amnicola Clay layers An addi-tional deeper clay here termed the D-E Clay forms alocal confining layer isolating a relatively low perme-ability section at the base of the Tulare Formation fromthe rest of the aquifer These characteristic deviationsin hydrostatic gradients indicates that the clays hydrau-lically separate individual aquifer layers in the area ofthis well

Farther northeast in wells API no 02974436 andAPI no 02974433 these deviations from hydrostaticgradient are much less noticeable especially across theAmnicola Clay This suggests that either (1) the claylayers are less confining south of Lost Hills or (2) thehydrologic regime near South Belridge has been alteredby production and injection into the Tulare Formationand the pressure responses in aquifer layers near the fieldin 1985 had not had sufficient time for their pressures

to reach equilibrium Recent drilling and multiple wellmonitoring site data (April 2018) off the eastern flank ofthe northern Lost Hills oil field indicates a 45 m (~15 ft)head difference across the Amnicola Clay with an upwardgradient of 0038 (measured May 31 2018) suggestingthat it is a confining layer in the northeastern part of thestudy area (US Geological Survey 2018a b)

Salinity Analysis

Log analysis from 30 wells in areas outside the oil fieldboundaries or within the oil fields but below the oilndashwater contact were used to create a map of the depthto the base of USDW The depth map to the base ofUSDW shows two trends (Figure 7) In the area be-tween Lost Hills and the Belridge oil fields depthto USDW increases toward the south from 150 m(~500 ft) between North Belridge and Lost Hills to490 m (~1600 ft) southeast of South Belridge In the

Figure 7 Map showing depth to base of underground sourcesof drinking water (USDW) (total dissolved solids = 10000 mgL)based on log analysis Red lines indicate faults (Young 1968California Division of Oil Gas and Geothermal Resources 1998)Black lines are oil field administrative boundaries Wells shown asblue dots are those used to determine the base of USDW Depthvalues in feet CI = contour interval No = North So = South

86 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

eastern part of the study area depth to base of USDWincreases sharply eastward from approximately 215 m(~700 ft) on the west flank of the Lost Hills anticline toas great as 550 m (~1800 ft) within 3 km (~2 mi)

Salinityndashdepth profiles based on chemical analysesof producedwater samples were compared to the depthto base USDW map estimated from borehole geo-physical log analysis However this comparison waslimited because most of the lab analyses come from oil-producing wells within the field boundaries rather thanin areas outside the fields The presence of oil in thesampled intervals within the fields precludes the useof log analysis to calculate the salinity in these samewells Plots of TDS versus depth of the samples in eachfield are shown in Figures 8 and 9 along with log-calculated TDS

Wood and Davis (1959) reported the groundwaterchemistry in this area as being an NaSO4 type howeveronly one Tulare Formation monitoring well Mackesseyno 1 (API no 02987363) was an NaSO4 type the restare NaCl type Because the calculated salinity fromArchie log analysis assumes NaCl water types the log-calculated and lab sample TDS values should be com-parable The two monitoring wells completed in thealluvium have higher SO4 concentrations relative tothe Tulare Formation wells however Cl is the domi-nant anion

Only four Tulare Formation lab samples (blue dots)in Lost Hills are in the California DOGGR databaseData from 14 monitoring wells in the central part ofthe Lost Hills field were used to enlarge the data set(Figure 8) With the exception of one sample TDSconcentrations in the Tulare Formation and alluviumare less than 10000 mgL indicating protected (USDW)aquifers These wells have top perforations ranging from40 to 290 m (133ndash950 ft) The well with a lab analysisindicating a salinity of 12295 mgL TDS in the TulareFormation had a top perforation at 300 m (984 ft) Thissuggests that the base of USDW lies somewhere be-tween 290 and 300 m (950 and 985 ft) within the fieldLog analysis suggests that the base may occur as deepas 580 m (~1900 ft) but many of the wells used for loganalysis come from areas east of the field boundarywhere the Tulare Formation is much thicker (200 ftwithin the field vs 2000 ft east of the field [Figure 4])Within the field interpolated contour lines from log datashow the depth to base USDWat 213m (~700 ft) alongthe western field boundary and 365 m (~1200 ft) alongthe eastern field boundary a trend that is consistentwith the more limited water sample data

Typical TDS concentrations in the underlyingEtchegoin Formation (red dots) range from 20000 to35000 mgL probably because of the presence oftrapped connate seawater within these marine depositsIn northern Lost Hills (Figure 8) two samples fromEtchegoin sands have TDS concentrations below 10000mgL However in this area an active steamflood isongoing within the Tulare and Etchegoin FormationsHere the Etchegoin Formation sands lie at shallowdepths (lt150 m [~500 ft]) immediately below pro-ducing intervals in the Tulare Formation The EtchegoinFormation samples in this area are much more dilute(5600ndash8000 mgL) than in other areas This may be be-cause the sands are in hydraulic communication with thefresher water sands of the overlying Tulare Formation orbecause the Etchegoin sands in this area are steamfloodedand the more saline formation water is diluted by theinjected steam Since the twoEtchegoinUSDWsamplesare post-steamflood initiation the latter explanation islikely

Only one chemical analysis sample from the TulareFormation is available in the North Belridge field (Fig-ure 9) The TDS concentration of the sample is 6055mgL from a depth interval of 157ndash211 m (515ndash692ft) It confirms that waters above approximately 211 m(~700 ft) are USDW consistent with the log-deriveddepth to USDW map which shows depth to USDWnear the well at 275 m (900 ft)

Many lab analyses are available for the Tulare For-mation at South Belridge (Figure 9) but they do notshow salinity values that are correlated with depth Thelog-generated depth to USDW map shows that salinitytrends along the northern part of the oil field are vari-able but the southern part of the field shows depth toUSDW increasing toward the southeast Thereforesamples from northern and southern South Belridgewere considered separately The boundary betweenthe two subareas is shown by the dashed line in Figure 9Because of the variability in sample TDS values withdepth comparisons of sample data to calculated salin-ities fromwell-log analyses can only bemade in a generalfashion

In the northern part of the South Belridge field nosamples below 365 m (1200 ft) are USDW suggestingthat the base of USDW lies above this depth The log-derived map shows depth to base of USDW at 335 m(1100 ft) in the north To the south the base of USDWincreases to 427 m (~1400 ft) Near the dividing linebetween the north and south parts of the field thedepth to base USDW decreases to 365 m (1200 ft)

Gillespie et al 87

Figure 8 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare and alluvium (blue) and Etchegoin (red) formations in and near the Lost Hills oilfield Gray dots represent TDS calculated from geophysical logs in the field The horizontal red linemarks 10000mgL and the vertical dashed red linemarks the approximate depth of thedeepest underground sources of drinking water (USDW) sample The adjacent map shows the location of the Tulare and alluviumwells sampled as blue dots and shallow (lt150m [500ft] deep) Etchegoin samples as red dots (deeper Etchegoin sample sites are not shown onmap) and the contours to base USDW from log analysis Lost Hills is the only area that containsbrackish water in the Etchegoin Formation However the brackish waters in the Etchegoin occur in an oil-producing zone that is steam flooded so it is considered to be an exempt aquiferand is not protected CI = contour interval

88Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

Figure 9 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare Formation in and near the North and South Belridge oil fields The northern andsouthern parts of the South Belridge field (marked by the red line on the map) are shown in separate graphs Blue dots show Tulare Formation (Fm) Measurements of TDS from labanalyses Gray dots show Tulare Fm The TDS calculated from log analysis Horizontal red lines on the graphs mark 10000 mgL and the dashed vertical red lines mark the approximatedepth of USDW from lab samples The adjacent map shows the location of the wells sampled (blue dots) and the contours to base underground sources of drinking water (USDW) (infeet) from log analysis Black lines are oil field administrative boundaries and red lines are faults CI = contour interval No = North So = South

Gillespieetal

89

Figure 10 Cross section showing three wells drilled and logged in different years near a produced water disposal pond (light blue polygon on inset map) Yellow shading onspontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivity curve (right track at scale of 0ndash20 ohmm) indicate resistivity of more than 3 ohmm Pinkbars indicate low resistivity sands affected by infiltration of saline produced water from the pond The API numbers at the top of the logs are used for well identification Year drilled andlogged in parentheses Depth scale in feet

90Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

This may be caused by the presence of an active waterdisposal field in the Tulare Formation near this areaOverall the log-derived map shows base of USDWapproximately 61m (~200 ft) deeper than lab analyseswould suggest As at Lost Hills this may be caused bythe geographic distribution of the two data sets the labsamples coming mainly from within the oil fieldboundaries whereas the log data are confined to thearea outside the oil field

In the southern part of South Belridge field a wellnear the southern tip of the field was sampled at mul-tiple intervals Samples as deep as 455 m (~1495 ft) areUSDW and the deepest sample from 480 m (1575 ft)has a TDS value of 13074 mgL suggesting that thebase of USDW in the southern part of South Belridge liesbetween 457 and 480 m (1495ndash1575 ft) (Figure 9) Thedepth to USDW map from log analysis in the southernpart of South Belridge shows that the base of USDW

ranges from 365 m (1200 ft) in the north to approxi-mately 487 m (~1600 ft) in the south

Effects of Oil Field Activities on Water Salinity

The effects of water disposal in surface ponds on ground-water salinity is evident by comparing geophysical logsfrom old wells near the pond locations to logs fromnewerwells drilled nearbyOlder geophysical logs (pre-1970s) indicate that the upper sands in the alluviumare desaturated (vadose zone) as indicated by areas ofcrossover on densityndashneutron logs and have high re-sistivity (gt20ohmmFigure 10)However geophysicallogs collected from wells drilled later in the area donot show densityndashneutron crossover characteristic of avadose zone andhave extremely low resistivity (lt1ohmm)in the upper alluvial sands because of the presence ofhigh salinityproducedwater thathasfilled thevadosezoneInvasion of low resistivity water is especially apparent

Figure 11 Two adjacent wells drilled and logged in different years (1990 and 2012) in an area containing numerous disposal wells on theeast flank of South Belridge Yellow shading on spontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivitycurve indicates resistivity greater than 3 ohm m Pink bars indicate low resistivity sands in the 2012 well (0403046566) affected by injectedsaline produced water The API numbers at the top of the logs are used for well identification Year drilled and logged in parentheses

Gillespie et al 91

where saline waters have infiltrated the 22K sandwhich lies directly above the CCE (well 03047481 inFigure 10) in the lower part of the alluvium The salinewater lies atop the CCE which appears to be pro-tecting the underlying Tulare aquifer from contami-nation Log-calculated andmeasuredTDS concentrationscan exceed 20000mgL in affected parts of the alluvium

The effects of surface disposal ponds are less evidentin Lost Hills and North Belridge compared to the areaadjacent to South Belridge Lost Hills in particular tends tohave highly saline water in the shallow parts of the aquifereven where disposal ponds are not present Swain andDuell (1989) noted TDS concentrations as high as 21700mgL from very shallowwells (6m [~20 ft]) along the eastflank of Lost Hills but did not note the cause of the highsalinity in the area

The effects of water disposal injection wells aremore subtle than that of the ponds and are especiallydifficult to identify in sands closer to the base of USDWwhere resistivity is naturally fairly low Geophysicallogs in sands affected by saline injection tend to have aconcave resistivity profile and resistivities are as small ashalf that in logs fromolder unaffectedwells (Figure 11)Injected saline water tends to follow individual sand

layers leaving some layers relatively unaffected In somecases only the lower part of an individual sand containssaline water whereas the waters in the upper part re-mainbrackishThis suggests that thewaters are stratifiedand mixing has not had time to occur Log-calculatedTDS concentrations show a relatively smooth increasewith depth inwells logged prior to the start of large-scaledisposal activities whereas recent logs in these areasshow a much more variable TDS concentration profilewith depth Additionally densityndashneutron curves inwells affected by disposal of produced waters show thinintervals (~1 m [~3 ft] thick) of crossover This may becaused by bacterial metabolism of trace amounts oforganic matter in the disposal water resulting in thegeneration of small amounts of biogenic methane(McMahon et al 2018)

The cross section in Figure 12 shows wells affectedby saline produced water disposal by injection (belowthe CCE) and percolation (above the CCE) along theeast flank of the South Belridge oil field Wells affectedby disposal of produced waters in saline ponds (n = 139)and by disposal in injection wells (n = 125) are alsoshown in the map in Figure 13 The effects of disposalponds are evident from changes in borehole resistivity

Figure 12 Cross section showing intervals affected by disposal of saline produced water (pink boxes) at South Belridge oil field Wellsidentified by API number at top of each well Year drilled and logged shown at bottom of eachwell Insetmap shows location of cross sectionProduced water disposal wells are highlighted in pink and disposal ponds are shown in blue in the inset map

92 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 5 Isochore maps of the (A) Amnicola Clay and (B) Corcoran Clay Equivalent layers in the study area Wells shown are those used to determine thickness Gray lines aretownship boundaries Red lines are faults (Young 1968 California Division of Oil Gas and Geothermal Resources 1998) Abd = abandoned Approx = Approximate

82Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

point-source data from a single depth interval and do nottell us at what depth the salinity changes However inactive petroleum-producing basins such as the SJV open-hole geophysical logs are abundant Geophysical logsprovide continuous vertical records of the properties ofthe formation matrix fluids adjacent to the boreholeand fluids within the borehole Using log analysis it ispossible to calculate the depths at which salinity changesoccur These depthsmay be verified by chemical analyseswhere available Of particular interest for determiningsalinity are the electrical logs (SP and resistivity) becausethe ability of water to conduct an electrical current isstrongly influenced by both the temperature and salinity ofthe formation water

Use of electrical logs to determine water salinity ismost common in the petroleum literature In oil-bearingreservoirs it is important to know the resistivity of thewater which occurs with the oil to determine the oilsaturation using the Archie (1942) equation or one ofits many variants

Fewer studies have evaluated the use of electricallogs to determine salinity in fresh to brackish aquifersHowells (1990) andHowells et al (1987) used well-loginterpretation to determine the location of the base ofmoderately saline (lt10000 mgL TDS) water in UtahLindner-Lunsford and Bruce (1995) used the ArchieEquation to determine salinity in aquifers with TDS lessthan 1000 mgL in southwest Wyoming Schnoebelenet al (1995) used geophysical logs to map the depth to10000 mgL TDS water in carbonate aquifers in Indianausing the SP log mud filtrate resistivity and the resis-tivity porosity method of Archie They compared theresults to sampled water analyses and found the resis-tivity porosity method to be most accurate More re-cently Hamlin and de la Rocha (2015) used the ArchieEquation to map fresh (lt1000 mgL TDS) slightlysaline (1000ndash3000mgL TDS) moderately saline (3000ndash10000mgL TDS) and very saline (gt10000mgL TDS)water in the CarrizondashWilcox aquifer of south TexasGillespie et al (2017) used log analysis to determinedepth to 10000 mgL TDS on a regional basis through-out much of the southern San Joaquin Basin but thewestern margin fields were not considered in theirstudy because the presence of shallow oil reservoirs requiremore extensive mapping to locate wet sands for loganalysis

In this study a variation of the Archie Equationwas used to estimate salinity (as NaCl equivalent)from the geophysical logs Archie (1942) related thein situ electrical resistivity of a fully water saturated

sedimentary rock (Ro) to its porosity (f) nondimen-sional factors related to matrix properties (the cemen-tation factor [m]and the tortuosity factor [a]) and theresistivity of the formation water (Rw) as shown inequation 1

Rw = Ropethfm=aTHORN (1)

Ro is the resistivity of a clean (minimal clay content) oil-free wet sand and can be obtained from the deepresistivity curve Sands were chosen based on an evalu-ation of nearby core mud log and drillers log data in-dicating that the sands analyzed did not contain oil orgas The porosity value is obtained from porosity logssuch as density neutron sonic or nuclear magneticresonance logs and core analysis For this study sonicporositieswere not used the poorly consolidated natureof the aquifer sediments causes additional slowing ofthe sonic wave from compression of the material re-sulting in artificially high porosity values

Because of the poorly sorted nature of the sedimentsand the presence of large amounts of clay and silt in somezones the average porosity used for salinity calcula-tions weighted density measurements twice as much asneutron values Neutron values for porosity are basedupon the abundance of hydrogen atoms in a formationBecause clays contain hydroxide groups within theircrystal lattice the neutron curve tends to overestimateporosity when clays are present The largest separationbetween the density and neutron curves on each logwasconsidered to be a zone with 100 clay The separationof the densityndashneutron curves for each evaluated in-terval was compared to the 100 clay separation valueto estimate clay content The Rw values were not cal-culated for intervals containing more than 25 clay

The exponentm (unitless) is related to the degree ofcementation in clastic rocks It typically ranges from 13to 26 (Wylie and Rose 1950) The value a (unitless) iscalled the tortuosity factor and is related to the lengththe current must travel through the formation Thevalue for a typically ranges from 05 to 15 For thisstudy special core analyses from five intervals in theTulare Formation were available in the CaliforniaDOGGRonline files from awater disposal well south ofthe Lost Hills field (American Petroleum Institute[API] well number [no] 0402974433) The value fora was kept constant at a value of 1 and m was allowedto vary The measured values for m ranged from 15 to18 with a mean value of 17 (standard deviation of015) and a median value of 18 For this study salinity

Gillespie et al 83

calculations used a = 1 andm = 17 based on the specialcore analysis results

Upon obtaining a value for Rw and calculating thetemperature of the zone of interest from a linear in-terpolation between bottom borehole temperature andair temperatureof 75degF the formationwater salinity canbeestimated from empirical charts such as those provided bySchlumberger (1997) or from equations such as thosein Bateman and Konen (1977) relating fluid resistivity toNaCl equivalent TDS concentration

Geochemical AnalysesThe California Division of Oil Gas and GeothermalResources (2016) maintains a website containing scan-ned copies of chemical analyses of oil field waters Thesedata are useful in determining the salinity of the deeperaquifers not normally used for drinking water or irriga-tion Data taken from these reports included dates oftesting source of water for test (when available) majorion concentrations and TDS concentrations The compileddata are available from Gillespie et al (2019) Chemicalanalysis of groundwater samples compiled from varioussources were also available (Metzger et al 2018) How-ever these samples were generally available from rela-tively shallow depths and had TDS ranging fromapproximately 1000 to 3000 mgL and therefore did notdirectly contribute to efforts to map the depth to USDW

Scanned copies of well completion reports are alsoavailable for the wells with chemical analysis from theCalifornia Division of Oil Gas and Geothermal Re-sources (2018) Well Finder online search engine Thecompletion reports for the wells provide informationincluding perforated intervals date perforated geolog-ical formation sampled (in some cases) scanned copiesof borehole geophysical logs pressure tests and bottom-hole temperatures These records were used to de-termine the depth interval and formation representedby the produced water chemical analysis data (Gillespieet al 2019)

