Abnormal Pressure in Hydrocarbon Environments.pdf

1INTRODUCTIONThrough the years there has been an evolution of

ideas or concepts concerning the cause(s) of abnormalpressure, as well as reasons for studying abnormalpressures. Most studies of abnormal pressures prior tothe mid-1980s were driven by the concern for drillingand completion practices, as well as safety considera-tions during drilling. While those concerns are stillimportant, abnormal pressures are now importantcomponents of hydrocarbon exploration, field develop-ment, and resource assessment.

Some of the earlier proposed causal mechanisms ofabnormal pressure, such as mineral transformations,osmosis, and tectonics have given way to additionalcauses such as compaction disequilibrium, hydrocar-bon generation, and aquathermal expansion. Withinthe last 15 years there has been a realization that, insome cases, the processes involved in the generation,expulsion, migration, and entrapment of hydrocarbonsare the same processes responsible for the developmentof abnormal fluid pressures. In addition, the concept ofpressure compartments with vertical and lateral sealsnow play a major role in the exploration for hydrocar-

bons. Therefore, the study of abnormal pressures is notonly important for purposes of hydrocarbon exploita-tion, but is also now recognized as an important com-ponent of hydrocarbon exploration.

As a consequence of these ongoing developments inthe evolution of abnormal pressure studies, this inves-tigation was initiated to provide information on theglobal distribution of abnormal pressures, examine therelationships among various attributes of abnormalpressure, and evaluate relationships that may occurbetween the occurrence of abnormal pressures and theoccurrence of hydrocarbon accumulations. The conclu-sions of this study are largely based on the evaluationof previously published literature and the authors col-lective experience.

ATTRIBUTES OF ABNORMAL PRESSURES

Global DistributionAbnormal pressures occur in a wide range of geo-

graphic and geologic conditions. Figure 1 shows the

AbstractAbnormal pressures, pressures above or below hydrostatic pressures, occur on all continents in a

wide range of geological conditions. According to a survey of published literature on abnormal pres-sures, compaction disequilibrium and hydrocarbon generation are the two most commonly cited caus-es of abnormally high pressure in petroleum provinces. In young (Tertiary) deltaic sequences, com-paction disequilibrium is the dominant cause of abnormal pressure. In older (pre-Tertiary) lithifiedrocks, hydrocarbon generation, aquathermal expansion, and tectonics are most often cited as the causesof abnormal pressure.

The association of abnormal pressures with hydrocarbon accumulations is statistically significant.Within abnormally pressured reservoirs, empirical evidence indicates that the bulk of economicallyrecoverable oil and gas occurs in reservoirs with pressure gradients less than 0.75 psi/ft (17.4 kPa/m)and there is very little production potential from reservoirs that exceed 0.85 psi/ft (19.6 kPa/m). Abnor-mally pressured rocks are also commonly associated with unconventional gas accumulations where thepressuring phase is gas of either a thermal or microbial origin. In underpressured, thermally maturerocks, the affected reservoirs have most often experienced a significant cooling history and probablyevolved from an originally overpressured system.

Law, B.E., and C.W. Spencer, 1998, Abnormal pressures in hydrocarbon environments, in Law, B.E., G.F. Ulmishek, and V.I. Slavin eds., Abnormalpressures in hydrocarbon environments: AAPG Memoir 70, p.111.

Chapter 1

Abnormal Pressure inHydrocarbon Environments

Ben E. Law1C. W. SpencerU.S. Geological SurveyDenver, Colorado, U.S.A.

1 Present Affiliation: Consulting Petroleum Geologist, Lakewood, Colorado, U.S.A.

global distribution of abnormal pressures. This dis-tribution reflects information available in the literatureand the experience of the authors. There are undoubt-edly many additional areas of abnormal pressureeither not identified or not reported in the literature. Inthis compilation, we have attempted to show onlythose regions associated with petroleum provinces. Inmany cases, the areal distribution of abnormal pres-sures is not known or was not defined in our sources ofinformation, so we have shown the entire basin or region.

Based on our compilation of the occurrence ofabnormal pressures, there are approximately 150 geo-graphic locations around the world known to beabnormally pressured (Figure 1). Hunt (1990) has indi-cated that abnormal pressures have been identified inabout 180 basins. In many of these areas, however,there are more than one abnormally pressured strati-graphic unit or zone. For example, in the U.S. GulfCoast region there are at least seven stratigraphic unitsranging in age from Jurassic to Recent that are abnor-mally pressured. Nearly all of the abnormally pres-sured regions shown are overpressured. Only about 12of the areas in Figure 1 are underpressured. Under-pressure is much more difficult to identify duringdrilling than overpressure, consequently more over-pressured systems have been identified than under-pressured systems.