Produced water chemical analyses from approxi-mately 150 oil production injection and observationwells in the study area were available from DOGGRrsquosonline chemical analysis database (California Divisionof Oil Gas and Geothermal Resources 2016) Addi-tionally 12 samples from the Tulare Formation and al-luvium were available from groundwater monitoringwells in the Lost Hills oil field on the Geotracker website(California State Water Resources Control Board2018a) The vintage of samples in the database rangesfrom 1932 to 2017

The sample analyses may not always representnatural aquifer conditions For example water samplesare commonly collected when a well has productionproblems or during the initial development of a well(Blondes et al 2016) Production problems may in-clude holes in well casings or annular cement that mayhave caused water from a different zone to enter theperforations In most cases few data exist regarding thesampling procedures and analysis techniques particu-larly for the older samples However older samples wereconsidered to be important for this study because theypredate the use of many EOR techniques such as waterflooding waste disposal and steam injection the large-scale use of which commenced in the early to mid-1960s These EOR processes may significantly alterthe salinity of the samples and cause the more recentsamples to be an inaccurate reflection of the originalaquifer conditions Samples taken at different timeintervals are helpful in determining temporal changes insalinity that may have occurred because of oil fieldactivities

One quality control method used in this study wasto perform a charge balance on the analyses Becausewater is electrically neutral the negative (anions) andpositive (cations) charged ions should sum to zero Thedifference between the total milliequivalents of thecations and anions is divided by the sum to give a chargebalance error We used a cutoff of ndash5 to determinewhich samples to discard in this analysis

For wells used for injection purposes the initiationof water injection relative to the date of sampling maybe important to consider Samples taken after the startof injection in the sampled well may not accuratelyrepresent the true formation water composition In mostcases it was not feasible to catalog the time of initiationof injection in nearby wells because of the large numberof injectionwells that could potentially affect a sampledwell Consequently the effect of subsurface injectionon sample TDS data remains a source of uncertainty inthe analysis

RESULTS

The methods and data discussed in the previous sec-tion were used to construct geologic maps showing thethickness and continuity of aquifers and confining unitsas well as to determine aquifer salinity and temporalchanges in salinity These results are discussed below

84 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 6 Graphs of shut-in pressure (from open-hole sequential formation testers) versus depth for three wells across the study area Red lines are used to emphasize the pressuregradient Tan areas showmajor clay layers the 22K sand is shown in yellow Base of Tulare and top of thewater table are also notedWells are identified by their API number and the yeardrilled and logged at the top of each graph

Gillespieetal

85

Geologic Mapping

The thickness of the Tulare Formation and overlyingalluvium in the study area (Figure 4) ranges from1200m (4000 ft) in the southeast part of the study areato 75 m (~250 ft) along the anticline crests in thenorthern parts of Lost Hills and North Belridge oilfields Because chemical analyses indicate that theprotected aquifers occur largely within the Tulare For-mation and overlying alluvium throughout the studyarea the map of the depth to the base of the TulareFormation defines the extent of potential protectedaquifers outside the oil field limits

The Amnicola Clay is thickest (67 m [~220 ft])along the northeastern and southeastern flank of theLost Hills anticline and east of the South Belridge an-ticline (Figure 5A) It thins to zero in the northwest partof the study area The clay is replaced by sandier sedi-ments east of the central part of the Lost Hills anticlineThe CCE thickens to 61m (~200 ft) in the northcentralpart of the study area in the Tulare lakebed but iscommonly less than30m(lt~100 ft) thick (Figure5B) Itis absent over the Lost Hills anticline crest and in thenorthwestern part of the study area These clay maps(Figure 5A B) are used to determine the presence ofpossible confining beds that in some cases may controlthe distribution of protected water resources and restrictthe migration of potential contaminants

Graphs of pressure versus depth collected in 1985from three wells along a northeast-trending transectfrom South Belridge to south of Lost Hills are shown inFigure 6 Marked changes in the hydrostatic pressuregradient in well API no 02974434 occur across theMiddle Tulare Clay and Amnicola Clay layers An addi-tional deeper clay here termed the D-E Clay forms alocal confining layer isolating a relatively low perme-ability section at the base of the Tulare Formation fromthe rest of the aquifer These characteristic deviationsin hydrostatic gradients indicates that the clays hydrau-lically separate individual aquifer layers in the area ofthis well

Farther northeast in wells API no 02974436 andAPI no 02974433 these deviations from hydrostaticgradient are much less noticeable especially across theAmnicola Clay This suggests that either (1) the claylayers are less confining south of Lost Hills or (2) thehydrologic regime near South Belridge has been alteredby production and injection into the Tulare Formationand the pressure responses in aquifer layers near the fieldin 1985 had not had sufficient time for their pressures

to reach equilibrium Recent drilling and multiple wellmonitoring site data (April 2018) off the eastern flank ofthe northern Lost Hills oil field indicates a 45 m (~15 ft)head difference across the Amnicola Clay with an upwardgradient of 0038 (measured May 31 2018) suggestingthat it is a confining layer in the northeastern part of thestudy area (US Geological Survey 2018a b)

Salinity Analysis

Log analysis from 30 wells in areas outside the oil fieldboundaries or within the oil fields but below the oilndashwater contact were used to create a map of the depthto the base of USDW The depth map to the base ofUSDW shows two trends (Figure 7) In the area be-tween Lost Hills and the Belridge oil fields depthto USDW increases toward the south from 150 m(~500 ft) between North Belridge and Lost Hills to490 m (~1600 ft) southeast of South Belridge In the

Figure 7 Map showing depth to base of underground sourcesof drinking water (USDW) (total dissolved solids = 10000 mgL)based on log analysis Red lines indicate faults (Young 1968California Division of Oil Gas and Geothermal Resources 1998)Black lines are oil field administrative boundaries Wells shown asblue dots are those used to determine the base of USDW Depthvalues in feet CI = contour interval No = North So = South

86 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

eastern part of the study area depth to base of USDWincreases sharply eastward from approximately 215 m(~700 ft) on the west flank of the Lost Hills anticline toas great as 550 m (~1800 ft) within 3 km (~2 mi)

Salinityndashdepth profiles based on chemical analysesof producedwater samples were compared to the depthto base USDW map estimated from borehole geo-physical log analysis However this comparison waslimited because most of the lab analyses come from oil-producing wells within the field boundaries rather thanin areas outside the fields The presence of oil in thesampled intervals within the fields precludes the useof log analysis to calculate the salinity in these samewells Plots of TDS versus depth of the samples in eachfield are shown in Figures 8 and 9 along with log-calculated TDS

Wood and Davis (1959) reported the groundwaterchemistry in this area as being an NaSO4 type howeveronly one Tulare Formation monitoring well Mackesseyno 1 (API no 02987363) was an NaSO4 type the restare NaCl type Because the calculated salinity fromArchie log analysis assumes NaCl water types the log-calculated and lab sample TDS values should be com-parable The two monitoring wells completed in thealluvium have higher SO4 concentrations relative tothe Tulare Formation wells however Cl is the domi-nant anion

Only four Tulare Formation lab samples (blue dots)in Lost Hills are in the California DOGGR databaseData from 14 monitoring wells in the central part ofthe Lost Hills field were used to enlarge the data set(Figure 8) With the exception of one sample TDSconcentrations in the Tulare Formation and alluviumare less than 10000 mgL indicating protected (USDW)aquifers These wells have top perforations ranging from40 to 290 m (133ndash950 ft) The well with a lab analysisindicating a salinity of 12295 mgL TDS in the TulareFormation had a top perforation at 300 m (984 ft) Thissuggests that the base of USDW lies somewhere be-tween 290 and 300 m (950 and 985 ft) within the fieldLog analysis suggests that the base may occur as deepas 580 m (~1900 ft) but many of the wells used for loganalysis come from areas east of the field boundarywhere the Tulare Formation is much thicker (200 ftwithin the field vs 2000 ft east of the field [Figure 4])Within the field interpolated contour lines from log datashow the depth to base USDWat 213m (~700 ft) alongthe western field boundary and 365 m (~1200 ft) alongthe eastern field boundary a trend that is consistentwith the more limited water sample data

Typical TDS concentrations in the underlyingEtchegoin Formation (red dots) range from 20000 to35000 mgL probably because of the presence oftrapped connate seawater within these marine depositsIn northern Lost Hills (Figure 8) two samples fromEtchegoin sands have TDS concentrations below 10000mgL However in this area an active steamflood isongoing within the Tulare and Etchegoin FormationsHere the Etchegoin Formation sands lie at shallowdepths (lt150 m [~500 ft]) immediately below pro-ducing intervals in the Tulare Formation The EtchegoinFormation samples in this area are much more dilute(5600ndash8000 mgL) than in other areas This may be be-cause the sands are in hydraulic communication with thefresher water sands of the overlying Tulare Formation orbecause the Etchegoin sands in this area are steamfloodedand the more saline formation water is diluted by theinjected steam Since the twoEtchegoinUSDWsamplesare post-steamflood initiation the latter explanation islikely

Only one chemical analysis sample from the TulareFormation is available in the North Belridge field (Fig-ure 9) The TDS concentration of the sample is 6055mgL from a depth interval of 157ndash211 m (515ndash692ft) It confirms that waters above approximately 211 m(~700 ft) are USDW consistent with the log-deriveddepth to USDW map which shows depth to USDWnear the well at 275 m (900 ft)

Many lab analyses are available for the Tulare For-mation at South Belridge (Figure 9) but they do notshow salinity values that are correlated with depth Thelog-generated depth to USDW map shows that salinitytrends along the northern part of the oil field are vari-able but the southern part of the field shows depth toUSDW increasing toward the southeast Thereforesamples from northern and southern South Belridgewere considered separately The boundary betweenthe two subareas is shown by the dashed line in Figure 9Because of the variability in sample TDS values withdepth comparisons of sample data to calculated salin-ities fromwell-log analyses can only bemade in a generalfashion

In the northern part of the South Belridge field nosamples below 365 m (1200 ft) are USDW suggestingthat the base of USDW lies above this depth The log-derived map shows depth to base of USDW at 335 m(1100 ft) in the north To the south the base of USDWincreases to 427 m (~1400 ft) Near the dividing linebetween the north and south parts of the field thedepth to base USDW decreases to 365 m (1200 ft)

Gillespie et al 87

Figure 8 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare and alluvium (blue) and Etchegoin (red) formations in and near the Lost Hills oilfield Gray dots represent TDS calculated from geophysical logs in the field The horizontal red linemarks 10000mgL and the vertical dashed red linemarks the approximate depth of thedeepest underground sources of drinking water (USDW) sample The adjacent map shows the location of the Tulare and alluviumwells sampled as blue dots and shallow (lt150m [500ft] deep) Etchegoin samples as red dots (deeper Etchegoin sample sites are not shown onmap) and the contours to base USDW from log analysis Lost Hills is the only area that containsbrackish water in the Etchegoin Formation However the brackish waters in the Etchegoin occur in an oil-producing zone that is steam flooded so it is considered to be an exempt aquiferand is not protected CI = contour interval

88Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

Figure 9 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare Formation in and near the North and South Belridge oil fields The northern andsouthern parts of the South Belridge field (marked by the red line on the map) are shown in separate graphs Blue dots show Tulare Formation (Fm) Measurements of TDS from labanalyses Gray dots show Tulare Fm The TDS calculated from log analysis Horizontal red lines on the graphs mark 10000 mgL and the dashed vertical red lines mark the approximatedepth of USDW from lab samples The adjacent map shows the location of the wells sampled (blue dots) and the contours to base underground sources of drinking water (USDW) (infeet) from log analysis Black lines are oil field administrative boundaries and red lines are faults CI = contour interval No = North So = South

Gillespieetal

89

Figure 10 Cross section showing three wells drilled and logged in different years near a produced water disposal pond (light blue polygon on inset map) Yellow shading onspontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivity curve (right track at scale of 0ndash20 ohmm) indicate resistivity of more than 3 ohmm Pinkbars indicate low resistivity sands affected by infiltration of saline produced water from the pond The API numbers at the top of the logs are used for well identification Year drilled andlogged in parentheses Depth scale in feet

90Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

This may be caused by the presence of an active waterdisposal field in the Tulare Formation near this areaOverall the log-derived map shows base of USDWapproximately 61m (~200 ft) deeper than lab analyseswould suggest As at Lost Hills this may be caused bythe geographic distribution of the two data sets the labsamples coming mainly from within the oil fieldboundaries whereas the log data are confined to thearea outside the oil field

In the southern part of South Belridge field a wellnear the southern tip of the field was sampled at mul-tiple intervals Samples as deep as 455 m (~1495 ft) areUSDW and the deepest sample from 480 m (1575 ft)has a TDS value of 13074 mgL suggesting that thebase of USDW in the southern part of South Belridge liesbetween 457 and 480 m (1495ndash1575 ft) (Figure 9) Thedepth to USDW map from log analysis in the southernpart of South Belridge shows that the base of USDW

ranges from 365 m (1200 ft) in the north to approxi-mately 487 m (~1600 ft) in the south

Effects of Oil Field Activities on Water Salinity

The effects of water disposal in surface ponds on ground-water salinity is evident by comparing geophysical logsfrom old wells near the pond locations to logs fromnewerwells drilled nearbyOlder geophysical logs (pre-1970s) indicate that the upper sands in the alluviumare desaturated (vadose zone) as indicated by areas ofcrossover on densityndashneutron logs and have high re-sistivity (gt20ohmmFigure 10)However geophysicallogs collected from wells drilled later in the area donot show densityndashneutron crossover characteristic of avadose zone andhave extremely low resistivity (lt1ohmm)in the upper alluvial sands because of the presence ofhigh salinityproducedwater thathasfilled thevadosezoneInvasion of low resistivity water is especially apparent

Figure 11 Two adjacent wells drilled and logged in different years (1990 and 2012) in an area containing numerous disposal wells on theeast flank of South Belridge Yellow shading on spontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivitycurve indicates resistivity greater than 3 ohm m Pink bars indicate low resistivity sands in the 2012 well (0403046566) affected by injectedsaline produced water The API numbers at the top of the logs are used for well identification Year drilled and logged in parentheses

Gillespie et al 91

where saline waters have infiltrated the 22K sandwhich lies directly above the CCE (well 03047481 inFigure 10) in the lower part of the alluvium The salinewater lies atop the CCE which appears to be pro-tecting the underlying Tulare aquifer from contami-nation Log-calculated andmeasuredTDS concentrationscan exceed 20000mgL in affected parts of the alluvium

The effects of surface disposal ponds are less evidentin Lost Hills and North Belridge compared to the areaadjacent to South Belridge Lost Hills in particular tends tohave highly saline water in the shallow parts of the aquifereven where disposal ponds are not present Swain andDuell (1989) noted TDS concentrations as high as 21700mgL from very shallowwells (6m [~20 ft]) along the eastflank of Lost Hills but did not note the cause of the highsalinity in the area

The effects of water disposal injection wells aremore subtle than that of the ponds and are especiallydifficult to identify in sands closer to the base of USDWwhere resistivity is naturally fairly low Geophysicallogs in sands affected by saline injection tend to have aconcave resistivity profile and resistivities are as small ashalf that in logs fromolder unaffectedwells (Figure 11)Injected saline water tends to follow individual sand

layers leaving some layers relatively unaffected In somecases only the lower part of an individual sand containssaline water whereas the waters in the upper part re-mainbrackishThis suggests that thewaters are stratifiedand mixing has not had time to occur Log-calculatedTDS concentrations show a relatively smooth increasewith depth inwells logged prior to the start of large-scaledisposal activities whereas recent logs in these areasshow a much more variable TDS concentration profilewith depth Additionally densityndashneutron curves inwells affected by disposal of produced waters show thinintervals (~1 m [~3 ft] thick) of crossover This may becaused by bacterial metabolism of trace amounts oforganic matter in the disposal water resulting in thegeneration of small amounts of biogenic methane(McMahon et al 2018)

The cross section in Figure 12 shows wells affectedby saline produced water disposal by injection (belowthe CCE) and percolation (above the CCE) along theeast flank of the South Belridge oil field Wells affectedby disposal of produced waters in saline ponds (n = 139)and by disposal in injection wells (n = 125) are alsoshown in the map in Figure 13 The effects of disposalponds are evident from changes in borehole resistivity

Figure 12 Cross section showing intervals affected by disposal of saline produced water (pink boxes) at South Belridge oil field Wellsidentified by API number at top of each well Year drilled and logged shown at bottom of eachwell Insetmap shows location of cross sectionProduced water disposal wells are highlighted in pink and disposal ponds are shown in blue in the inset map

92 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

point-source data from a single depth interval and do nottell us at what depth the salinity changes However inactive petroleum-producing basins such as the SJV open-hole geophysical logs are abundant Geophysical logsprovide continuous vertical records of the properties ofthe formation matrix fluids adjacent to the boreholeand fluids within the borehole Using log analysis it ispossible to calculate the depths at which salinity changesoccur These depthsmay be verified by chemical analyseswhere available Of particular interest for determiningsalinity are the electrical logs (SP and resistivity) becausethe ability of water to conduct an electrical current isstrongly influenced by both the temperature and salinity ofthe formation water

Use of electrical logs to determine water salinity ismost common in the petroleum literature In oil-bearingreservoirs it is important to know the resistivity of thewater which occurs with the oil to determine the oilsaturation using the Archie (1942) equation or one ofits many variants

Fewer studies have evaluated the use of electricallogs to determine salinity in fresh to brackish aquifersHowells (1990) andHowells et al (1987) used well-loginterpretation to determine the location of the base ofmoderately saline (lt10000 mgL TDS) water in UtahLindner-Lunsford and Bruce (1995) used the ArchieEquation to determine salinity in aquifers with TDS lessthan 1000 mgL in southwest Wyoming Schnoebelenet al (1995) used geophysical logs to map the depth to10000 mgL TDS water in carbonate aquifers in Indianausing the SP log mud filtrate resistivity and the resis-tivity porosity method of Archie They compared theresults to sampled water analyses and found the resis-tivity porosity method to be most accurate More re-cently Hamlin and de la Rocha (2015) used the ArchieEquation to map fresh (lt1000 mgL TDS) slightlysaline (1000ndash3000mgL TDS) moderately saline (3000ndash10000mgL TDS) and very saline (gt10000mgL TDS)water in the CarrizondashWilcox aquifer of south TexasGillespie et al (2017) used log analysis to determinedepth to 10000 mgL TDS on a regional basis through-out much of the southern San Joaquin Basin but thewestern margin fields were not considered in theirstudy because the presence of shallow oil reservoirs requiremore extensive mapping to locate wet sands for loganalysis

In this study a variation of the Archie Equationwas used to estimate salinity (as NaCl equivalent)from the geophysical logs Archie (1942) related thein situ electrical resistivity of a fully water saturated

sedimentary rock (Ro) to its porosity (f) nondimen-sional factors related to matrix properties (the cemen-tation factor [m]and the tortuosity factor [a]) and theresistivity of the formation water (Rw) as shown inequation 1

Rw = Ropethfm=aTHORN (1)

Ro is the resistivity of a clean (minimal clay content) oil-free wet sand and can be obtained from the deepresistivity curve Sands were chosen based on an evalu-ation of nearby core mud log and drillers log data in-dicating that the sands analyzed did not contain oil orgas The porosity value is obtained from porosity logssuch as density neutron sonic or nuclear magneticresonance logs and core analysis For this study sonicporositieswere not used the poorly consolidated natureof the aquifer sediments causes additional slowing ofthe sonic wave from compression of the material re-sulting in artificially high porosity values

Because of the poorly sorted nature of the sedimentsand the presence of large amounts of clay and silt in somezones the average porosity used for salinity calcula-tions weighted density measurements twice as much asneutron values Neutron values for porosity are basedupon the abundance of hydrogen atoms in a formationBecause clays contain hydroxide groups within theircrystal lattice the neutron curve tends to overestimateporosity when clays are present The largest separationbetween the density and neutron curves on each logwasconsidered to be a zone with 100 clay The separationof the densityndashneutron curves for each evaluated in-terval was compared to the 100 clay separation valueto estimate clay content The Rw values were not cal-culated for intervals containing more than 25 clay

The exponentm (unitless) is related to the degree ofcementation in clastic rocks It typically ranges from 13to 26 (Wylie and Rose 1950) The value a (unitless) iscalled the tortuosity factor and is related to the lengththe current must travel through the formation Thevalue for a typically ranges from 05 to 15 For thisstudy special core analyses from five intervals in theTulare Formation were available in the CaliforniaDOGGRonline files from awater disposal well south ofthe Lost Hills field (American Petroleum Institute[API] well number [no] 0402974433) The value fora was kept constant at a value of 1 and m was allowedto vary The measured values for m ranged from 15 to18 with a mean value of 17 (standard deviation of015) and a median value of 18 For this study salinity

Gillespie et al 83

calculations used a = 1 andm = 17 based on the specialcore analysis results

Upon obtaining a value for Rw and calculating thetemperature of the zone of interest from a linear in-terpolation between bottom borehole temperature andair temperatureof 75degF the formationwater salinity canbeestimated from empirical charts such as those provided bySchlumberger (1997) or from equations such as thosein Bateman and Konen (1977) relating fluid resistivity toNaCl equivalent TDS concentration

Geochemical AnalysesThe California Division of Oil Gas and GeothermalResources (2016) maintains a website containing scan-ned copies of chemical analyses of oil field waters Thesedata are useful in determining the salinity of the deeperaquifers not normally used for drinking water or irriga-tion Data taken from these reports included dates oftesting source of water for test (when available) majorion concentrations and TDS concentrations The compileddata are available from Gillespie et al (2019) Chemicalanalysis of groundwater samples compiled from varioussources were also available (Metzger et al 2018) How-ever these samples were generally available from rela-tively shallow depths and had TDS ranging fromapproximately 1000 to 3000 mgL and therefore did notdirectly contribute to efforts to map the depth to USDW

Scanned copies of well completion reports are alsoavailable for the wells with chemical analysis from theCalifornia Division of Oil Gas and Geothermal Re-sources (2018) Well Finder online search engine Thecompletion reports for the wells provide informationincluding perforated intervals date perforated geolog-ical formation sampled (in some cases) scanned copiesof borehole geophysical logs pressure tests and bottom-hole temperatures These records were used to de-termine the depth interval and formation representedby the produced water chemical analysis data (Gillespieet al 2019)

Produced water chemical analyses from approxi-mately 150 oil production injection and observationwells in the study area were available from DOGGRrsquosonline chemical analysis database (California Divisionof Oil Gas and Geothermal Resources 2016) Addi-tionally 12 samples from the Tulare Formation and al-luvium were available from groundwater monitoringwells in the Lost Hills oil field on the Geotracker website(California State Water Resources Control Board2018a) The vintage of samples in the database rangesfrom 1932 to 2017

The sample analyses may not always representnatural aquifer conditions For example water samplesare commonly collected when a well has productionproblems or during the initial development of a well(Blondes et al 2016) Production problems may in-clude holes in well casings or annular cement that mayhave caused water from a different zone to enter theperforations In most cases few data exist regarding thesampling procedures and analysis techniques particu-larly for the older samples However older samples wereconsidered to be important for this study because theypredate the use of many EOR techniques such as waterflooding waste disposal and steam injection the large-scale use of which commenced in the early to mid-1960s These EOR processes may significantly alterthe salinity of the samples and cause the more recentsamples to be an inaccurate reflection of the originalaquifer conditions Samples taken at different timeintervals are helpful in determining temporal changes insalinity that may have occurred because of oil fieldactivities

One quality control method used in this study wasto perform a charge balance on the analyses Becausewater is electrically neutral the negative (anions) andpositive (cations) charged ions should sum to zero Thedifference between the total milliequivalents of thecations and anions is divided by the sum to give a chargebalance error We used a cutoff of ndash5 to determinewhich samples to discard in this analysis

For wells used for injection purposes the initiationof water injection relative to the date of sampling maybe important to consider Samples taken after the startof injection in the sampled well may not accuratelyrepresent the true formation water composition In mostcases it was not feasible to catalog the time of initiationof injection in nearby wells because of the large numberof injectionwells that could potentially affect a sampledwell Consequently the effect of subsurface injectionon sample TDS data remains a source of uncertainty inthe analysis

RESULTS

The methods and data discussed in the previous sec-tion were used to construct geologic maps showing thethickness and continuity of aquifers and confining unitsas well as to determine aquifer salinity and temporalchanges in salinity These results are discussed below

84 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 6 Graphs of shut-in pressure (from open-hole sequential formation testers) versus depth for three wells across the study area Red lines are used to emphasize the pressuregradient Tan areas showmajor clay layers the 22K sand is shown in yellow Base of Tulare and top of thewater table are also notedWells are identified by their API number and the yeardrilled and logged at the top of each graph

Gillespieetal

85

Geologic Mapping

The thickness of the Tulare Formation and overlyingalluvium in the study area (Figure 4) ranges from1200m (4000 ft) in the southeast part of the study areato 75 m (~250 ft) along the anticline crests in thenorthern parts of Lost Hills and North Belridge oilfields Because chemical analyses indicate that theprotected aquifers occur largely within the Tulare For-mation and overlying alluvium throughout the studyarea the map of the depth to the base of the TulareFormation defines the extent of potential protectedaquifers outside the oil field limits

The Amnicola Clay is thickest (67 m [~220 ft])along the northeastern and southeastern flank of theLost Hills anticline and east of the South Belridge an-ticline (Figure 5A) It thins to zero in the northwest partof the study area The clay is replaced by sandier sedi-ments east of the central part of the Lost Hills anticlineThe CCE thickens to 61m (~200 ft) in the northcentralpart of the study area in the Tulare lakebed but iscommonly less than30m(lt~100 ft) thick (Figure5B) Itis absent over the Lost Hills anticline crest and in thenorthwestern part of the study area These clay maps(Figure 5A B) are used to determine the presence ofpossible confining beds that in some cases may controlthe distribution of protected water resources and restrictthe migration of potential contaminants

Graphs of pressure versus depth collected in 1985from three wells along a northeast-trending transectfrom South Belridge to south of Lost Hills are shown inFigure 6 Marked changes in the hydrostatic pressuregradient in well API no 02974434 occur across theMiddle Tulare Clay and Amnicola Clay layers An addi-tional deeper clay here termed the D-E Clay forms alocal confining layer isolating a relatively low perme-ability section at the base of the Tulare Formation fromthe rest of the aquifer These characteristic deviationsin hydrostatic gradients indicates that the clays hydrau-lically separate individual aquifer layers in the area ofthis well

Farther northeast in wells API no 02974436 andAPI no 02974433 these deviations from hydrostaticgradient are much less noticeable especially across theAmnicola Clay This suggests that either (1) the claylayers are less confining south of Lost Hills or (2) thehydrologic regime near South Belridge has been alteredby production and injection into the Tulare Formationand the pressure responses in aquifer layers near the fieldin 1985 had not had sufficient time for their pressures

to reach equilibrium Recent drilling and multiple wellmonitoring site data (April 2018) off the eastern flank ofthe northern Lost Hills oil field indicates a 45 m (~15 ft)head difference across the Amnicola Clay with an upwardgradient of 0038 (measured May 31 2018) suggestingthat it is a confining layer in the northeastern part of thestudy area (US Geological Survey 2018a b)

Salinity Analysis

Log analysis from 30 wells in areas outside the oil fieldboundaries or within the oil fields but below the oilndashwater contact were used to create a map of the depthto the base of USDW The depth map to the base ofUSDW shows two trends (Figure 7) In the area be-tween Lost Hills and the Belridge oil fields depthto USDW increases toward the south from 150 m(~500 ft) between North Belridge and Lost Hills to490 m (~1600 ft) southeast of South Belridge In the

Figure 7 Map showing depth to base of underground sourcesof drinking water (USDW) (total dissolved solids = 10000 mgL)based on log analysis Red lines indicate faults (Young 1968California Division of Oil Gas and Geothermal Resources 1998)Black lines are oil field administrative boundaries Wells shown asblue dots are those used to determine the base of USDW Depthvalues in feet CI = contour interval No = North So = South

86 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

eastern part of the study area depth to base of USDWincreases sharply eastward from approximately 215 m(~700 ft) on the west flank of the Lost Hills anticline toas great as 550 m (~1800 ft) within 3 km (~2 mi)

Salinityndashdepth profiles based on chemical analysesof producedwater samples were compared to the depthto base USDW map estimated from borehole geo-physical log analysis However this comparison waslimited because most of the lab analyses come from oil-producing wells within the field boundaries rather thanin areas outside the fields The presence of oil in thesampled intervals within the fields precludes the useof log analysis to calculate the salinity in these samewells Plots of TDS versus depth of the samples in eachfield are shown in Figures 8 and 9 along with log-calculated TDS

Wood and Davis (1959) reported the groundwaterchemistry in this area as being an NaSO4 type howeveronly one Tulare Formation monitoring well Mackesseyno 1 (API no 02987363) was an NaSO4 type the restare NaCl type Because the calculated salinity fromArchie log analysis assumes NaCl water types the log-calculated and lab sample TDS values should be com-parable The two monitoring wells completed in thealluvium have higher SO4 concentrations relative tothe Tulare Formation wells however Cl is the domi-nant anion

Only four Tulare Formation lab samples (blue dots)in Lost Hills are in the California DOGGR databaseData from 14 monitoring wells in the central part ofthe Lost Hills field were used to enlarge the data set(Figure 8) With the exception of one sample TDSconcentrations in the Tulare Formation and alluviumare less than 10000 mgL indicating protected (USDW)aquifers These wells have top perforations ranging from40 to 290 m (133ndash950 ft) The well with a lab analysisindicating a salinity of 12295 mgL TDS in the TulareFormation had a top perforation at 300 m (984 ft) Thissuggests that the base of USDW lies somewhere be-tween 290 and 300 m (950 and 985 ft) within the fieldLog analysis suggests that the base may occur as deepas 580 m (~1900 ft) but many of the wells used for loganalysis come from areas east of the field boundarywhere the Tulare Formation is much thicker (200 ftwithin the field vs 2000 ft east of the field [Figure 4])Within the field interpolated contour lines from log datashow the depth to base USDWat 213m (~700 ft) alongthe western field boundary and 365 m (~1200 ft) alongthe eastern field boundary a trend that is consistentwith the more limited water sample data

Typical TDS concentrations in the underlyingEtchegoin Formation (red dots) range from 20000 to35000 mgL probably because of the presence oftrapped connate seawater within these marine depositsIn northern Lost Hills (Figure 8) two samples fromEtchegoin sands have TDS concentrations below 10000mgL However in this area an active steamflood isongoing within the Tulare and Etchegoin FormationsHere the Etchegoin Formation sands lie at shallowdepths (lt150 m [~500 ft]) immediately below pro-ducing intervals in the Tulare Formation The EtchegoinFormation samples in this area are much more dilute(5600ndash8000 mgL) than in other areas This may be be-cause the sands are in hydraulic communication with thefresher water sands of the overlying Tulare Formation orbecause the Etchegoin sands in this area are steamfloodedand the more saline formation water is diluted by theinjected steam Since the twoEtchegoinUSDWsamplesare post-steamflood initiation the latter explanation islikely

Only one chemical analysis sample from the TulareFormation is available in the North Belridge field (Fig-ure 9) The TDS concentration of the sample is 6055mgL from a depth interval of 157ndash211 m (515ndash692ft) It confirms that waters above approximately 211 m(~700 ft) are USDW consistent with the log-deriveddepth to USDW map which shows depth to USDWnear the well at 275 m (900 ft)

Many lab analyses are available for the Tulare For-mation at South Belridge (Figure 9) but they do notshow salinity values that are correlated with depth Thelog-generated depth to USDW map shows that salinitytrends along the northern part of the oil field are vari-able but the southern part of the field shows depth toUSDW increasing toward the southeast Thereforesamples from northern and southern South Belridgewere considered separately The boundary betweenthe two subareas is shown by the dashed line in Figure 9Because of the variability in sample TDS values withdepth comparisons of sample data to calculated salin-ities fromwell-log analyses can only bemade in a generalfashion

In the northern part of the South Belridge field nosamples below 365 m (1200 ft) are USDW suggestingthat the base of USDW lies above this depth The log-derived map shows depth to base of USDW at 335 m(1100 ft) in the north To the south the base of USDWincreases to 427 m (~1400 ft) Near the dividing linebetween the north and south parts of the field thedepth to base USDW decreases to 365 m (1200 ft)

Gillespie et al 87

Figure 8 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare and alluvium (blue) and Etchegoin (red) formations in and near the Lost Hills oilfield Gray dots represent TDS calculated from geophysical logs in the field The horizontal red linemarks 10000mgL and the vertical dashed red linemarks the approximate depth of thedeepest underground sources of drinking water (USDW) sample The adjacent map shows the location of the Tulare and alluviumwells sampled as blue dots and shallow (lt150m [500ft] deep) Etchegoin samples as red dots (deeper Etchegoin sample sites are not shown onmap) and the contours to base USDW from log analysis Lost Hills is the only area that containsbrackish water in the Etchegoin Formation However the brackish waters in the Etchegoin occur in an oil-producing zone that is steam flooded so it is considered to be an exempt aquiferand is not protected CI = contour interval

88Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

Figure 9 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare Formation in and near the North and South Belridge oil fields The northern andsouthern parts of the South Belridge field (marked by the red line on the map) are shown in separate graphs Blue dots show Tulare Formation (Fm) Measurements of TDS from labanalyses Gray dots show Tulare Fm The TDS calculated from log analysis Horizontal red lines on the graphs mark 10000 mgL and the dashed vertical red lines mark the approximatedepth of USDW from lab samples The adjacent map shows the location of the wells sampled (blue dots) and the contours to base underground sources of drinking water (USDW) (infeet) from log analysis Black lines are oil field administrative boundaries and red lines are faults CI = contour interval No = North So = South

Gillespieetal

89

Figure 10 Cross section showing three wells drilled and logged in different years near a produced water disposal pond (light blue polygon on inset map) Yellow shading onspontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivity curve (right track at scale of 0ndash20 ohmm) indicate resistivity of more than 3 ohmm Pinkbars indicate low resistivity sands affected by infiltration of saline produced water from the pond The API numbers at the top of the logs are used for well identification Year drilled andlogged in parentheses Depth scale in feet

90Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

This may be caused by the presence of an active waterdisposal field in the Tulare Formation near this areaOverall the log-derived map shows base of USDWapproximately 61m (~200 ft) deeper than lab analyseswould suggest As at Lost Hills this may be caused bythe geographic distribution of the two data sets the labsamples coming mainly from within the oil fieldboundaries whereas the log data are confined to thearea outside the oil field

In the southern part of South Belridge field a wellnear the southern tip of the field was sampled at mul-tiple intervals Samples as deep as 455 m (~1495 ft) areUSDW and the deepest sample from 480 m (1575 ft)has a TDS value of 13074 mgL suggesting that thebase of USDW in the southern part of South Belridge liesbetween 457 and 480 m (1495ndash1575 ft) (Figure 9) Thedepth to USDW map from log analysis in the southernpart of South Belridge shows that the base of USDW

ranges from 365 m (1200 ft) in the north to approxi-mately 487 m (~1600 ft) in the south

Effects of Oil Field Activities on Water Salinity

The effects of water disposal in surface ponds on ground-water salinity is evident by comparing geophysical logsfrom old wells near the pond locations to logs fromnewerwells drilled nearbyOlder geophysical logs (pre-1970s) indicate that the upper sands in the alluviumare desaturated (vadose zone) as indicated by areas ofcrossover on densityndashneutron logs and have high re-sistivity (gt20ohmmFigure 10)However geophysicallogs collected from wells drilled later in the area donot show densityndashneutron crossover characteristic of avadose zone andhave extremely low resistivity (lt1ohmm)in the upper alluvial sands because of the presence ofhigh salinityproducedwater thathasfilled thevadosezoneInvasion of low resistivity water is especially apparent

Figure 11 Two adjacent wells drilled and logged in different years (1990 and 2012) in an area containing numerous disposal wells on theeast flank of South Belridge Yellow shading on spontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivitycurve indicates resistivity greater than 3 ohm m Pink bars indicate low resistivity sands in the 2012 well (0403046566) affected by injectedsaline produced water The API numbers at the top of the logs are used for well identification Year drilled and logged in parentheses

Gillespie et al 91

where saline waters have infiltrated the 22K sandwhich lies directly above the CCE (well 03047481 inFigure 10) in the lower part of the alluvium The salinewater lies atop the CCE which appears to be pro-tecting the underlying Tulare aquifer from contami-nation Log-calculated andmeasuredTDS concentrationscan exceed 20000mgL in affected parts of the alluvium

The effects of surface disposal ponds are less evidentin Lost Hills and North Belridge compared to the areaadjacent to South Belridge Lost Hills in particular tends tohave highly saline water in the shallow parts of the aquifereven where disposal ponds are not present Swain andDuell (1989) noted TDS concentrations as high as 21700mgL from very shallowwells (6m [~20 ft]) along the eastflank of Lost Hills but did not note the cause of the highsalinity in the area