The distribution of abnormal pressures (Figure 1)appears to favor the northern hemisphere, even thoughthere are no readily apparent reasons why there shouldbe a preferential occurrence of abnormal pressuresthere. We suspect that this unequal distribution merelyreflects the relatively larger number of investigationsconducted in the northern hemisphere. For example,the large number of abnormally pressured areas shownon Figure 1 in the Rocky Mountain region of the Unit-ed States is a consequence of several, detailed investi-gations of abnormally pressured, unconventional gasreservoirs. In this region and elsewhere in North Amer-ica, investigators have noted the close association ofhydrocarbon accumulations, particularly unconven-tional gas accumulations, and abnormal pressures.Conversely, the relatively few number of abnormallypressured areas in the Andean region of South Ameri-ca, probably reflects differences in exploration objec-tives and perhaps an unawareness of the association ofabnormal pressures and hydrocarbons.

Causal Mechanisms of Abnormal Pressure

While it is not our intention to review all aspects ofabnormal pressures, we have tabulated some of themore important attributes of abnormally pressuredrocks (Table 1) in an attempt to identify those attribut-es that may have a bearing on the cause(s) of abnormalpressure. From an examination of this compilation,attributes such as depth to the top of abnormal pres-sure and structural province do not appear to renderany useful information regarding the cause of abnor-

mal pressure, other than documenting the variabilityof depth and structural settings within which abnor-mally pressured rocks occur. However, attributes suchas the geologic age of abnormally pressured rocks,their depositional setting, maximum pressure, natureof the seal, temperature, and thermal maturity doreveal useful information concerning the cause(s) ofabnormal pressuring.

Because of this wide range of variability, the cause(s)of abnormal pressure are often difficult to determineand may involve more than one process. Swarbrickand Osborne (1998-this volume) provide a comprehen-sive list and discussion of the mechanisms of abnormalpressure. Some of the more notable publishedoverviews of the different mechanisms of abnormalpressures include those by Fertl (1976), Mouchet andMitchell (1989), and Fertl et al. (1994).

Of all the causes of abnormal pressures referred to inthe literature: compaction disequilibrium, aquathermalexpansion, hydrocarbon generation, mineral transfor-mations, tectonics, and osmosis; the most commonlycited cause of abnormally high pressure is compactiondisequilibrium. And in nearly all cases where com-paction disequilibrium has been determined to be theprimary cause of overpressuring, the age of the rocks isgeologically young. Examples of areas where com-paction disequilibrium is cited as the primary cause ofabnormal pressure include the U.S. Gulf Coast, NigerDelta, Mahakam Delta, MacKenzie River Delta, NorthSea, Adriatic Sea, the Nile Delta, and the PotwarPlateau of Pakistan (Figure 1, Table 1). In these areas,the age of the abnormally pressured rocks is Tertiary,the depositional setting is dominantly deltaic, and thelithology is dominantly shale. A notable exception isthe highly overpressured Neogene rock sequence inthe Potwar Plateau of Pakistan (Figure 1), where thedominant lithology is sandstone (Law et al., 1998-thisvolume). The most commonly cited depositional envi-ronment for abnormally pressured rocks is deltaic.

In pre-Tertiary rocks, the main causes of abnormalpressure include hydrocarbon generation, aquather-mal expansion, mineral transformations, and tectonicdeformationwith hydrocarbon generation cited as themost common cause. In our judgment, hydrocarbongeneration as a cause of abnormal pressure has beenunder-evaluated.

The relationship between the cause of abnormalpressuring in young versus old rocks suggests thatthere may be a continuum of processes responsible forthe development of abnormal pressure. We are of theopinion that pressures are time transient and that pres-sure causing mechanisms are also transient. For exam-ple, Law and Dickinson (1985) presented a conceptualmodel for the origin of abnormal pressures in low-per-meability rocks that involved hydrocarbon generation.In their model, abnormal high pressures were initiallycaused by hydrocarbon generation. With subsequentchanges of structural uplift, erosion, and temperaturereduction during the burial and thermal history, the

2 Law and Spencer

overpressured rocks evolved into an underpressuredphase. And finally, at an even later burial history, Lawand Dickinson theorized that the underpressured rockswould evolve into a normally pressured system. Inves-tigations by Dickey and Cox (1977) and Dor andJensen (1996) have also called on uplift, erosion, andcooling as a cause of underpressuring, but have notproposed an earlier pressure history of overpressuring.