The effects of water disposal injection wells aremore subtle than that of the ponds and are especiallydifficult to identify in sands closer to the base of USDWwhere resistivity is naturally fairly low Geophysicallogs in sands affected by saline injection tend to have aconcave resistivity profile and resistivities are as small ashalf that in logs fromolder unaffectedwells (Figure 11)Injected saline water tends to follow individual sand

layers leaving some layers relatively unaffected In somecases only the lower part of an individual sand containssaline water whereas the waters in the upper part re-mainbrackishThis suggests that thewaters are stratifiedand mixing has not had time to occur Log-calculatedTDS concentrations show a relatively smooth increasewith depth inwells logged prior to the start of large-scaledisposal activities whereas recent logs in these areasshow a much more variable TDS concentration profilewith depth Additionally densityndashneutron curves inwells affected by disposal of produced waters show thinintervals (~1 m [~3 ft] thick) of crossover This may becaused by bacterial metabolism of trace amounts oforganic matter in the disposal water resulting in thegeneration of small amounts of biogenic methane(McMahon et al 2018)

The cross section in Figure 12 shows wells affectedby saline produced water disposal by injection (belowthe CCE) and percolation (above the CCE) along theeast flank of the South Belridge oil field Wells affectedby disposal of produced waters in saline ponds (n = 139)and by disposal in injection wells (n = 125) are alsoshown in the map in Figure 13 The effects of disposalponds are evident from changes in borehole resistivity

Figure 12 Cross section showing intervals affected by disposal of saline produced water (pink boxes) at South Belridge oil field Wellsidentified by API number at top of each well Year drilled and logged shown at bottom of eachwell Insetmap shows location of cross sectionProduced water disposal wells are highlighted in pink and disposal ponds are shown in blue in the inset map

92 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

calculations used a = 1 andm = 17 based on the specialcore analysis results

Upon obtaining a value for Rw and calculating thetemperature of the zone of interest from a linear in-terpolation between bottom borehole temperature andair temperatureof 75degF the formationwater salinity canbeestimated from empirical charts such as those provided bySchlumberger (1997) or from equations such as thosein Bateman and Konen (1977) relating fluid resistivity toNaCl equivalent TDS concentration

Geochemical AnalysesThe California Division of Oil Gas and GeothermalResources (2016) maintains a website containing scan-ned copies of chemical analyses of oil field waters Thesedata are useful in determining the salinity of the deeperaquifers not normally used for drinking water or irriga-tion Data taken from these reports included dates oftesting source of water for test (when available) majorion concentrations and TDS concentrations The compileddata are available from Gillespie et al (2019) Chemicalanalysis of groundwater samples compiled from varioussources were also available (Metzger et al 2018) How-ever these samples were generally available from rela-tively shallow depths and had TDS ranging fromapproximately 1000 to 3000 mgL and therefore did notdirectly contribute to efforts to map the depth to USDW

Scanned copies of well completion reports are alsoavailable for the wells with chemical analysis from theCalifornia Division of Oil Gas and Geothermal Re-sources (2018) Well Finder online search engine Thecompletion reports for the wells provide informationincluding perforated intervals date perforated geolog-ical formation sampled (in some cases) scanned copiesof borehole geophysical logs pressure tests and bottom-hole temperatures These records were used to de-termine the depth interval and formation representedby the produced water chemical analysis data (Gillespieet al 2019)

Produced water chemical analyses from approxi-mately 150 oil production injection and observationwells in the study area were available from DOGGRrsquosonline chemical analysis database (California Divisionof Oil Gas and Geothermal Resources 2016) Addi-tionally 12 samples from the Tulare Formation and al-luvium were available from groundwater monitoringwells in the Lost Hills oil field on the Geotracker website(California State Water Resources Control Board2018a) The vintage of samples in the database rangesfrom 1932 to 2017

The sample analyses may not always representnatural aquifer conditions For example water samplesare commonly collected when a well has productionproblems or during the initial development of a well(Blondes et al 2016) Production problems may in-clude holes in well casings or annular cement that mayhave caused water from a different zone to enter theperforations In most cases few data exist regarding thesampling procedures and analysis techniques particu-larly for the older samples However older samples wereconsidered to be important for this study because theypredate the use of many EOR techniques such as waterflooding waste disposal and steam injection the large-scale use of which commenced in the early to mid-1960s These EOR processes may significantly alterthe salinity of the samples and cause the more recentsamples to be an inaccurate reflection of the originalaquifer conditions Samples taken at different timeintervals are helpful in determining temporal changes insalinity that may have occurred because of oil fieldactivities

One quality control method used in this study wasto perform a charge balance on the analyses Becausewater is electrically neutral the negative (anions) andpositive (cations) charged ions should sum to zero Thedifference between the total milliequivalents of thecations and anions is divided by the sum to give a chargebalance error We used a cutoff of ndash5 to determinewhich samples to discard in this analysis

For wells used for injection purposes the initiationof water injection relative to the date of sampling maybe important to consider Samples taken after the startof injection in the sampled well may not accuratelyrepresent the true formation water composition In mostcases it was not feasible to catalog the time of initiationof injection in nearby wells because of the large numberof injectionwells that could potentially affect a sampledwell Consequently the effect of subsurface injectionon sample TDS data remains a source of uncertainty inthe analysis

RESULTS

The methods and data discussed in the previous sec-tion were used to construct geologic maps showing thethickness and continuity of aquifers and confining unitsas well as to determine aquifer salinity and temporalchanges in salinity These results are discussed below

84 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 6 Graphs of shut-in pressure (from open-hole sequential formation testers) versus depth for three wells across the study area Red lines are used to emphasize the pressuregradient Tan areas showmajor clay layers the 22K sand is shown in yellow Base of Tulare and top of thewater table are also notedWells are identified by their API number and the yeardrilled and logged at the top of each graph

Gillespieetal

85

Geologic Mapping

The thickness of the Tulare Formation and overlyingalluvium in the study area (Figure 4) ranges from1200m (4000 ft) in the southeast part of the study areato 75 m (~250 ft) along the anticline crests in thenorthern parts of Lost Hills and North Belridge oilfields Because chemical analyses indicate that theprotected aquifers occur largely within the Tulare For-mation and overlying alluvium throughout the studyarea the map of the depth to the base of the TulareFormation defines the extent of potential protectedaquifers outside the oil field limits

The Amnicola Clay is thickest (67 m [~220 ft])along the northeastern and southeastern flank of theLost Hills anticline and east of the South Belridge an-ticline (Figure 5A) It thins to zero in the northwest partof the study area The clay is replaced by sandier sedi-ments east of the central part of the Lost Hills anticlineThe CCE thickens to 61m (~200 ft) in the northcentralpart of the study area in the Tulare lakebed but iscommonly less than30m(lt~100 ft) thick (Figure5B) Itis absent over the Lost Hills anticline crest and in thenorthwestern part of the study area These clay maps(Figure 5A B) are used to determine the presence ofpossible confining beds that in some cases may controlthe distribution of protected water resources and restrictthe migration of potential contaminants

Graphs of pressure versus depth collected in 1985from three wells along a northeast-trending transectfrom South Belridge to south of Lost Hills are shown inFigure 6 Marked changes in the hydrostatic pressuregradient in well API no 02974434 occur across theMiddle Tulare Clay and Amnicola Clay layers An addi-tional deeper clay here termed the D-E Clay forms alocal confining layer isolating a relatively low perme-ability section at the base of the Tulare Formation fromthe rest of the aquifer These characteristic deviationsin hydrostatic gradients indicates that the clays hydrau-lically separate individual aquifer layers in the area ofthis well

Farther northeast in wells API no 02974436 andAPI no 02974433 these deviations from hydrostaticgradient are much less noticeable especially across theAmnicola Clay This suggests that either (1) the claylayers are less confining south of Lost Hills or (2) thehydrologic regime near South Belridge has been alteredby production and injection into the Tulare Formationand the pressure responses in aquifer layers near the fieldin 1985 had not had sufficient time for their pressures

to reach equilibrium Recent drilling and multiple wellmonitoring site data (April 2018) off the eastern flank ofthe northern Lost Hills oil field indicates a 45 m (~15 ft)head difference across the Amnicola Clay with an upwardgradient of 0038 (measured May 31 2018) suggestingthat it is a confining layer in the northeastern part of thestudy area (US Geological Survey 2018a b)

Salinity Analysis

Log analysis from 30 wells in areas outside the oil fieldboundaries or within the oil fields but below the oilndashwater contact were used to create a map of the depthto the base of USDW The depth map to the base ofUSDW shows two trends (Figure 7) In the area be-tween Lost Hills and the Belridge oil fields depthto USDW increases toward the south from 150 m(~500 ft) between North Belridge and Lost Hills to490 m (~1600 ft) southeast of South Belridge In the

Figure 7 Map showing depth to base of underground sourcesof drinking water (USDW) (total dissolved solids = 10000 mgL)based on log analysis Red lines indicate faults (Young 1968California Division of Oil Gas and Geothermal Resources 1998)Black lines are oil field administrative boundaries Wells shown asblue dots are those used to determine the base of USDW Depthvalues in feet CI = contour interval No = North So = South

86 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

eastern part of the study area depth to base of USDWincreases sharply eastward from approximately 215 m(~700 ft) on the west flank of the Lost Hills anticline toas great as 550 m (~1800 ft) within 3 km (~2 mi)

Salinityndashdepth profiles based on chemical analysesof producedwater samples were compared to the depthto base USDW map estimated from borehole geo-physical log analysis However this comparison waslimited because most of the lab analyses come from oil-producing wells within the field boundaries rather thanin areas outside the fields The presence of oil in thesampled intervals within the fields precludes the useof log analysis to calculate the salinity in these samewells Plots of TDS versus depth of the samples in eachfield are shown in Figures 8 and 9 along with log-calculated TDS

Wood and Davis (1959) reported the groundwaterchemistry in this area as being an NaSO4 type howeveronly one Tulare Formation monitoring well Mackesseyno 1 (API no 02987363) was an NaSO4 type the restare NaCl type Because the calculated salinity fromArchie log analysis assumes NaCl water types the log-calculated and lab sample TDS values should be com-parable The two monitoring wells completed in thealluvium have higher SO4 concentrations relative tothe Tulare Formation wells however Cl is the domi-nant anion

Only four Tulare Formation lab samples (blue dots)in Lost Hills are in the California DOGGR databaseData from 14 monitoring wells in the central part ofthe Lost Hills field were used to enlarge the data set(Figure 8) With the exception of one sample TDSconcentrations in the Tulare Formation and alluviumare less than 10000 mgL indicating protected (USDW)aquifers These wells have top perforations ranging from40 to 290 m (133ndash950 ft) The well with a lab analysisindicating a salinity of 12295 mgL TDS in the TulareFormation had a top perforation at 300 m (984 ft) Thissuggests that the base of USDW lies somewhere be-tween 290 and 300 m (950 and 985 ft) within the fieldLog analysis suggests that the base may occur as deepas 580 m (~1900 ft) but many of the wells used for loganalysis come from areas east of the field boundarywhere the Tulare Formation is much thicker (200 ftwithin the field vs 2000 ft east of the field [Figure 4])Within the field interpolated contour lines from log datashow the depth to base USDWat 213m (~700 ft) alongthe western field boundary and 365 m (~1200 ft) alongthe eastern field boundary a trend that is consistentwith the more limited water sample data

Typical TDS concentrations in the underlyingEtchegoin Formation (red dots) range from 20000 to35000 mgL probably because of the presence oftrapped connate seawater within these marine depositsIn northern Lost Hills (Figure 8) two samples fromEtchegoin sands have TDS concentrations below 10000mgL However in this area an active steamflood isongoing within the Tulare and Etchegoin FormationsHere the Etchegoin Formation sands lie at shallowdepths (lt150 m [~500 ft]) immediately below pro-ducing intervals in the Tulare Formation The EtchegoinFormation samples in this area are much more dilute(5600ndash8000 mgL) than in other areas This may be be-cause the sands are in hydraulic communication with thefresher water sands of the overlying Tulare Formation orbecause the Etchegoin sands in this area are steamfloodedand the more saline formation water is diluted by theinjected steam Since the twoEtchegoinUSDWsamplesare post-steamflood initiation the latter explanation islikely

Only one chemical analysis sample from the TulareFormation is available in the North Belridge field (Fig-ure 9) The TDS concentration of the sample is 6055mgL from a depth interval of 157ndash211 m (515ndash692ft) It confirms that waters above approximately 211 m(~700 ft) are USDW consistent with the log-deriveddepth to USDW map which shows depth to USDWnear the well at 275 m (900 ft)

Many lab analyses are available for the Tulare For-mation at South Belridge (Figure 9) but they do notshow salinity values that are correlated with depth Thelog-generated depth to USDW map shows that salinitytrends along the northern part of the oil field are vari-able but the southern part of the field shows depth toUSDW increasing toward the southeast Thereforesamples from northern and southern South Belridgewere considered separately The boundary betweenthe two subareas is shown by the dashed line in Figure 9Because of the variability in sample TDS values withdepth comparisons of sample data to calculated salin-ities fromwell-log analyses can only bemade in a generalfashion

In the northern part of the South Belridge field nosamples below 365 m (1200 ft) are USDW suggestingthat the base of USDW lies above this depth The log-derived map shows depth to base of USDW at 335 m(1100 ft) in the north To the south the base of USDWincreases to 427 m (~1400 ft) Near the dividing linebetween the north and south parts of the field thedepth to base USDW decreases to 365 m (1200 ft)

Gillespie et al 87

Figure 8 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare and alluvium (blue) and Etchegoin (red) formations in and near the Lost Hills oilfield Gray dots represent TDS calculated from geophysical logs in the field The horizontal red linemarks 10000mgL and the vertical dashed red linemarks the approximate depth of thedeepest underground sources of drinking water (USDW) sample The adjacent map shows the location of the Tulare and alluviumwells sampled as blue dots and shallow (lt150m [500ft] deep) Etchegoin samples as red dots (deeper Etchegoin sample sites are not shown onmap) and the contours to base USDW from log analysis Lost Hills is the only area that containsbrackish water in the Etchegoin Formation However the brackish waters in the Etchegoin occur in an oil-producing zone that is steam flooded so it is considered to be an exempt aquiferand is not protected CI = contour interval

88Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

Figure 9 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare Formation in and near the North and South Belridge oil fields The northern andsouthern parts of the South Belridge field (marked by the red line on the map) are shown in separate graphs Blue dots show Tulare Formation (Fm) Measurements of TDS from labanalyses Gray dots show Tulare Fm The TDS calculated from log analysis Horizontal red lines on the graphs mark 10000 mgL and the dashed vertical red lines mark the approximatedepth of USDW from lab samples The adjacent map shows the location of the wells sampled (blue dots) and the contours to base underground sources of drinking water (USDW) (infeet) from log analysis Black lines are oil field administrative boundaries and red lines are faults CI = contour interval No = North So = South

Gillespieetal

89

Figure 10 Cross section showing three wells drilled and logged in different years near a produced water disposal pond (light blue polygon on inset map) Yellow shading onspontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivity curve (right track at scale of 0ndash20 ohmm) indicate resistivity of more than 3 ohmm Pinkbars indicate low resistivity sands affected by infiltration of saline produced water from the pond The API numbers at the top of the logs are used for well identification Year drilled andlogged in parentheses Depth scale in feet

90Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

This may be caused by the presence of an active waterdisposal field in the Tulare Formation near this areaOverall the log-derived map shows base of USDWapproximately 61m (~200 ft) deeper than lab analyseswould suggest As at Lost Hills this may be caused bythe geographic distribution of the two data sets the labsamples coming mainly from within the oil fieldboundaries whereas the log data are confined to thearea outside the oil field

In the southern part of South Belridge field a wellnear the southern tip of the field was sampled at mul-tiple intervals Samples as deep as 455 m (~1495 ft) areUSDW and the deepest sample from 480 m (1575 ft)has a TDS value of 13074 mgL suggesting that thebase of USDW in the southern part of South Belridge liesbetween 457 and 480 m (1495ndash1575 ft) (Figure 9) Thedepth to USDW map from log analysis in the southernpart of South Belridge shows that the base of USDW

ranges from 365 m (1200 ft) in the north to approxi-mately 487 m (~1600 ft) in the south

Effects of Oil Field Activities on Water Salinity

The effects of water disposal in surface ponds on ground-water salinity is evident by comparing geophysical logsfrom old wells near the pond locations to logs fromnewerwells drilled nearbyOlder geophysical logs (pre-1970s) indicate that the upper sands in the alluviumare desaturated (vadose zone) as indicated by areas ofcrossover on densityndashneutron logs and have high re-sistivity (gt20ohmmFigure 10)However geophysicallogs collected from wells drilled later in the area donot show densityndashneutron crossover characteristic of avadose zone andhave extremely low resistivity (lt1ohmm)in the upper alluvial sands because of the presence ofhigh salinityproducedwater thathasfilled thevadosezoneInvasion of low resistivity water is especially apparent

Figure 11 Two adjacent wells drilled and logged in different years (1990 and 2012) in an area containing numerous disposal wells on theeast flank of South Belridge Yellow shading on spontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivitycurve indicates resistivity greater than 3 ohm m Pink bars indicate low resistivity sands in the 2012 well (0403046566) affected by injectedsaline produced water The API numbers at the top of the logs are used for well identification Year drilled and logged in parentheses

Gillespie et al 91

where saline waters have infiltrated the 22K sandwhich lies directly above the CCE (well 03047481 inFigure 10) in the lower part of the alluvium The salinewater lies atop the CCE which appears to be pro-tecting the underlying Tulare aquifer from contami-nation Log-calculated andmeasuredTDS concentrationscan exceed 20000mgL in affected parts of the alluvium

The effects of surface disposal ponds are less evidentin Lost Hills and North Belridge compared to the areaadjacent to South Belridge Lost Hills in particular tends tohave highly saline water in the shallow parts of the aquifereven where disposal ponds are not present Swain andDuell (1989) noted TDS concentrations as high as 21700mgL from very shallowwells (6m [~20 ft]) along the eastflank of Lost Hills but did not note the cause of the highsalinity in the area

The effects of water disposal injection wells aremore subtle than that of the ponds and are especiallydifficult to identify in sands closer to the base of USDWwhere resistivity is naturally fairly low Geophysicallogs in sands affected by saline injection tend to have aconcave resistivity profile and resistivities are as small ashalf that in logs fromolder unaffectedwells (Figure 11)Injected saline water tends to follow individual sand

layers leaving some layers relatively unaffected In somecases only the lower part of an individual sand containssaline water whereas the waters in the upper part re-mainbrackishThis suggests that thewaters are stratifiedand mixing has not had time to occur Log-calculatedTDS concentrations show a relatively smooth increasewith depth inwells logged prior to the start of large-scaledisposal activities whereas recent logs in these areasshow a much more variable TDS concentration profilewith depth Additionally densityndashneutron curves inwells affected by disposal of produced waters show thinintervals (~1 m [~3 ft] thick) of crossover This may becaused by bacterial metabolism of trace amounts oforganic matter in the disposal water resulting in thegeneration of small amounts of biogenic methane(McMahon et al 2018)

The cross section in Figure 12 shows wells affectedby saline produced water disposal by injection (belowthe CCE) and percolation (above the CCE) along theeast flank of the South Belridge oil field Wells affectedby disposal of produced waters in saline ponds (n = 139)and by disposal in injection wells (n = 125) are alsoshown in the map in Figure 13 The effects of disposalponds are evident from changes in borehole resistivity

Figure 12 Cross section showing intervals affected by disposal of saline produced water (pink boxes) at South Belridge oil field Wellsidentified by API number at top of each well Year drilled and logged shown at bottom of eachwell Insetmap shows location of cross sectionProduced water disposal wells are highlighted in pink and disposal ponds are shown in blue in the inset map

92 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 6 Graphs of shut-in pressure (from open-hole sequential formation testers) versus depth for three wells across the study area Red lines are used to emphasize the pressuregradient Tan areas showmajor clay layers the 22K sand is shown in yellow Base of Tulare and top of thewater table are also notedWells are identified by their API number and the yeardrilled and logged at the top of each graph

Gillespieetal

85

Geologic Mapping

The thickness of the Tulare Formation and overlyingalluvium in the study area (Figure 4) ranges from1200m (4000 ft) in the southeast part of the study areato 75 m (~250 ft) along the anticline crests in thenorthern parts of Lost Hills and North Belridge oilfields Because chemical analyses indicate that theprotected aquifers occur largely within the Tulare For-mation and overlying alluvium throughout the studyarea the map of the depth to the base of the TulareFormation defines the extent of potential protectedaquifers outside the oil field limits

The Amnicola Clay is thickest (67 m [~220 ft])along the northeastern and southeastern flank of theLost Hills anticline and east of the South Belridge an-ticline (Figure 5A) It thins to zero in the northwest partof the study area The clay is replaced by sandier sedi-ments east of the central part of the Lost Hills anticlineThe CCE thickens to 61m (~200 ft) in the northcentralpart of the study area in the Tulare lakebed but iscommonly less than30m(lt~100 ft) thick (Figure5B) Itis absent over the Lost Hills anticline crest and in thenorthwestern part of the study area These clay maps(Figure 5A B) are used to determine the presence ofpossible confining beds that in some cases may controlthe distribution of protected water resources and restrictthe migration of potential contaminants

Graphs of pressure versus depth collected in 1985from three wells along a northeast-trending transectfrom South Belridge to south of Lost Hills are shown inFigure 6 Marked changes in the hydrostatic pressuregradient in well API no 02974434 occur across theMiddle Tulare Clay and Amnicola Clay layers An addi-tional deeper clay here termed the D-E Clay forms alocal confining layer isolating a relatively low perme-ability section at the base of the Tulare Formation fromthe rest of the aquifer These characteristic deviationsin hydrostatic gradients indicates that the clays hydrau-lically separate individual aquifer layers in the area ofthis well

Farther northeast in wells API no 02974436 andAPI no 02974433 these deviations from hydrostaticgradient are much less noticeable especially across theAmnicola Clay This suggests that either (1) the claylayers are less confining south of Lost Hills or (2) thehydrologic regime near South Belridge has been alteredby production and injection into the Tulare Formationand the pressure responses in aquifer layers near the fieldin 1985 had not had sufficient time for their pressures

to reach equilibrium Recent drilling and multiple wellmonitoring site data (April 2018) off the eastern flank ofthe northern Lost Hills oil field indicates a 45 m (~15 ft)head difference across the Amnicola Clay with an upwardgradient of 0038 (measured May 31 2018) suggestingthat it is a confining layer in the northeastern part of thestudy area (US Geological Survey 2018a b)

Salinity Analysis

Log analysis from 30 wells in areas outside the oil fieldboundaries or within the oil fields but below the oilndashwater contact were used to create a map of the depthto the base of USDW The depth map to the base ofUSDW shows two trends (Figure 7) In the area be-tween Lost Hills and the Belridge oil fields depthto USDW increases toward the south from 150 m(~500 ft) between North Belridge and Lost Hills to490 m (~1600 ft) southeast of South Belridge In the

Figure 7 Map showing depth to base of underground sourcesof drinking water (USDW) (total dissolved solids = 10000 mgL)based on log analysis Red lines indicate faults (Young 1968California Division of Oil Gas and Geothermal Resources 1998)Black lines are oil field administrative boundaries Wells shown asblue dots are those used to determine the base of USDW Depthvalues in feet CI = contour interval No = North So = South

86 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

eastern part of the study area depth to base of USDWincreases sharply eastward from approximately 215 m(~700 ft) on the west flank of the Lost Hills anticline toas great as 550 m (~1800 ft) within 3 km (~2 mi)

Salinityndashdepth profiles based on chemical analysesof producedwater samples were compared to the depthto base USDW map estimated from borehole geo-physical log analysis However this comparison waslimited because most of the lab analyses come from oil-producing wells within the field boundaries rather thanin areas outside the fields The presence of oil in thesampled intervals within the fields precludes the useof log analysis to calculate the salinity in these samewells Plots of TDS versus depth of the samples in eachfield are shown in Figures 8 and 9 along with log-calculated TDS

Wood and Davis (1959) reported the groundwaterchemistry in this area as being an NaSO4 type howeveronly one Tulare Formation monitoring well Mackesseyno 1 (API no 02987363) was an NaSO4 type the restare NaCl type Because the calculated salinity fromArchie log analysis assumes NaCl water types the log-calculated and lab sample TDS values should be com-parable The two monitoring wells completed in thealluvium have higher SO4 concentrations relative tothe Tulare Formation wells however Cl is the domi-nant anion

Only four Tulare Formation lab samples (blue dots)in Lost Hills are in the California DOGGR databaseData from 14 monitoring wells in the central part ofthe Lost Hills field were used to enlarge the data set(Figure 8) With the exception of one sample TDSconcentrations in the Tulare Formation and alluviumare less than 10000 mgL indicating protected (USDW)aquifers These wells have top perforations ranging from40 to 290 m (133ndash950 ft) The well with a lab analysisindicating a salinity of 12295 mgL TDS in the TulareFormation had a top perforation at 300 m (984 ft) Thissuggests that the base of USDW lies somewhere be-tween 290 and 300 m (950 and 985 ft) within the fieldLog analysis suggests that the base may occur as deepas 580 m (~1900 ft) but many of the wells used for loganalysis come from areas east of the field boundarywhere the Tulare Formation is much thicker (200 ftwithin the field vs 2000 ft east of the field [Figure 4])Within the field interpolated contour lines from log datashow the depth to base USDWat 213m (~700 ft) alongthe western field boundary and 365 m (~1200 ft) alongthe eastern field boundary a trend that is consistentwith the more limited water sample data

Typical TDS concentrations in the underlyingEtchegoin Formation (red dots) range from 20000 to35000 mgL probably because of the presence oftrapped connate seawater within these marine depositsIn northern Lost Hills (Figure 8) two samples fromEtchegoin sands have TDS concentrations below 10000mgL However in this area an active steamflood isongoing within the Tulare and Etchegoin FormationsHere the Etchegoin Formation sands lie at shallowdepths (lt150 m [~500 ft]) immediately below pro-ducing intervals in the Tulare Formation The EtchegoinFormation samples in this area are much more dilute(5600ndash8000 mgL) than in other areas This may be be-cause the sands are in hydraulic communication with thefresher water sands of the overlying Tulare Formation orbecause the Etchegoin sands in this area are steamfloodedand the more saline formation water is diluted by theinjected steam Since the twoEtchegoinUSDWsamplesare post-steamflood initiation the latter explanation islikely

Only one chemical analysis sample from the TulareFormation is available in the North Belridge field (Fig-ure 9) The TDS concentration of the sample is 6055mgL from a depth interval of 157ndash211 m (515ndash692ft) It confirms that waters above approximately 211 m(~700 ft) are USDW consistent with the log-deriveddepth to USDW map which shows depth to USDWnear the well at 275 m (900 ft)

Many lab analyses are available for the Tulare For-mation at South Belridge (Figure 9) but they do notshow salinity values that are correlated with depth Thelog-generated depth to USDW map shows that salinitytrends along the northern part of the oil field are vari-able but the southern part of the field shows depth toUSDW increasing toward the southeast Thereforesamples from northern and southern South Belridgewere considered separately The boundary betweenthe two subareas is shown by the dashed line in Figure 9Because of the variability in sample TDS values withdepth comparisons of sample data to calculated salin-ities fromwell-log analyses can only bemade in a generalfashion

In the northern part of the South Belridge field nosamples below 365 m (1200 ft) are USDW suggestingthat the base of USDW lies above this depth The log-derived map shows depth to base of USDW at 335 m(1100 ft) in the north To the south the base of USDWincreases to 427 m (~1400 ft) Near the dividing linebetween the north and south parts of the field thedepth to base USDW decreases to 365 m (1200 ft)

Gillespie et al 87

Figure 8 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare and alluvium (blue) and Etchegoin (red) formations in and near the Lost Hills oilfield Gray dots represent TDS calculated from geophysical logs in the field The horizontal red linemarks 10000mgL and the vertical dashed red linemarks the approximate depth of thedeepest underground sources of drinking water (USDW) sample The adjacent map shows the location of the Tulare and alluviumwells sampled as blue dots and shallow (lt150m [500ft] deep) Etchegoin samples as red dots (deeper Etchegoin sample sites are not shown onmap) and the contours to base USDW from log analysis Lost Hills is the only area that containsbrackish water in the Etchegoin Formation However the brackish waters in the Etchegoin occur in an oil-producing zone that is steam flooded so it is considered to be an exempt aquiferand is not protected CI = contour interval

88Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

Figure 9 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare Formation in and near the North and South Belridge oil fields The northern andsouthern parts of the South Belridge field (marked by the red line on the map) are shown in separate graphs Blue dots show Tulare Formation (Fm) Measurements of TDS from labanalyses Gray dots show Tulare Fm The TDS calculated from log analysis Horizontal red lines on the graphs mark 10000 mgL and the dashed vertical red lines mark the approximatedepth of USDW from lab samples The adjacent map shows the location of the wells sampled (blue dots) and the contours to base underground sources of drinking water (USDW) (infeet) from log analysis Black lines are oil field administrative boundaries and red lines are faults CI = contour interval No = North So = South

Gillespieetal

89

Figure 10 Cross section showing three wells drilled and logged in different years near a produced water disposal pond (light blue polygon on inset map) Yellow shading onspontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivity curve (right track at scale of 0ndash20 ohmm) indicate resistivity of more than 3 ohmm Pinkbars indicate low resistivity sands affected by infiltration of saline produced water from the pond The API numbers at the top of the logs are used for well identification Year drilled andlogged in parentheses Depth scale in feet

90Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

This may be caused by the presence of an active waterdisposal field in the Tulare Formation near this areaOverall the log-derived map shows base of USDWapproximately 61m (~200 ft) deeper than lab analyseswould suggest As at Lost Hills this may be caused bythe geographic distribution of the two data sets the labsamples coming mainly from within the oil fieldboundaries whereas the log data are confined to thearea outside the oil field

In the southern part of South Belridge field a wellnear the southern tip of the field was sampled at mul-tiple intervals Samples as deep as 455 m (~1495 ft) areUSDW and the deepest sample from 480 m (1575 ft)has a TDS value of 13074 mgL suggesting that thebase of USDW in the southern part of South Belridge liesbetween 457 and 480 m (1495ndash1575 ft) (Figure 9) Thedepth to USDW map from log analysis in the southernpart of South Belridge shows that the base of USDW

ranges from 365 m (1200 ft) in the north to approxi-mately 487 m (~1600 ft) in the south

Effects of Oil Field Activities on Water Salinity

The effects of water disposal in surface ponds on ground-water salinity is evident by comparing geophysical logsfrom old wells near the pond locations to logs fromnewerwells drilled nearbyOlder geophysical logs (pre-1970s) indicate that the upper sands in the alluviumare desaturated (vadose zone) as indicated by areas ofcrossover on densityndashneutron logs and have high re-sistivity (gt20ohmmFigure 10)However geophysicallogs collected from wells drilled later in the area donot show densityndashneutron crossover characteristic of avadose zone andhave extremely low resistivity (lt1ohmm)in the upper alluvial sands because of the presence ofhigh salinityproducedwater thathasfilled thevadosezoneInvasion of low resistivity water is especially apparent

Figure 11 Two adjacent wells drilled and logged in different years (1990 and 2012) in an area containing numerous disposal wells on theeast flank of South Belridge Yellow shading on spontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivitycurve indicates resistivity greater than 3 ohm m Pink bars indicate low resistivity sands in the 2012 well (0403046566) affected by injectedsaline produced water The API numbers at the top of the logs are used for well identification Year drilled and logged in parentheses

Gillespie et al 91

where saline waters have infiltrated the 22K sandwhich lies directly above the CCE (well 03047481 inFigure 10) in the lower part of the alluvium The salinewater lies atop the CCE which appears to be pro-tecting the underlying Tulare aquifer from contami-nation Log-calculated andmeasuredTDS concentrationscan exceed 20000mgL in affected parts of the alluvium

The effects of surface disposal ponds are less evidentin Lost Hills and North Belridge compared to the areaadjacent to South Belridge Lost Hills in particular tends tohave highly saline water in the shallow parts of the aquifereven where disposal ponds are not present Swain andDuell (1989) noted TDS concentrations as high as 21700mgL from very shallowwells (6m [~20 ft]) along the eastflank of Lost Hills but did not note the cause of the highsalinity in the area

The effects of water disposal injection wells aremore subtle than that of the ponds and are especiallydifficult to identify in sands closer to the base of USDWwhere resistivity is naturally fairly low Geophysicallogs in sands affected by saline injection tend to have aconcave resistivity profile and resistivities are as small ashalf that in logs fromolder unaffectedwells (Figure 11)Injected saline water tends to follow individual sand

layers leaving some layers relatively unaffected In somecases only the lower part of an individual sand containssaline water whereas the waters in the upper part re-mainbrackishThis suggests that thewaters are stratifiedand mixing has not had time to occur Log-calculatedTDS concentrations show a relatively smooth increasewith depth inwells logged prior to the start of large-scaledisposal activities whereas recent logs in these areasshow a much more variable TDS concentration profilewith depth Additionally densityndashneutron curves inwells affected by disposal of produced waters show thinintervals (~1 m [~3 ft] thick) of crossover This may becaused by bacterial metabolism of trace amounts oforganic matter in the disposal water resulting in thegeneration of small amounts of biogenic methane(McMahon et al 2018)

The cross section in Figure 12 shows wells affectedby saline produced water disposal by injection (belowthe CCE) and percolation (above the CCE) along theeast flank of the South Belridge oil field Wells affectedby disposal of produced waters in saline ponds (n = 139)and by disposal in injection wells (n = 125) are alsoshown in the map in Figure 13 The effects of disposalponds are evident from changes in borehole resistivity

Figure 12 Cross section showing intervals affected by disposal of saline produced water (pink boxes) at South Belridge oil field Wellsidentified by API number at top of each well Year drilled and logged shown at bottom of eachwell Insetmap shows location of cross sectionProduced water disposal wells are highlighted in pink and disposal ponds are shown in blue in the inset map

92 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Geologic Mapping

The thickness of the Tulare Formation and overlyingalluvium in the study area (Figure 4) ranges from1200m (4000 ft) in the southeast part of the study areato 75 m (~250 ft) along the anticline crests in thenorthern parts of Lost Hills and North Belridge oilfields Because chemical analyses indicate that theprotected aquifers occur largely within the Tulare For-mation and overlying alluvium throughout the studyarea the map of the depth to the base of the TulareFormation defines the extent of potential protectedaquifers outside the oil field limits

The Amnicola Clay is thickest (67 m [~220 ft])along the northeastern and southeastern flank of theLost Hills anticline and east of the South Belridge an-ticline (Figure 5A) It thins to zero in the northwest partof the study area The clay is replaced by sandier sedi-ments east of the central part of the Lost Hills anticlineThe CCE thickens to 61m (~200 ft) in the northcentralpart of the study area in the Tulare lakebed but iscommonly less than30m(lt~100 ft) thick (Figure5B) Itis absent over the Lost Hills anticline crest and in thenorthwestern part of the study area These clay maps(Figure 5A B) are used to determine the presence ofpossible confining beds that in some cases may controlthe distribution of protected water resources and restrictthe migration of potential contaminants

Graphs of pressure versus depth collected in 1985from three wells along a northeast-trending transectfrom South Belridge to south of Lost Hills are shown inFigure 6 Marked changes in the hydrostatic pressuregradient in well API no 02974434 occur across theMiddle Tulare Clay and Amnicola Clay layers An addi-tional deeper clay here termed the D-E Clay forms alocal confining layer isolating a relatively low perme-ability section at the base of the Tulare Formation fromthe rest of the aquifer These characteristic deviationsin hydrostatic gradients indicates that the clays hydrau-lically separate individual aquifer layers in the area ofthis well

Farther northeast in wells API no 02974436 andAPI no 02974433 these deviations from hydrostaticgradient are much less noticeable especially across theAmnicola Clay This suggests that either (1) the claylayers are less confining south of Lost Hills or (2) thehydrologic regime near South Belridge has been alteredby production and injection into the Tulare Formationand the pressure responses in aquifer layers near the fieldin 1985 had not had sufficient time for their pressures

to reach equilibrium Recent drilling and multiple wellmonitoring site data (April 2018) off the eastern flank ofthe northern Lost Hills oil field indicates a 45 m (~15 ft)head difference across the Amnicola Clay with an upwardgradient of 0038 (measured May 31 2018) suggestingthat it is a confining layer in the northeastern part of thestudy area (US Geological Survey 2018a b)

Salinity Analysis

Log analysis from 30 wells in areas outside the oil fieldboundaries or within the oil fields but below the oilndashwater contact were used to create a map of the depthto the base of USDW The depth map to the base ofUSDW shows two trends (Figure 7) In the area be-tween Lost Hills and the Belridge oil fields depthto USDW increases toward the south from 150 m(~500 ft) between North Belridge and Lost Hills to490 m (~1600 ft) southeast of South Belridge In the

Figure 7 Map showing depth to base of underground sourcesof drinking water (USDW) (total dissolved solids = 10000 mgL)based on log analysis Red lines indicate faults (Young 1968California Division of Oil Gas and Geothermal Resources 1998)Black lines are oil field administrative boundaries Wells shown asblue dots are those used to determine the base of USDW Depthvalues in feet CI = contour interval No = North So = South

86 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

eastern part of the study area depth to base of USDWincreases sharply eastward from approximately 215 m(~700 ft) on the west flank of the Lost Hills anticline toas great as 550 m (~1800 ft) within 3 km (~2 mi)