We speculate that in some cases, such as in deltaicsystems with high rates of deposition, abnormal pres-sures may be initiated by compaction disequilibrium.As these deltaic sediments are buried deeper and expe-rience higher temperatures, hydrocarbon generationmay supplant compaction disequilbrium as the maincause of abnormally high pressure. In deltaic rocksequences where the hydrocarbon source rock occursstratigraphically below the compaction disequilibri-um-affected sediments, the generation of hydrocar-bons from these source rocks may result in the devel-opment of overpressure which could be physicallytransferred upward, via the development of a pressuregradient, into the region of compaction disequilibrium.

Hunt et al. (1994; 1998-this volume) have proposedan abnormal pressure mechanism of hydrocarbon gen-eration for the U.S. Gulf Coast. In our opinion, theobservations by Leach (1993a, b, c) of the close associa-tion of productive oil and gas fields and the top ofoverpressure in southern Louisiana are also suggestiveof the role of hydrocarbon generation in the develop-ment of overpressure. Alternatively, basin modeling byBurrus (1998-this volume) attributes the origin of over-pressuring in the U.S. Gulf Coast almost exclusively tocompaction disequilibrium.

HYDROCARBON ACCUMULATIONSAND ABNORMAL PRESSURES

Hydrocarbon accumulations are frequently found inclose association with abnormal pressures. In abnor-mally pressured, conventionally trapped oil and gasaccumulations, pressures above hydrostatic are com-mon. However, as Chapman (1994) points out, some ofthese abnormal pressures are normal for their fluidsand are a function of the densities of the fluid and theheight of the oil and gas column above the oil-gas/watercontact. Therefore, such abnormal pressures are notdue to processes such as compaction disequilibrium,aquathermal expansion, or hydrocarbon generationand are not considered here to be abnormally pres-sured. Discounting these abnormally pressuredhydrocarbon accumulations, the association of trulyabnormal pressures and hydrocarbon accumulationshave been noted in several studies of conventionallyand unconventionally trapped hydrocarbons.

In the U.S. Gulf Coast, Burst (1969) noted thathydrocarbon production was evenly distributed abouta depth 1,500 ft (460 m) above the depths of his 2nddehydration stage of clays (top of overpressure). Sub-

sequent studies by Fertl and Leach (1990) and Leach(1993a, b, c) in southern Louisiana have also shownspatial relationships between the top of overpressuringand the accumulation of oil and gas fields. A statisticalevaluation by Leach (1993a) of oil and gas productionfrom Tertiary reservoirs in southern Louisiana showedthat almost half (46.1%) of the oil production was froman interval 2,000 ft (610 m) above the top of overpres-suring and that nearly half of the gas production camefrom a 2,000 ft (610 m) interval centered around the topof overpressuring.

Other studies in the U.S. Gulf Coast by Timko andFertl (1971) and Leach (1993a, b, c) noted that oil andgas production decreased with increasing pressure,and at pressure gradients approaching 0.85 psi/ft (19.6kPa/m) there was a marked decrease in production.Leach (1993b) concluded that gradients of 0.85 psi/ft(19.6 kPa/m) or higher exceed the fracture gradients ofmost sandstone reservoirs. Consequently, hydrocar-bons that may have originally been trapped in thesehigh-pressure reservoirs may have been lost throughpressure-induced fractures. Similar observations of therelationship between the distribution of hydrocarbonsand abnormal pressures have been proposed by Dow(1984) in the U.S. Gulf Coast and by Schaar (1976) in theBaram Delta of Sarawak. In the Nile Delta and NorthSinai basins of Egypt, Nashaat (1998-this volume) con-cluded that hydrocarbon production is precluded inreservoirs that exceed 0.85 psi/ft (19.6 kPa/m). Hep-pard et al. (1998-this volume) noted that oil and gasproduction in the Trinidad, West Indies area wasrestricted to reservoirs with pore pressures gradientsless than 0.73 psi/ft (16.9 kPa/m). In the former SovietUnion, Belonin and Slavin (1998-this volume) observedthat most oil and gas production from abnormally pres-sured reservoirs occurred at abnormality coefficients(measured pore pressure divided by hydrostatic pres-sure) less than 1.8 (assuming a hydrostatic gradient of0.45 psi/ft [10.2 kPa/m], an abnormality coefficient of1.8 is equal to about 0.81 psi/ft [18.7 kPa/m]). In theSichuan Basin of China, Da-jun and Yun-ho (1994) relat-ed the presence of natural fractures to the magnitude ofpressure. They presented pressure data from gas-pro-ductive, Permian carbonate reservoirs showing thatgradients greater than 0.63 psi/ft (14.2 kPa/m) areindicative of relatively small fields.