Salinityndashdepth profiles based on chemical analysesof producedwater samples were compared to the depthto base USDW map estimated from borehole geo-physical log analysis However this comparison waslimited because most of the lab analyses come from oil-producing wells within the field boundaries rather thanin areas outside the fields The presence of oil in thesampled intervals within the fields precludes the useof log analysis to calculate the salinity in these samewells Plots of TDS versus depth of the samples in eachfield are shown in Figures 8 and 9 along with log-calculated TDS

Wood and Davis (1959) reported the groundwaterchemistry in this area as being an NaSO4 type howeveronly one Tulare Formation monitoring well Mackesseyno 1 (API no 02987363) was an NaSO4 type the restare NaCl type Because the calculated salinity fromArchie log analysis assumes NaCl water types the log-calculated and lab sample TDS values should be com-parable The two monitoring wells completed in thealluvium have higher SO4 concentrations relative tothe Tulare Formation wells however Cl is the domi-nant anion

Only four Tulare Formation lab samples (blue dots)in Lost Hills are in the California DOGGR databaseData from 14 monitoring wells in the central part ofthe Lost Hills field were used to enlarge the data set(Figure 8) With the exception of one sample TDSconcentrations in the Tulare Formation and alluviumare less than 10000 mgL indicating protected (USDW)aquifers These wells have top perforations ranging from40 to 290 m (133ndash950 ft) The well with a lab analysisindicating a salinity of 12295 mgL TDS in the TulareFormation had a top perforation at 300 m (984 ft) Thissuggests that the base of USDW lies somewhere be-tween 290 and 300 m (950 and 985 ft) within the fieldLog analysis suggests that the base may occur as deepas 580 m (~1900 ft) but many of the wells used for loganalysis come from areas east of the field boundarywhere the Tulare Formation is much thicker (200 ftwithin the field vs 2000 ft east of the field [Figure 4])Within the field interpolated contour lines from log datashow the depth to base USDWat 213m (~700 ft) alongthe western field boundary and 365 m (~1200 ft) alongthe eastern field boundary a trend that is consistentwith the more limited water sample data

Typical TDS concentrations in the underlyingEtchegoin Formation (red dots) range from 20000 to35000 mgL probably because of the presence oftrapped connate seawater within these marine depositsIn northern Lost Hills (Figure 8) two samples fromEtchegoin sands have TDS concentrations below 10000mgL However in this area an active steamflood isongoing within the Tulare and Etchegoin FormationsHere the Etchegoin Formation sands lie at shallowdepths (lt150 m [~500 ft]) immediately below pro-ducing intervals in the Tulare Formation The EtchegoinFormation samples in this area are much more dilute(5600ndash8000 mgL) than in other areas This may be be-cause the sands are in hydraulic communication with thefresher water sands of the overlying Tulare Formation orbecause the Etchegoin sands in this area are steamfloodedand the more saline formation water is diluted by theinjected steam Since the twoEtchegoinUSDWsamplesare post-steamflood initiation the latter explanation islikely

Only one chemical analysis sample from the TulareFormation is available in the North Belridge field (Fig-ure 9) The TDS concentration of the sample is 6055mgL from a depth interval of 157ndash211 m (515ndash692ft) It confirms that waters above approximately 211 m(~700 ft) are USDW consistent with the log-deriveddepth to USDW map which shows depth to USDWnear the well at 275 m (900 ft)

Many lab analyses are available for the Tulare For-mation at South Belridge (Figure 9) but they do notshow salinity values that are correlated with depth Thelog-generated depth to USDW map shows that salinitytrends along the northern part of the oil field are vari-able but the southern part of the field shows depth toUSDW increasing toward the southeast Thereforesamples from northern and southern South Belridgewere considered separately The boundary betweenthe two subareas is shown by the dashed line in Figure 9Because of the variability in sample TDS values withdepth comparisons of sample data to calculated salin-ities fromwell-log analyses can only bemade in a generalfashion

In the northern part of the South Belridge field nosamples below 365 m (1200 ft) are USDW suggestingthat the base of USDW lies above this depth The log-derived map shows depth to base of USDW at 335 m(1100 ft) in the north To the south the base of USDWincreases to 427 m (~1400 ft) Near the dividing linebetween the north and south parts of the field thedepth to base USDW decreases to 365 m (1200 ft)

Gillespie et al 87

Figure 8 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare and alluvium (blue) and Etchegoin (red) formations in and near the Lost Hills oilfield Gray dots represent TDS calculated from geophysical logs in the field The horizontal red linemarks 10000mgL and the vertical dashed red linemarks the approximate depth of thedeepest underground sources of drinking water (USDW) sample The adjacent map shows the location of the Tulare and alluviumwells sampled as blue dots and shallow (lt150m [500ft] deep) Etchegoin samples as red dots (deeper Etchegoin sample sites are not shown onmap) and the contours to base USDW from log analysis Lost Hills is the only area that containsbrackish water in the Etchegoin Formation However the brackish waters in the Etchegoin occur in an oil-producing zone that is steam flooded so it is considered to be an exempt aquiferand is not protected CI = contour interval

88Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

Figure 9 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare Formation in and near the North and South Belridge oil fields The northern andsouthern parts of the South Belridge field (marked by the red line on the map) are shown in separate graphs Blue dots show Tulare Formation (Fm) Measurements of TDS from labanalyses Gray dots show Tulare Fm The TDS calculated from log analysis Horizontal red lines on the graphs mark 10000 mgL and the dashed vertical red lines mark the approximatedepth of USDW from lab samples The adjacent map shows the location of the wells sampled (blue dots) and the contours to base underground sources of drinking water (USDW) (infeet) from log analysis Black lines are oil field administrative boundaries and red lines are faults CI = contour interval No = North So = South

Gillespieetal

89

Figure 10 Cross section showing three wells drilled and logged in different years near a produced water disposal pond (light blue polygon on inset map) Yellow shading onspontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivity curve (right track at scale of 0ndash20 ohmm) indicate resistivity of more than 3 ohmm Pinkbars indicate low resistivity sands affected by infiltration of saline produced water from the pond The API numbers at the top of the logs are used for well identification Year drilled andlogged in parentheses Depth scale in feet

90Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

This may be caused by the presence of an active waterdisposal field in the Tulare Formation near this areaOverall the log-derived map shows base of USDWapproximately 61m (~200 ft) deeper than lab analyseswould suggest As at Lost Hills this may be caused bythe geographic distribution of the two data sets the labsamples coming mainly from within the oil fieldboundaries whereas the log data are confined to thearea outside the oil field

In the southern part of South Belridge field a wellnear the southern tip of the field was sampled at mul-tiple intervals Samples as deep as 455 m (~1495 ft) areUSDW and the deepest sample from 480 m (1575 ft)has a TDS value of 13074 mgL suggesting that thebase of USDW in the southern part of South Belridge liesbetween 457 and 480 m (1495ndash1575 ft) (Figure 9) Thedepth to USDW map from log analysis in the southernpart of South Belridge shows that the base of USDW

ranges from 365 m (1200 ft) in the north to approxi-mately 487 m (~1600 ft) in the south

Effects of Oil Field Activities on Water Salinity

The effects of water disposal in surface ponds on ground-water salinity is evident by comparing geophysical logsfrom old wells near the pond locations to logs fromnewerwells drilled nearbyOlder geophysical logs (pre-1970s) indicate that the upper sands in the alluviumare desaturated (vadose zone) as indicated by areas ofcrossover on densityndashneutron logs and have high re-sistivity (gt20ohmmFigure 10)However geophysicallogs collected from wells drilled later in the area donot show densityndashneutron crossover characteristic of avadose zone andhave extremely low resistivity (lt1ohmm)in the upper alluvial sands because of the presence ofhigh salinityproducedwater thathasfilled thevadosezoneInvasion of low resistivity water is especially apparent

Figure 11 Two adjacent wells drilled and logged in different years (1990 and 2012) in an area containing numerous disposal wells on theeast flank of South Belridge Yellow shading on spontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivitycurve indicates resistivity greater than 3 ohm m Pink bars indicate low resistivity sands in the 2012 well (0403046566) affected by injectedsaline produced water The API numbers at the top of the logs are used for well identification Year drilled and logged in parentheses

Gillespie et al 91

where saline waters have infiltrated the 22K sandwhich lies directly above the CCE (well 03047481 inFigure 10) in the lower part of the alluvium The salinewater lies atop the CCE which appears to be pro-tecting the underlying Tulare aquifer from contami-nation Log-calculated andmeasuredTDS concentrationscan exceed 20000mgL in affected parts of the alluvium

The effects of surface disposal ponds are less evidentin Lost Hills and North Belridge compared to the areaadjacent to South Belridge Lost Hills in particular tends tohave highly saline water in the shallow parts of the aquifereven where disposal ponds are not present Swain andDuell (1989) noted TDS concentrations as high as 21700mgL from very shallowwells (6m [~20 ft]) along the eastflank of Lost Hills but did not note the cause of the highsalinity in the area

The effects of water disposal injection wells aremore subtle than that of the ponds and are especiallydifficult to identify in sands closer to the base of USDWwhere resistivity is naturally fairly low Geophysicallogs in sands affected by saline injection tend to have aconcave resistivity profile and resistivities are as small ashalf that in logs fromolder unaffectedwells (Figure 11)Injected saline water tends to follow individual sand

layers leaving some layers relatively unaffected In somecases only the lower part of an individual sand containssaline water whereas the waters in the upper part re-mainbrackishThis suggests that thewaters are stratifiedand mixing has not had time to occur Log-calculatedTDS concentrations show a relatively smooth increasewith depth inwells logged prior to the start of large-scaledisposal activities whereas recent logs in these areasshow a much more variable TDS concentration profilewith depth Additionally densityndashneutron curves inwells affected by disposal of produced waters show thinintervals (~1 m [~3 ft] thick) of crossover This may becaused by bacterial metabolism of trace amounts oforganic matter in the disposal water resulting in thegeneration of small amounts of biogenic methane(McMahon et al 2018)

The cross section in Figure 12 shows wells affectedby saline produced water disposal by injection (belowthe CCE) and percolation (above the CCE) along theeast flank of the South Belridge oil field Wells affectedby disposal of produced waters in saline ponds (n = 139)and by disposal in injection wells (n = 125) are alsoshown in the map in Figure 13 The effects of disposalponds are evident from changes in borehole resistivity

Figure 12 Cross section showing intervals affected by disposal of saline produced water (pink boxes) at South Belridge oil field Wellsidentified by API number at top of each well Year drilled and logged shown at bottom of eachwell Insetmap shows location of cross sectionProduced water disposal wells are highlighted in pink and disposal ponds are shown in blue in the inset map

92 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

eastern part of the study area depth to base of USDWincreases sharply eastward from approximately 215 m(~700 ft) on the west flank of the Lost Hills anticline toas great as 550 m (~1800 ft) within 3 km (~2 mi)

Salinityndashdepth profiles based on chemical analysesof producedwater samples were compared to the depthto base USDW map estimated from borehole geo-physical log analysis However this comparison waslimited because most of the lab analyses come from oil-producing wells within the field boundaries rather thanin areas outside the fields The presence of oil in thesampled intervals within the fields precludes the useof log analysis to calculate the salinity in these samewells Plots of TDS versus depth of the samples in eachfield are shown in Figures 8 and 9 along with log-calculated TDS

Wood and Davis (1959) reported the groundwaterchemistry in this area as being an NaSO4 type howeveronly one Tulare Formation monitoring well Mackesseyno 1 (API no 02987363) was an NaSO4 type the restare NaCl type Because the calculated salinity fromArchie log analysis assumes NaCl water types the log-calculated and lab sample TDS values should be com-parable The two monitoring wells completed in thealluvium have higher SO4 concentrations relative tothe Tulare Formation wells however Cl is the domi-nant anion

Only four Tulare Formation lab samples (blue dots)in Lost Hills are in the California DOGGR databaseData from 14 monitoring wells in the central part ofthe Lost Hills field were used to enlarge the data set(Figure 8) With the exception of one sample TDSconcentrations in the Tulare Formation and alluviumare less than 10000 mgL indicating protected (USDW)aquifers These wells have top perforations ranging from40 to 290 m (133ndash950 ft) The well with a lab analysisindicating a salinity of 12295 mgL TDS in the TulareFormation had a top perforation at 300 m (984 ft) Thissuggests that the base of USDW lies somewhere be-tween 290 and 300 m (950 and 985 ft) within the fieldLog analysis suggests that the base may occur as deepas 580 m (~1900 ft) but many of the wells used for loganalysis come from areas east of the field boundarywhere the Tulare Formation is much thicker (200 ftwithin the field vs 2000 ft east of the field [Figure 4])Within the field interpolated contour lines from log datashow the depth to base USDWat 213m (~700 ft) alongthe western field boundary and 365 m (~1200 ft) alongthe eastern field boundary a trend that is consistentwith the more limited water sample data

Typical TDS concentrations in the underlyingEtchegoin Formation (red dots) range from 20000 to35000 mgL probably because of the presence oftrapped connate seawater within these marine depositsIn northern Lost Hills (Figure 8) two samples fromEtchegoin sands have TDS concentrations below 10000mgL However in this area an active steamflood isongoing within the Tulare and Etchegoin FormationsHere the Etchegoin Formation sands lie at shallowdepths (lt150 m [~500 ft]) immediately below pro-ducing intervals in the Tulare Formation The EtchegoinFormation samples in this area are much more dilute(5600ndash8000 mgL) than in other areas This may be be-cause the sands are in hydraulic communication with thefresher water sands of the overlying Tulare Formation orbecause the Etchegoin sands in this area are steamfloodedand the more saline formation water is diluted by theinjected steam Since the twoEtchegoinUSDWsamplesare post-steamflood initiation the latter explanation islikely

Only one chemical analysis sample from the TulareFormation is available in the North Belridge field (Fig-ure 9) The TDS concentration of the sample is 6055mgL from a depth interval of 157ndash211 m (515ndash692ft) It confirms that waters above approximately 211 m(~700 ft) are USDW consistent with the log-deriveddepth to USDW map which shows depth to USDWnear the well at 275 m (900 ft)

Many lab analyses are available for the Tulare For-mation at South Belridge (Figure 9) but they do notshow salinity values that are correlated with depth Thelog-generated depth to USDW map shows that salinitytrends along the northern part of the oil field are vari-able but the southern part of the field shows depth toUSDW increasing toward the southeast Thereforesamples from northern and southern South Belridgewere considered separately The boundary betweenthe two subareas is shown by the dashed line in Figure 9Because of the variability in sample TDS values withdepth comparisons of sample data to calculated salin-ities fromwell-log analyses can only bemade in a generalfashion

In the northern part of the South Belridge field nosamples below 365 m (1200 ft) are USDW suggestingthat the base of USDW lies above this depth The log-derived map shows depth to base of USDW at 335 m(1100 ft) in the north To the south the base of USDWincreases to 427 m (~1400 ft) Near the dividing linebetween the north and south parts of the field thedepth to base USDW decreases to 365 m (1200 ft)

Gillespie et al 87

Figure 8 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare and alluvium (blue) and Etchegoin (red) formations in and near the Lost Hills oilfield Gray dots represent TDS calculated from geophysical logs in the field The horizontal red linemarks 10000mgL and the vertical dashed red linemarks the approximate depth of thedeepest underground sources of drinking water (USDW) sample The adjacent map shows the location of the Tulare and alluviumwells sampled as blue dots and shallow (lt150m [500ft] deep) Etchegoin samples as red dots (deeper Etchegoin sample sites are not shown onmap) and the contours to base USDW from log analysis Lost Hills is the only area that containsbrackish water in the Etchegoin Formation However the brackish waters in the Etchegoin occur in an oil-producing zone that is steam flooded so it is considered to be an exempt aquiferand is not protected CI = contour interval

88Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

Figure 9 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare Formation in and near the North and South Belridge oil fields The northern andsouthern parts of the South Belridge field (marked by the red line on the map) are shown in separate graphs Blue dots show Tulare Formation (Fm) Measurements of TDS from labanalyses Gray dots show Tulare Fm The TDS calculated from log analysis Horizontal red lines on the graphs mark 10000 mgL and the dashed vertical red lines mark the approximatedepth of USDW from lab samples The adjacent map shows the location of the wells sampled (blue dots) and the contours to base underground sources of drinking water (USDW) (infeet) from log analysis Black lines are oil field administrative boundaries and red lines are faults CI = contour interval No = North So = South

Gillespieetal

89

Figure 10 Cross section showing three wells drilled and logged in different years near a produced water disposal pond (light blue polygon on inset map) Yellow shading onspontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivity curve (right track at scale of 0ndash20 ohmm) indicate resistivity of more than 3 ohmm Pinkbars indicate low resistivity sands affected by infiltration of saline produced water from the pond The API numbers at the top of the logs are used for well identification Year drilled andlogged in parentheses Depth scale in feet

90Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

This may be caused by the presence of an active waterdisposal field in the Tulare Formation near this areaOverall the log-derived map shows base of USDWapproximately 61m (~200 ft) deeper than lab analyseswould suggest As at Lost Hills this may be caused bythe geographic distribution of the two data sets the labsamples coming mainly from within the oil fieldboundaries whereas the log data are confined to thearea outside the oil field

In the southern part of South Belridge field a wellnear the southern tip of the field was sampled at mul-tiple intervals Samples as deep as 455 m (~1495 ft) areUSDW and the deepest sample from 480 m (1575 ft)has a TDS value of 13074 mgL suggesting that thebase of USDW in the southern part of South Belridge liesbetween 457 and 480 m (1495ndash1575 ft) (Figure 9) Thedepth to USDW map from log analysis in the southernpart of South Belridge shows that the base of USDW

ranges from 365 m (1200 ft) in the north to approxi-mately 487 m (~1600 ft) in the south

Effects of Oil Field Activities on Water Salinity

The effects of water disposal in surface ponds on ground-water salinity is evident by comparing geophysical logsfrom old wells near the pond locations to logs fromnewerwells drilled nearbyOlder geophysical logs (pre-1970s) indicate that the upper sands in the alluviumare desaturated (vadose zone) as indicated by areas ofcrossover on densityndashneutron logs and have high re-sistivity (gt20ohmmFigure 10)However geophysicallogs collected from wells drilled later in the area donot show densityndashneutron crossover characteristic of avadose zone andhave extremely low resistivity (lt1ohmm)in the upper alluvial sands because of the presence ofhigh salinityproducedwater thathasfilled thevadosezoneInvasion of low resistivity water is especially apparent

Figure 11 Two adjacent wells drilled and logged in different years (1990 and 2012) in an area containing numerous disposal wells on theeast flank of South Belridge Yellow shading on spontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivitycurve indicates resistivity greater than 3 ohm m Pink bars indicate low resistivity sands in the 2012 well (0403046566) affected by injectedsaline produced water The API numbers at the top of the logs are used for well identification Year drilled and logged in parentheses

Gillespie et al 91

where saline waters have infiltrated the 22K sandwhich lies directly above the CCE (well 03047481 inFigure 10) in the lower part of the alluvium The salinewater lies atop the CCE which appears to be pro-tecting the underlying Tulare aquifer from contami-nation Log-calculated andmeasuredTDS concentrationscan exceed 20000mgL in affected parts of the alluvium

The effects of surface disposal ponds are less evidentin Lost Hills and North Belridge compared to the areaadjacent to South Belridge Lost Hills in particular tends tohave highly saline water in the shallow parts of the aquifereven where disposal ponds are not present Swain andDuell (1989) noted TDS concentrations as high as 21700mgL from very shallowwells (6m [~20 ft]) along the eastflank of Lost Hills but did not note the cause of the highsalinity in the area

The effects of water disposal injection wells aremore subtle than that of the ponds and are especiallydifficult to identify in sands closer to the base of USDWwhere resistivity is naturally fairly low Geophysicallogs in sands affected by saline injection tend to have aconcave resistivity profile and resistivities are as small ashalf that in logs fromolder unaffectedwells (Figure 11)Injected saline water tends to follow individual sand

layers leaving some layers relatively unaffected In somecases only the lower part of an individual sand containssaline water whereas the waters in the upper part re-mainbrackishThis suggests that thewaters are stratifiedand mixing has not had time to occur Log-calculatedTDS concentrations show a relatively smooth increasewith depth inwells logged prior to the start of large-scaledisposal activities whereas recent logs in these areasshow a much more variable TDS concentration profilewith depth Additionally densityndashneutron curves inwells affected by disposal of produced waters show thinintervals (~1 m [~3 ft] thick) of crossover This may becaused by bacterial metabolism of trace amounts oforganic matter in the disposal water resulting in thegeneration of small amounts of biogenic methane(McMahon et al 2018)

The cross section in Figure 12 shows wells affectedby saline produced water disposal by injection (belowthe CCE) and percolation (above the CCE) along theeast flank of the South Belridge oil field Wells affectedby disposal of produced waters in saline ponds (n = 139)and by disposal in injection wells (n = 125) are alsoshown in the map in Figure 13 The effects of disposalponds are evident from changes in borehole resistivity

Figure 12 Cross section showing intervals affected by disposal of saline produced water (pink boxes) at South Belridge oil field Wellsidentified by API number at top of each well Year drilled and logged shown at bottom of eachwell Insetmap shows location of cross sectionProduced water disposal wells are highlighted in pink and disposal ponds are shown in blue in the inset map

92 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 8 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare and alluvium (blue) and Etchegoin (red) formations in and near the Lost Hills oilfield Gray dots represent TDS calculated from geophysical logs in the field The horizontal red linemarks 10000mgL and the vertical dashed red linemarks the approximate depth of thedeepest underground sources of drinking water (USDW) sample The adjacent map shows the location of the Tulare and alluviumwells sampled as blue dots and shallow (lt150m [500ft] deep) Etchegoin samples as red dots (deeper Etchegoin sample sites are not shown onmap) and the contours to base USDW from log analysis Lost Hills is the only area that containsbrackish water in the Etchegoin Formation However the brackish waters in the Etchegoin occur in an oil-producing zone that is steam flooded so it is considered to be an exempt aquiferand is not protected CI = contour interval

88Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

Figure 9 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare Formation in and near the North and South Belridge oil fields The northern andsouthern parts of the South Belridge field (marked by the red line on the map) are shown in separate graphs Blue dots show Tulare Formation (Fm) Measurements of TDS from labanalyses Gray dots show Tulare Fm The TDS calculated from log analysis Horizontal red lines on the graphs mark 10000 mgL and the dashed vertical red lines mark the approximatedepth of USDW from lab samples The adjacent map shows the location of the wells sampled (blue dots) and the contours to base underground sources of drinking water (USDW) (infeet) from log analysis Black lines are oil field administrative boundaries and red lines are faults CI = contour interval No = North So = South

Gillespieetal

89

Figure 10 Cross section showing three wells drilled and logged in different years near a produced water disposal pond (light blue polygon on inset map) Yellow shading onspontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivity curve (right track at scale of 0ndash20 ohmm) indicate resistivity of more than 3 ohmm Pinkbars indicate low resistivity sands affected by infiltration of saline produced water from the pond The API numbers at the top of the logs are used for well identification Year drilled andlogged in parentheses Depth scale in feet

90Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

This may be caused by the presence of an active waterdisposal field in the Tulare Formation near this areaOverall the log-derived map shows base of USDWapproximately 61m (~200 ft) deeper than lab analyseswould suggest As at Lost Hills this may be caused bythe geographic distribution of the two data sets the labsamples coming mainly from within the oil fieldboundaries whereas the log data are confined to thearea outside the oil field

In the southern part of South Belridge field a wellnear the southern tip of the field was sampled at mul-tiple intervals Samples as deep as 455 m (~1495 ft) areUSDW and the deepest sample from 480 m (1575 ft)has a TDS value of 13074 mgL suggesting that thebase of USDW in the southern part of South Belridge liesbetween 457 and 480 m (1495ndash1575 ft) (Figure 9) Thedepth to USDW map from log analysis in the southernpart of South Belridge shows that the base of USDW

ranges from 365 m (1200 ft) in the north to approxi-mately 487 m (~1600 ft) in the south

Effects of Oil Field Activities on Water Salinity

The effects of water disposal in surface ponds on ground-water salinity is evident by comparing geophysical logsfrom old wells near the pond locations to logs fromnewerwells drilled nearbyOlder geophysical logs (pre-1970s) indicate that the upper sands in the alluviumare desaturated (vadose zone) as indicated by areas ofcrossover on densityndashneutron logs and have high re-sistivity (gt20ohmmFigure 10)However geophysicallogs collected from wells drilled later in the area donot show densityndashneutron crossover characteristic of avadose zone andhave extremely low resistivity (lt1ohmm)in the upper alluvial sands because of the presence ofhigh salinityproducedwater thathasfilled thevadosezoneInvasion of low resistivity water is especially apparent

Figure 11 Two adjacent wells drilled and logged in different years (1990 and 2012) in an area containing numerous disposal wells on theeast flank of South Belridge Yellow shading on spontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivitycurve indicates resistivity greater than 3 ohm m Pink bars indicate low resistivity sands in the 2012 well (0403046566) affected by injectedsaline produced water The API numbers at the top of the logs are used for well identification Year drilled and logged in parentheses

Gillespie et al 91

where saline waters have infiltrated the 22K sandwhich lies directly above the CCE (well 03047481 inFigure 10) in the lower part of the alluvium The salinewater lies atop the CCE which appears to be pro-tecting the underlying Tulare aquifer from contami-nation Log-calculated andmeasuredTDS concentrationscan exceed 20000mgL in affected parts of the alluvium

The effects of surface disposal ponds are less evidentin Lost Hills and North Belridge compared to the areaadjacent to South Belridge Lost Hills in particular tends tohave highly saline water in the shallow parts of the aquifereven where disposal ponds are not present Swain andDuell (1989) noted TDS concentrations as high as 21700mgL from very shallowwells (6m [~20 ft]) along the eastflank of Lost Hills but did not note the cause of the highsalinity in the area

The effects of water disposal injection wells aremore subtle than that of the ponds and are especiallydifficult to identify in sands closer to the base of USDWwhere resistivity is naturally fairly low Geophysicallogs in sands affected by saline injection tend to have aconcave resistivity profile and resistivities are as small ashalf that in logs fromolder unaffectedwells (Figure 11)Injected saline water tends to follow individual sand

layers leaving some layers relatively unaffected In somecases only the lower part of an individual sand containssaline water whereas the waters in the upper part re-mainbrackishThis suggests that thewaters are stratifiedand mixing has not had time to occur Log-calculatedTDS concentrations show a relatively smooth increasewith depth inwells logged prior to the start of large-scaledisposal activities whereas recent logs in these areasshow a much more variable TDS concentration profilewith depth Additionally densityndashneutron curves inwells affected by disposal of produced waters show thinintervals (~1 m [~3 ft] thick) of crossover This may becaused by bacterial metabolism of trace amounts oforganic matter in the disposal water resulting in thegeneration of small amounts of biogenic methane(McMahon et al 2018)

The cross section in Figure 12 shows wells affectedby saline produced water disposal by injection (belowthe CCE) and percolation (above the CCE) along theeast flank of the South Belridge oil field Wells affectedby disposal of produced waters in saline ponds (n = 139)and by disposal in injection wells (n = 125) are alsoshown in the map in Figure 13 The effects of disposalponds are evident from changes in borehole resistivity

Figure 12 Cross section showing intervals affected by disposal of saline produced water (pink boxes) at South Belridge oil field Wellsidentified by API number at top of each well Year drilled and logged shown at bottom of eachwell Insetmap shows location of cross sectionProduced water disposal wells are highlighted in pink and disposal ponds are shown in blue in the inset map

92 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 9 Graph of total dissolved solids (TDS) versus depth of geochemical samples from the Tulare Formation in and near the North and South Belridge oil fields The northern andsouthern parts of the South Belridge field (marked by the red line on the map) are shown in separate graphs Blue dots show Tulare Formation (Fm) Measurements of TDS from labanalyses Gray dots show Tulare Fm The TDS calculated from log analysis Horizontal red lines on the graphs mark 10000 mgL and the dashed vertical red lines mark the approximatedepth of USDW from lab samples The adjacent map shows the location of the wells sampled (blue dots) and the contours to base underground sources of drinking water (USDW) (infeet) from log analysis Black lines are oil field administrative boundaries and red lines are faults CI = contour interval No = North So = South

Gillespieetal

89

Figure 10 Cross section showing three wells drilled and logged in different years near a produced water disposal pond (light blue polygon on inset map) Yellow shading onspontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivity curve (right track at scale of 0ndash20 ohmm) indicate resistivity of more than 3 ohmm Pinkbars indicate low resistivity sands affected by infiltration of saline produced water from the pond The API numbers at the top of the logs are used for well identification Year drilled andlogged in parentheses Depth scale in feet

90Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

This may be caused by the presence of an active waterdisposal field in the Tulare Formation near this areaOverall the log-derived map shows base of USDWapproximately 61m (~200 ft) deeper than lab analyseswould suggest As at Lost Hills this may be caused bythe geographic distribution of the two data sets the labsamples coming mainly from within the oil fieldboundaries whereas the log data are confined to thearea outside the oil field

In the southern part of South Belridge field a wellnear the southern tip of the field was sampled at mul-tiple intervals Samples as deep as 455 m (~1495 ft) areUSDW and the deepest sample from 480 m (1575 ft)has a TDS value of 13074 mgL suggesting that thebase of USDW in the southern part of South Belridge liesbetween 457 and 480 m (1495ndash1575 ft) (Figure 9) Thedepth to USDW map from log analysis in the southernpart of South Belridge shows that the base of USDW

ranges from 365 m (1200 ft) in the north to approxi-mately 487 m (~1600 ft) in the south

Effects of Oil Field Activities on Water Salinity

The effects of water disposal in surface ponds on ground-water salinity is evident by comparing geophysical logsfrom old wells near the pond locations to logs fromnewerwells drilled nearbyOlder geophysical logs (pre-1970s) indicate that the upper sands in the alluviumare desaturated (vadose zone) as indicated by areas ofcrossover on densityndashneutron logs and have high re-sistivity (gt20ohmmFigure 10)However geophysicallogs collected from wells drilled later in the area donot show densityndashneutron crossover characteristic of avadose zone andhave extremely low resistivity (lt1ohmm)in the upper alluvial sands because of the presence ofhigh salinityproducedwater thathasfilled thevadosezoneInvasion of low resistivity water is especially apparent

Figure 11 Two adjacent wells drilled and logged in different years (1990 and 2012) in an area containing numerous disposal wells on theeast flank of South Belridge Yellow shading on spontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivitycurve indicates resistivity greater than 3 ohm m Pink bars indicate low resistivity sands in the 2012 well (0403046566) affected by injectedsaline produced water The API numbers at the top of the logs are used for well identification Year drilled and logged in parentheses

Gillespie et al 91

where saline waters have infiltrated the 22K sandwhich lies directly above the CCE (well 03047481 inFigure 10) in the lower part of the alluvium The salinewater lies atop the CCE which appears to be pro-tecting the underlying Tulare aquifer from contami-nation Log-calculated andmeasuredTDS concentrationscan exceed 20000mgL in affected parts of the alluvium

The effects of surface disposal ponds are less evidentin Lost Hills and North Belridge compared to the areaadjacent to South Belridge Lost Hills in particular tends tohave highly saline water in the shallow parts of the aquifereven where disposal ponds are not present Swain andDuell (1989) noted TDS concentrations as high as 21700mgL from very shallowwells (6m [~20 ft]) along the eastflank of Lost Hills but did not note the cause of the highsalinity in the area

The effects of water disposal injection wells aremore subtle than that of the ponds and are especiallydifficult to identify in sands closer to the base of USDWwhere resistivity is naturally fairly low Geophysicallogs in sands affected by saline injection tend to have aconcave resistivity profile and resistivities are as small ashalf that in logs fromolder unaffectedwells (Figure 11)Injected saline water tends to follow individual sand

layers leaving some layers relatively unaffected In somecases only the lower part of an individual sand containssaline water whereas the waters in the upper part re-mainbrackishThis suggests that thewaters are stratifiedand mixing has not had time to occur Log-calculatedTDS concentrations show a relatively smooth increasewith depth inwells logged prior to the start of large-scaledisposal activities whereas recent logs in these areasshow a much more variable TDS concentration profilewith depth Additionally densityndashneutron curves inwells affected by disposal of produced waters show thinintervals (~1 m [~3 ft] thick) of crossover This may becaused by bacterial metabolism of trace amounts oforganic matter in the disposal water resulting in thegeneration of small amounts of biogenic methane(McMahon et al 2018)

The cross section in Figure 12 shows wells affectedby saline produced water disposal by injection (belowthe CCE) and percolation (above the CCE) along theeast flank of the South Belridge oil field Wells affectedby disposal of produced waters in saline ponds (n = 139)and by disposal in injection wells (n = 125) are alsoshown in the map in Figure 13 The effects of disposalponds are evident from changes in borehole resistivity

Figure 12 Cross section showing intervals affected by disposal of saline produced water (pink boxes) at South Belridge oil field Wellsidentified by API number at top of each well Year drilled and logged shown at bottom of eachwell Insetmap shows location of cross sectionProduced water disposal wells are highlighted in pink and disposal ponds are shown in blue in the inset map

92 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Figure 10 Cross section showing three wells drilled and logged in different years near a produced water disposal pond (light blue polygon on inset map) Yellow shading onspontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivity curve (right track at scale of 0ndash20 ohmm) indicate resistivity of more than 3 ohmm Pinkbars indicate low resistivity sands affected by infiltration of saline produced water from the pond The API numbers at the top of the logs are used for well identification Year drilled andlogged in parentheses Depth scale in feet

90Groundwater

SalinityatLostHillsndashBelridge

OilFieldsKern

CountyCalifornia

This may be caused by the presence of an active waterdisposal field in the Tulare Formation near this areaOverall the log-derived map shows base of USDWapproximately 61m (~200 ft) deeper than lab analyseswould suggest As at Lost Hills this may be caused bythe geographic distribution of the two data sets the labsamples coming mainly from within the oil fieldboundaries whereas the log data are confined to thearea outside the oil field

In the southern part of South Belridge field a wellnear the southern tip of the field was sampled at mul-tiple intervals Samples as deep as 455 m (~1495 ft) areUSDW and the deepest sample from 480 m (1575 ft)has a TDS value of 13074 mgL suggesting that thebase of USDW in the southern part of South Belridge liesbetween 457 and 480 m (1495ndash1575 ft) (Figure 9) Thedepth to USDW map from log analysis in the southernpart of South Belridge shows that the base of USDW

ranges from 365 m (1200 ft) in the north to approxi-mately 487 m (~1600 ft) in the south

Effects of Oil Field Activities on Water Salinity

The effects of water disposal in surface ponds on ground-water salinity is evident by comparing geophysical logsfrom old wells near the pond locations to logs fromnewerwells drilled nearbyOlder geophysical logs (pre-1970s) indicate that the upper sands in the alluviumare desaturated (vadose zone) as indicated by areas ofcrossover on densityndashneutron logs and have high re-sistivity (gt20ohmmFigure 10)However geophysicallogs collected from wells drilled later in the area donot show densityndashneutron crossover characteristic of avadose zone andhave extremely low resistivity (lt1ohmm)in the upper alluvial sands because of the presence ofhigh salinityproducedwater thathasfilled thevadosezoneInvasion of low resistivity water is especially apparent

Figure 11 Two adjacent wells drilled and logged in different years (1990 and 2012) in an area containing numerous disposal wells on theeast flank of South Belridge Yellow shading on spontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivitycurve indicates resistivity greater than 3 ohm m Pink bars indicate low resistivity sands in the 2012 well (0403046566) affected by injectedsaline produced water The API numbers at the top of the logs are used for well identification Year drilled and logged in parentheses

Gillespie et al 91

where saline waters have infiltrated the 22K sandwhich lies directly above the CCE (well 03047481 inFigure 10) in the lower part of the alluvium The salinewater lies atop the CCE which appears to be pro-tecting the underlying Tulare aquifer from contami-nation Log-calculated andmeasuredTDS concentrationscan exceed 20000mgL in affected parts of the alluvium

The effects of surface disposal ponds are less evidentin Lost Hills and North Belridge compared to the areaadjacent to South Belridge Lost Hills in particular tends tohave highly saline water in the shallow parts of the aquifereven where disposal ponds are not present Swain andDuell (1989) noted TDS concentrations as high as 21700mgL from very shallowwells (6m [~20 ft]) along the eastflank of Lost Hills but did not note the cause of the highsalinity in the area

The effects of water disposal injection wells aremore subtle than that of the ponds and are especiallydifficult to identify in sands closer to the base of USDWwhere resistivity is naturally fairly low Geophysicallogs in sands affected by saline injection tend to have aconcave resistivity profile and resistivities are as small ashalf that in logs fromolder unaffectedwells (Figure 11)Injected saline water tends to follow individual sand

layers leaving some layers relatively unaffected In somecases only the lower part of an individual sand containssaline water whereas the waters in the upper part re-mainbrackishThis suggests that thewaters are stratifiedand mixing has not had time to occur Log-calculatedTDS concentrations show a relatively smooth increasewith depth inwells logged prior to the start of large-scaledisposal activities whereas recent logs in these areasshow a much more variable TDS concentration profilewith depth Additionally densityndashneutron curves inwells affected by disposal of produced waters show thinintervals (~1 m [~3 ft] thick) of crossover This may becaused by bacterial metabolism of trace amounts oforganic matter in the disposal water resulting in thegeneration of small amounts of biogenic methane(McMahon et al 2018)