The association of hydrocarbon accumulations andabnormal pressure is even more evident in unconven-tional gas accumulations. For example, coalbedmethane, shale gas, basin-centered gas, and low-per-meability microbial gas are nearly always associatedwith abnormal pressures.

Gas in shale and coal are self-sourced reservoirs thatare commonly abnormally pressured. In the Appal-achian Basin, oil and gas are produced from organical-ly-rich, Devonian shale (de Witt, 1984; Reeves et al.,1996). In some productive regions in the AppalachianBasin, oil and gas are produced from fractured, under-pressured shale (Hunter, 1962; de Witt, 1984). Some

Abnormal Pressures in Hydrocarbon Environments 3

4Law

and Spencer

Figure 1. Map showing the global distribution of abnormal pressures. Heavier shaded, diagonally ruled patterns are used to avoid masking of darker pat-terned areas listed on Table 1. Index numbers adjacent to selected abnormally pressured areas refer to additional data provided in Table 1.

Table 1. Selected attributes of abnormally pressured regions of the world. Locations of the regions are shown on Figure 1 and are linked by the IndexNumber of the region.

Abnorm

al Pressures in H

ydrocarbon Environm

ents 5

ThermalDepth Maximum Temperature Maturity

Age of to Top of or Minimum Top of Top of AssociatedAbnormally Abnormal Pressure Abnormal Abnormal Fluid Hydrocarbon

Index Pressured Depositional Structural Pressure Gradients Pressure Pressure Pressure Hydrocarbon AccumulationRegion No. Rocks System Setting ft (m) psi/ft (kPa/m) Seal F (C) %Ro Phase Source & Age Type Cause(s) References Remarks

Alaska National Wildlife 1 Cretaceous marine foreland 10,000 0.84 150-163 0.55 uncertain Gautier et al. (1987) Conclusion based on 2 wells.Refuge nonmarine (3,000) (19.4) (66-73)Alaska, U.S.A. deltaic/fluvial

Beaufort Mackenzie 2 Paleozoic & deltaic passive 6,550-16,400 0.85 faults highly 0.75 water/gas structure Hitchon et al. (1990)Canada Miocene margin (2,000-5,000) (19.4) variable

Alberta Deep Basin 3 Triassic, Jurassic deltaic foreland >3,000 water 0.7-1.0 gas Cretaceous basin-center HC Masters (1979, 1984) Under & overpressured. SeeCanada Cretaceous marginal (>1,000) block* Type III OM gas Welte et al. (1984) Masters (1984) Fig. 18. Very little

marine pressure data, largely interpretive.

Jeanne dArc Basin 4 Jurassic marine failed 9,500-15,100 0.99 shale water/gas* Jurassic structure uncertain Rogers & Yassir (1993)Canada rift (2,900-4,600) (21.3)Scotian Shelf 5 Jurassic deltaic rift >14,750 0.87 >248 0.8 Jurassic- structure CD/HC Mudford & Best (1989)Canada (>4,500) (20.1) (>120) Cretaceous Rogers & Yassir (1993)Columbia Basin 6 Tertiary fluvial foreland 9,000-10,000 >0.80 water 200 0.8 gas Tertiary basin-center HC Law et al. (1994) Overpressured sequence coveredU.S.A. (2,700-3,000) (>18.5) block* (93) gas with thick basalts.Williston Basin 7 Mississippian marine cratonic 9,000 0.73 190 0.4-0.5 oil Mississippian basin-center HC Meissner (1978) Type I organic material.U.S.A. Devonian basin (2,740) (16.9) (88) Devonian oilPowder River 8 Cretaceous dominantly foreland 10,000 0.8 unconformity 200 0.5 gas & Cretaceous stratigraphic HC Spencer (1987)U.S.A. marine (3,000) (18.5) water block* (96) condensate Surdam et al. (1994)Big Horn Basin 9 Cretaceous fluvial & foreland 11,000 >0.6 ? water 200 >1.2 gas Cretaceous Spencer (1987) Experienced cooling associatedU.S.A. marine (3,350) (13.6 ?) block* (96) with uplift and erosion.Wind River Basin 10 Mississippian marine foreland variable 0.8 ? water block* 1.1 gas Cretaceous basin-center HC Bilyeu (1978)U.S.A. Cretaceous fluvial (18.5 ?) shale gas Johnson et al. (1996)

Tertiary

Greater Green River 11 Tertiary marine- foreland 8,000 0.9 water block 180-200 0.75-0.85 gas Cretaceous basin-center HC McPeek (1981) Underpressured gas accumulation atBasin, U.S.A. Cretaceous nonmarine (2,440) (20.8) (82-96) gas Law (1984) shallow depths above deeper,

deltaic/fluvial Spencer (1987) overpressured rocks.Piceance Basin 12 Mississippian fluvial to foreland 6,000-8,000 0.82 water block 165-200 >0.8 gas Cretaceous basin-center HC Spencer (1987) Locally overpressured CO2 inU.S.A. Cretaceous marine (1,830-2,440) (18.9) (74-96) gas Johnson et al. (1987) Mississippian carbonates, see

Wilson et al. (this volume).Uinta Basin 13 Tertiary lacustrine foreland 10,000 0.83 water 210 oil Tertiary basin-center HC Lucas & Drexler (1976)U.S.A. fluvial (3,000) (19.2) block (98) Type I OM oil Spencer (1987)Sacramento Basin 14 Cretaceous fluvial- forearc 3,900-10,000 0.85 gas* TC/AE Berry (1973) Diminished shale gas productionU.S.A. Tertiary deltaic (1,190-3,050) (19.7) Lico & Kharaka (1983) when pressure gradient >0.71 psi/ft

(>16.3 kPa/m).Anadarko Basin 15 Mississippian fluvial foreland 9,000-10,000 0.9 diagenetic 140-155 0.81-0.89 gas Devonian stratigraphic HC/O/CD Breeze (1970) Types II & III kerogen, Burrus &U.S.A. Pennsylvanian fluvial deltaic (2,740-3,050) (20.8) (60-66) Mississippian basin-center* Al-Shaieb et al. (1994) Hatch (1989).

marine, deltaic Pennsylvanian

Appalachian Basin 16 Silurian fluvial foreland >2,500 0.36* water gas Ordovician ? basin-center HC Russell (1972), Davis Underpressured, locally over-U.S.A. deltaic (>760) (8.37*) block* (1984), Zagorski (1988) pressured at depths >10,000 ft

marine Law & Spencer (1993) (>3,000 m).Gulf Coast 17 Jurassic marine passive 6,000-16,00 near shale oil/gas/water structure CD/HC Dickinson (1953) Numerous overpressured strati-U.S.A. Cretaceous deltaic margin (1,800-4,875) lithostatic Bradley (1975) graphic units. The worlds most

Tertiary extensively studied abnormallypressured region.

6Law

and Spencer

ThermalDepth Maximum Temperature Maturity

Age of to Top of or Minimum Top of Top of AssociatedAbnormally Abnormal Pressure Abnormal Abnormal Fluid Hydrocarbon

Index Pressured Depositional Structural Pressure Gradients Pressure Pressure Pressure Hydrocarbon AccumulationRegion No. Rocks System Setting ft (m) psi/ft (kPa/m) Seal F (C) %Ro Phase Source & Age Type Cause(s) References Remarks

Trinidad 18 Cretaceous marine active 1,500-12,000 0.95 faults water/gas/oil Upper structure CD/HC?/ Heppard et al. Mud diapirs and mud volcanoes.West Indies to Tertiary to deltaic margin (460-3,650) (21.9) shale Cretaceous AE?/TC? (this volume) Max. pressure gradient in productive

fields is 0.73 psi/ft (16.9 kPa/m).North Sea 19 Triassic fluvial failed >5,900 0.87 marls oil/gas/water Jurassic structure CD/HC/TC/AE Buhrig (1989) Examples of pore pressure