The cross section in Figure 12 shows wells affectedby saline produced water disposal by injection (belowthe CCE) and percolation (above the CCE) along theeast flank of the South Belridge oil field Wells affectedby disposal of produced waters in saline ponds (n = 139)and by disposal in injection wells (n = 125) are alsoshown in the map in Figure 13 The effects of disposalponds are evident from changes in borehole resistivity

Figure 12 Cross section showing intervals affected by disposal of saline produced water (pink boxes) at South Belridge oil field Wellsidentified by API number at top of each well Year drilled and logged shown at bottom of eachwell Insetmap shows location of cross sectionProduced water disposal wells are highlighted in pink and disposal ponds are shown in blue in the inset map

92 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

This may be caused by the presence of an active waterdisposal field in the Tulare Formation near this areaOverall the log-derived map shows base of USDWapproximately 61m (~200 ft) deeper than lab analyseswould suggest As at Lost Hills this may be caused bythe geographic distribution of the two data sets the labsamples coming mainly from within the oil fieldboundaries whereas the log data are confined to thearea outside the oil field

In the southern part of South Belridge field a wellnear the southern tip of the field was sampled at mul-tiple intervals Samples as deep as 455 m (~1495 ft) areUSDW and the deepest sample from 480 m (1575 ft)has a TDS value of 13074 mgL suggesting that thebase of USDW in the southern part of South Belridge liesbetween 457 and 480 m (1495ndash1575 ft) (Figure 9) Thedepth to USDW map from log analysis in the southernpart of South Belridge shows that the base of USDW

ranges from 365 m (1200 ft) in the north to approxi-mately 487 m (~1600 ft) in the south

Effects of Oil Field Activities on Water Salinity

The effects of water disposal in surface ponds on ground-water salinity is evident by comparing geophysical logsfrom old wells near the pond locations to logs fromnewerwells drilled nearbyOlder geophysical logs (pre-1970s) indicate that the upper sands in the alluviumare desaturated (vadose zone) as indicated by areas ofcrossover on densityndashneutron logs and have high re-sistivity (gt20ohmmFigure 10)However geophysicallogs collected from wells drilled later in the area donot show densityndashneutron crossover characteristic of avadose zone andhave extremely low resistivity (lt1ohmm)in the upper alluvial sands because of the presence ofhigh salinityproducedwater thathasfilled thevadosezoneInvasion of low resistivity water is especially apparent

Figure 11 Two adjacent wells drilled and logged in different years (1990 and 2012) in an area containing numerous disposal wells on theeast flank of South Belridge Yellow shading on spontaneous potential (SP) curve (left track) indicate sand layers Yellow shading on resistivitycurve indicates resistivity greater than 3 ohm m Pink bars indicate low resistivity sands in the 2012 well (0403046566) affected by injectedsaline produced water The API numbers at the top of the logs are used for well identification Year drilled and logged in parentheses

Gillespie et al 91

where saline waters have infiltrated the 22K sandwhich lies directly above the CCE (well 03047481 inFigure 10) in the lower part of the alluvium The salinewater lies atop the CCE which appears to be pro-tecting the underlying Tulare aquifer from contami-nation Log-calculated andmeasuredTDS concentrationscan exceed 20000mgL in affected parts of the alluvium

The effects of surface disposal ponds are less evidentin Lost Hills and North Belridge compared to the areaadjacent to South Belridge Lost Hills in particular tends tohave highly saline water in the shallow parts of the aquifereven where disposal ponds are not present Swain andDuell (1989) noted TDS concentrations as high as 21700mgL from very shallowwells (6m [~20 ft]) along the eastflank of Lost Hills but did not note the cause of the highsalinity in the area

The effects of water disposal injection wells aremore subtle than that of the ponds and are especiallydifficult to identify in sands closer to the base of USDWwhere resistivity is naturally fairly low Geophysicallogs in sands affected by saline injection tend to have aconcave resistivity profile and resistivities are as small ashalf that in logs fromolder unaffectedwells (Figure 11)Injected saline water tends to follow individual sand

layers leaving some layers relatively unaffected In somecases only the lower part of an individual sand containssaline water whereas the waters in the upper part re-mainbrackishThis suggests that thewaters are stratifiedand mixing has not had time to occur Log-calculatedTDS concentrations show a relatively smooth increasewith depth inwells logged prior to the start of large-scaledisposal activities whereas recent logs in these areasshow a much more variable TDS concentration profilewith depth Additionally densityndashneutron curves inwells affected by disposal of produced waters show thinintervals (~1 m [~3 ft] thick) of crossover This may becaused by bacterial metabolism of trace amounts oforganic matter in the disposal water resulting in thegeneration of small amounts of biogenic methane(McMahon et al 2018)

The cross section in Figure 12 shows wells affectedby saline produced water disposal by injection (belowthe CCE) and percolation (above the CCE) along theeast flank of the South Belridge oil field Wells affectedby disposal of produced waters in saline ponds (n = 139)and by disposal in injection wells (n = 125) are alsoshown in the map in Figure 13 The effects of disposalponds are evident from changes in borehole resistivity

Figure 12 Cross section showing intervals affected by disposal of saline produced water (pink boxes) at South Belridge oil field Wellsidentified by API number at top of each well Year drilled and logged shown at bottom of eachwell Insetmap shows location of cross sectionProduced water disposal wells are highlighted in pink and disposal ponds are shown in blue in the inset map

92 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

where saline waters have infiltrated the 22K sandwhich lies directly above the CCE (well 03047481 inFigure 10) in the lower part of the alluvium The salinewater lies atop the CCE which appears to be pro-tecting the underlying Tulare aquifer from contami-nation Log-calculated andmeasuredTDS concentrationscan exceed 20000mgL in affected parts of the alluvium

The effects of surface disposal ponds are less evidentin Lost Hills and North Belridge compared to the areaadjacent to South Belridge Lost Hills in particular tends tohave highly saline water in the shallow parts of the aquifereven where disposal ponds are not present Swain andDuell (1989) noted TDS concentrations as high as 21700mgL from very shallowwells (6m [~20 ft]) along the eastflank of Lost Hills but did not note the cause of the highsalinity in the area

The effects of water disposal injection wells aremore subtle than that of the ponds and are especiallydifficult to identify in sands closer to the base of USDWwhere resistivity is naturally fairly low Geophysicallogs in sands affected by saline injection tend to have aconcave resistivity profile and resistivities are as small ashalf that in logs fromolder unaffectedwells (Figure 11)Injected saline water tends to follow individual sand

layers leaving some layers relatively unaffected In somecases only the lower part of an individual sand containssaline water whereas the waters in the upper part re-mainbrackishThis suggests that thewaters are stratifiedand mixing has not had time to occur Log-calculatedTDS concentrations show a relatively smooth increasewith depth inwells logged prior to the start of large-scaledisposal activities whereas recent logs in these areasshow a much more variable TDS concentration profilewith depth Additionally densityndashneutron curves inwells affected by disposal of produced waters show thinintervals (~1 m [~3 ft] thick) of crossover This may becaused by bacterial metabolism of trace amounts oforganic matter in the disposal water resulting in thegeneration of small amounts of biogenic methane(McMahon et al 2018)

The cross section in Figure 12 shows wells affectedby saline produced water disposal by injection (belowthe CCE) and percolation (above the CCE) along theeast flank of the South Belridge oil field Wells affectedby disposal of produced waters in saline ponds (n = 139)and by disposal in injection wells (n = 125) are alsoshown in the map in Figure 13 The effects of disposalponds are evident from changes in borehole resistivity

Figure 12 Cross section showing intervals affected by disposal of saline produced water (pink boxes) at South Belridge oil field Wellsidentified by API number at top of each well Year drilled and logged shown at bottom of eachwell Insetmap shows location of cross sectionProduced water disposal wells are highlighted in pink and disposal ponds are shown in blue in the inset map

92 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

at least 1525 m (5000 ft) in a downgradient direction(east) from some of the ponds These borehole geo-physical log analyses help fill in a more detailed un-derstanding of the areal and vertical extent of salinewater movement near historical disposal ponds Watersample data from monitoring wells downgradient ofselected historical ponds support the log analysis andindicate saline water movement above the CCE of upto several kilometers to the east (California State WaterResources Control Board 2018a) The effects of in-jection wells can be observed in geophysical logs fromnew wells (ca 2013ndash2015) drilled within the oil fieldboundary at least 550 m (~1800 ft) west of the mainTulare water disposal wellfield along the central partof the east flank of South Belridge No new wells areavailable east (downgradient) of the disposal field be-cause that area is outside the oil field limits and newdrilling has not occurred Existing monitoring wellsare not deep enough to determine the effects of injec-tion disposal east of the field

CONCLUSIONS

The Tulare Formation and overlying alluvium con-tain most of the groundwater classified as USDW in thestudy area The thickness of these formations ranges from

approximately 75 m (~250 ft) in North Belridge andnorthern Lost Hills to more than 1200 m (gt~4000 ft) inthe southeastern part of the study area Log-generatedmaps of groundwater salinity and geochemical analysesindicate that depth to protected waters ranges from150 m (~500 ft) in the northwestern part of the studyarea to 490 m (1600 ft) in the southeast and 550 m(~1800 ft) east of Lost Hills

TheTulare Formation containsmultiple clay layersthe largest and most extensive being the CCE MiddleTulare and Amnicola clays Pressure gradients changeacross the confining layers particularly near the SouthBelridge field The larger shifts in pressure gradientsacross the clay layers near South Belridge may indicategreater confining ability of the clays near the fieldhowever the shift in gradient may also indicate thatproduction and injection activities within the variousaquifer layers near the field have not allowed sufficienttime for the pressure to reach equilibrium throughoutthe formation

Disposal of produced waters in both surface pondsand injection wells have affected water salinity over timenear the oil fields Disposal in surface ponds has mainlyaffected the alluvial aquifer above the Tulare FormationWater with high salinity has filled the vadose zone abovethe water table at a distance of at least 1525 m (5000 ft)downgradient of the disposal ponds at South Belridge The

Figure 13 Wells highlighted in pink are wells affected by saline produced water from (A) disposal ponds and (B) water disposal wells inthe study area R20ndashR22E = Range 20ndash22 East T26ndash28S = Township 26ndash28 South

Gillespie et al 93

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

REFERENCES CITED

Archie G E 1942 The electrical resistivity log as an aid in de-termining some reservoir characteristics Transactions of theAIME v 146 p 54ndash62

Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

California Division of Oil Gas and Geothermal Resources 2019Annual reports of the state oil and gas supervisor accessedJune 30 2018 httpwwwconservationcagovdogpubs_statsannual_reportsPagesannual_reportsaspx

California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

California State Water Resources Control Board 2018b Water qualityin areas of oil and gas production ndash Rregional groundwater moni-toring accessed January 12 2018 httpswwwwaterboardscagovwater_issuesprogramsgroundwatersb4regional_monitoringindexshtml

Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

CCE appears to protect the underlying Tulare For-mation from the downward percolation of water fromdisposal ponds within the area analyzed The effects ofproduced water injection are mainly evident withinthe Tulare Formation and can be noted as far as 550 m(1800 ft) from the main group of disposal wells lo-cated along the east flank of South Belridge no recentgeophysical logs farther downgradient were availablefor analysis

Geophysical logs from newer wells with sands af-fected by produced water injection have resistivitiesapproximately half that of geophysical logs from wellsdrilled prior to or early in the course of produced waterinjection This saline water tends to occur within thelower parts of affected sands because of gravity segregationand is associated with thin areas of densityndashneutroncrossover perhaps indicative of methane producedin situ by microbial degradation of organic matter inthe produced water This analysis demonstrates theutility of borehole geophysical data in understandinggroundwater salinities at different spatial and tem-poral scales and an approach for constructing regionalpatterns based on site-specific information

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Atwater T 1970 Implications of plate tectonics for the Cenozoictectonic evolution of western North America Geological So-ciety of America Bulletin v 81 no 12 p 3513ndash3536 doi1011300016-7606(1970)81[3513IOPTFT]20CO2

Bailey W C 1939 North Belridge oilfield California Division ofOil and Gas Summary of Operations v 24 no 3 p 72ndash77

Bartow JA 1991TheCenozoic evolution of theSan JoaquinValleyCalifornia Reston Virginia US Geological Survey ProfessionalPaper 1501 40 p doi103133pp1501

Bateman R M and C E Konen 1977 The log analyst and theprogrammable pocket calculator Log Analyst v 18 no 5p 3ndash11

Belitz K and F J Heimes 1990 Character and evolution of theground-water flow system in the central part of the western SanJoaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 2348 28 p

Berryman W M 1973 Lithologic characteristics of Pliocene rockscored at Elk Hills Kern County California Reston Virginia USGeological Survey Bulletin 1332-D 54 p

Blondes M S K D Gans E L Rowan J J Thordsen M EReidy M A Engle Y K Kharaka and B Thomas 2016US Geological Survey national produced waters geo-chemical database v 22 (provisional) accessed June 172016 httpenergyusgsgovEnvironmentalAspectsEnvironmentalAspectsofEnergyProductionandUseProducedWatersaspx3822349-data

Burow K R J L Shelton J A Hevesi and G S Weissmann 2004Hydrogeologic characterization of the Modesto area SanJoaquin Valley California Reston Virginia US GeologicalSurvey Scientific Investigations Report 2004-5232 54 p doi103133sir20045232

California Division of Oil Gas and Geothermal Resources 1998California oil and gas fields volume 1-Central California Sac-ramento California California Department of ConservationDivision of Oil Gas and Geothermal Resources 507 p accessedMay 26 2018 ftpftpconsrvcagovpuboilpublicationsDatasheetsDtasheet_vol_1pdf

California Division of Oil Gas and Geothermal Resources 2015Summary of operations reports accessed June 30 2018 ftpftpconsrvcagovpuboilSummary_of_Operations

California Division of Oil Gas and Geothermal Resources 2016Produced water geochemical database accessed May 26 2018ftpftpconsrvcagovpuboilchemical_analysis

California Division of Oil Gas and Geothermal Resources 2017 2016Report of California oil and gas production statistics accessedMay 26 2018 ftpftpconsrvcagovpuboilannual_reports20162016_Annual_Report_Final_Corrected2pdf

California Division of Oil Gas and Geothermal Resources 2018Wellfinder accessed May 25 2018 httpwwwconservationcagovdogPagesWellfinderaspx

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California State Water Resources Control Board 2018a Geotrackeradvanced search oil and gas monitoring produced water pondsaccessed October 12 2018 httpsgeotrackerwaterboardscagovsearch

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Coburn M G and J M Gillespie 2002 A hydrogeologic study tooptimize steamflood permformance in a giant oilfield KernRiver field California AAPGBulletin v 86 no 8 p 1489ndash1505doi10130661EEDCD8-173E-11D7-8645000102C1865D

Croft M G 1972 Subsurface geology of the late Tertiary andQuaternary water-bearing deposits of the southern part of theSan Joaquin Valley California Reston Virginia US GeologicalSurvey Water Supply Paper 1999H 29 p

Dale B A G M Narahara and R M Stevens 1996 A case historyof reservoir subsidence and wellbore damage management inthe South Belridge diatomite field Society of Petroleum Engi-neers Western Regional Meeting Anchorage Alaska May22ndash24 1996 SPE-35658-MS p 101ndash113

Eastern Municipal Water District 2019 Water supply desalina-tion infrastructure Perris II desalter accessed August 92019 httpswwwemwdorgsitesmainfilesfile-attachmentsperrisiidesalterpdf

FauntCC ed 2009Groundwater availability of theCentralValleyaquifer California Reston Virginia US Geological SurveyProfessional Paper 1766 225 p

Frink J W and H A Kues 1954 Corcoran ClaymdashA Pleistocenelacustrine deposit in San Joaquin Valley California AAPGv 38 no 11 p 2357ndash2371

Gillespie J D Kong and S D Anderson 2017 Groundwatersalinity in the southern San Joaquin Valley AAPG Bulletinv 101 no 8 p 1239ndash1261 doi10130609021616043

94 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California

Gillespie JM TA Davis LB Ball PJ Herrera Z Wolpe VMedrano M Bobbitt and MJ Stephens 2019 Geologicalgeochemical and geophysical data from the Lost Hills andBelridge oil fields Reston Virginia US Geological Survey DataRelease 14 p doi105066P90QH6CI

Graham S A 1987 Tectonic controls on petroleum occurrence inCalifornia in R V Ingersoll and W G Ernst eds The geo-tectonic development of California Rubey 6th ed EnglewoodCliffs New Jersey Prentice-Hall 47ndash63

Hamlin H S and L de la Rocha 2015 Using electric logs to es-timate groundwater salinity and map brackish groundwaterresources in the Carrizo-Wilcox aquifer in south Texas (abs)Gulf Coast Association of Geological Societies HoustonTexas September 20ndash22 2015 accessed December 15 2017httpwwwsearchanddiscoverycomabstractshtml201590219gcagsabstracts91html

Howells L 1990 Base of moderately saline ground water in SanJuan County Utah Salt Lake City Utah State of Utah De-partment of Natural Resources Technical Publication 94 35 p

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Land P E 1984 Lost Hills oil field Sacramento California Cal-ifornia Division of Oil and Gas Publication TR32 p 3ndash16

Laudon J and K Belitz 1991 Texture and depositional history oflate Pleistocene-Holocene alluvium in the central part of thewestern San Joaquin Valley California Bulletin of the Associ-ation of Engineering Geologists v 28 no 1 p 73ndash88

Leitz F and W Boegli 2011 Evaluation of the Port HuenemeDemonstration Plant An analysis of 1 MGD reverse osmosisnanofiltration and electrodialysis reversal plants under nearlyidentical conditions Springfield Virginia US Bureau of Rec-lamation Desalination and Water Purification Research andDevelopment Program Report 65 accessed January 12 2018httpswwwusbrgovresearchdwprreportpdfsreport065pdf

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Williamson A K D E Prudic and L A Swain 1989 Ground-water flow in the Central Valley California Regional AquiferSystemAnalysismdashCentralValleyCalifornia RestonVirginiaUSGeological Survey Professional Paper 1401-D 127 p

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Reston Virginia US Geological SurveyWater Supply Paper1457 141 p

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WylieMR J andWDRose 1950Some theoretical considerationsrelated to the quantitative evaluation of the physical characteris-tics of reservoir rocks from electric log data Journal of Petro-leum technology v 2 no 4 SPE-950105-G 14 p

Young R J 1968 West Side oilfields Subsurface map commit-tee in S E Karp ed Guidebook geology and oilfields Westside southern San Joaquin Valley AAPG Pacific Sectionp 56ndash85

96 Groundwater Salinity at Lost HillsndashBelridge Oil Fields Kern County California