Jurassic paralic rift (>1,800) (20.1) shale Leonard (1993) compartment.Cretaceous marine faults Gaarenstroom (1993)Paleocene claystone Holm (this volume)

Bekes Basin 20 Neogene fluvial deltaic back-arc >5,900 0.86 257 0.45 gas/water/ Neogene structure CD/HC/CO2 Spencer et al. (1994) High heat flow. Hungary lacustrine rift (>1,800) (20.0) (125) condensate Cretaceous ? stratigraphic Rapid deposition.Adriatic Basin 21 Pliocene turbidite foreland variable, hard near faults/ water/gas/oil Lower structure CD Carlin & Dainelli Biogenic gas to 13,100 ft (4,000 m).Italy pressure at lithostatic shale Triassic (this volume) Also thermogenic gas.

>7,500 (>2,300) for oilNile Delta/North 22 Jurassic- pre-Tertiary- passive 1,700-12,140 0.89 evaporites 185 gas/water/ Jurassic structure CD/AE/HC Nashaat Temperature & pressure limitsSinai Basins Pliocene marine, fluvial margin (520-3,700) (20.6) shales (85) condensate/ Oligocene- (this volume) of oil & gas production indicatedEgypt Tertiary - deltaic faults oil Miocene in Nashaat (this volume).

marine

Niger Delta 23 Tertiary deltaic passive >9,000* shale 200 0.6* water plus Eocene structure CD Evamy et al. (1978) Nearly all productive fields at orNigeria margin (>2,740*) (96) dissolved gas* Chukwu (1991) near top of overpressure.

Ejedawe et al. (1986)Timan-Pechora 24 Devonian marine foreland highly variable 0.38 shale/ 0.8-0.9 oil/gas/water Permian structure HC in Law et al. (1996) Overpressured in Devonian &Basin Carboniferous deltaic >1,000 Permian evaporites for basin- reefs, basin- Permian Carboniferous. Underpressured inRussia Permian fluvial (>300) (8.8) water block center center in Permian.

Permian

Dnieper-Donets- 25 Carboniferous marine to rift 1,500-14,750 0.79 water block* 212-230 0.9 gas Carboniferous basin-center* HC Polutranko (this volume) Overpressured in Dnieper-DonetskDonbas paludal (450-4,500) (18.3) diagenetic (100-110) Law et al. (1997) & underpressured in Donbas region.Ukraine evaporites

South Caspian 26 Tertiary flysch & foreland >3,000 0.85 shale* water & structure* CD Bredehoeft et al. (1988) Contains numerous mud volcanoesBasin molasse (>1,000) (19.6) dissolved gas Gurevich & Chilingar see Durmishyan (1972) for discuss-Kazachstan (1995) sion of origin & relationship of mud

volcanoes to HC accumulation.

Potwar Plateau 27 Paleozoic marine foreland extremely near uncertain 110 (43) in 0.8-0.9 water in Eocene ? structure CD in Law et al. (this volume) Two pressure regimes - NeogenePakistan to Neogene fluvial deltaic variable lithostatic Neogene & in pre- Neogene & Eocambrian ? Neogene & pre-Neogene.

200-1,000 in Neogene & 180 (82) in Neogene oil/gas & HC/AE/TC in(60-300) 0.7 (15.8) in older rocks water in pre-Neogene

pre-Neogene pre-Neogene

Sichuan Basin 28 Permian marine foreland 0.93 carbonates gas Permian structure TC/O/TC Da-jun & Yun-ho (1994) Over- and underpressured China Triassic (21.4) carbonates.Nagaoka Plain 29 Miocene volcanics active >4,600 0.69 mudstone oil/gas/water Miocene structure CD Magara (1968) Overpressured rock are composedJapan margin (>1,400) (16.0) of lava, tuff breccia & agglomerates.Mahakam Delta 30 Miocene marine active 11,500-13,100 0.87 shale 200-248 0.6 water/gas Miocene structure CD Burrus et al. (1992) Coal is hydrocarbon source rock.Indonesia deltaic margin (3,500-4,000) (20.1) (100-120) Oudin & Picard (1982)

Burrus (this volume)Dampier sub-Basin 31 Jurassic passive >2,870 0.85 CD/MT Nyein et al. (1977) Only one well used in study.Australia Cretaceous margin (>875) (19.7)

Cause(s) codes used: CD = Compaction Disequilibrium, HC = Hydrocarbon Generation, AE = Aquathermal Expansion, TC = Tectonic Compression, O = Osmosis, MT = Mineral Transformations, CO2 = Carbon Dioxide Generation* = Authors Interpretation

Table 1 (continued). Selected attributes of abnormally pressured regions of the world. Locations of the regions are shown on Figure 1 and are linked bythe Index Number of the region.

evidence also exists of locally occurring, overpressuredDevonian shale in the Appalachian Basin (Milici, per-sonal. communication., 1996). The Cretaceous BarnettShale in Texas is another example of a gas-productiveabnormally pressured shale (Reeves et al., 1996).

Another major self-sourced reservoir is coal. Abnor-mally low and high pressures have been described incoal-gas reservoirs in the Upper Cretaceous FruitlandFormation of New Mexico and Colorado (Meissner,1984; Kaiser et al., 1991). In the Powder River Basin ofWyoming, microbial gas is produced from thick(65100 ft, 2030 m), underpressured coal beds in thePaleocene Tongue River Formation (Law et al., 1991).In both the Timan-Pechora Basin of Russia and theDonbas region of Ukraine, gas is vented to the atmos-phere from underpressured Permian and Carbonifer-ous coal beds, respectively.

The presence of abnormally high or low pressures isone of the more important attributes of basin-centeredgas accumulations. Examples of abnormally pressured,basin-centered gas accumulations include the AlbertaBasin of Canada (Masters, 1979, 1984), the GreaterGreen River Basin of Wyoming, Colorado, and Utah(Law et al., 1979; McPeek, 1981; Law, 1984; Spencer,1987; Law et al., 1989), the Piceance Basin of Colorado(Johnson, 1989; Johnson et al., 1987; Spencer, 1987), theSan Juan Basin of New Mexico and Colorado (Berry,1959; Meissner, 1984), and the Appalachian Basin ofOhio, Pennsylvania, New York, and West Virginia(Davis, 1984; Zagorsky, 1988; Law and Spencer, 1993).In countries other than those in North America, theconcept of abnormally pressured basin-centered gasaccumulations is not well known and very little pub-lished information is available. In Russia, a large,underpressured basin-centered gas accumulation hasbeen identified in Permian rocks in the Timan-PechoraBasin. A basin-centered gas accumulation in Carbonif-erous age rocks of the Dnieper-Donets Basin of Ukrainehas also recently been identified (Law et al., 1997). InSouth America, a probable basin-centered gas accumu-lation has been identified in Devonian rocks in theChaco Basin of Bolivia by Williams et al., (1995). InJordan, in the Middle East, gas is produced fromunderpressured, Ordovician sandstone reservoirs(Ahlbrandt et al., 1996, 1997). And there are undoubt-edly many more unidentified abnormally pressured,basin-centered gas accumulations distributed aroundthe world.

Low-permeability, shallow, underpressured, micro-bial gas accumulations in the northern Great Plains ofthe United States and Canada have been described byRice and Schurr (1980). Shallow, underpressured gasaccumulations in Cretaceous reservoirs are also knownto occur in eastern Colorado, and western Kansas.

Curiously, the fluid phase of nearly all abnormallypressured hydrocarbon accumulations is gas. Notableexceptions include the organic-rich Mississippian andDevonian Bakken Shale in the Williston Basin of NorthDakota and Montana (Meissner, 1978) and the Pale-

ocene and Eocene Wasatch, Colton, and Green RiverFormations in the Uinta Basin of Utah (Lucas andDrexler, 1976; Spencer, 1987; Fouch et al., 1992). Thesetwo overpressured systems are basin-centered oil accu-mulations. The reason for the disproportionately fewoccurrences of basin-centered oil accumulations is notknown. We suggest that in abnormally pressured, ther-mally over-mature reservoirs, originally trapped oilmight be expected to have been thermally cracked togas. This explanation may partially account for thesmall number of basin-centered oil accumulations.

SUMMARY

Abnormally pressured rocks are globally distrib-uted in a wide range of geologic conditions. An evalu-ation of causal mechanisms cited in the literature indi-cates that compaction disequilibrium is the mostcommonly cited mechanism, followed closely byhydrocarbon generation. In young, rapidly depositedsediments, compaction disequilbrium is most com-monly cited as the principle cause of abnormally highpressure, while in older rocks, the most commonlycited overpressure mechanism is hydrocarbon genera-tion. In thermally mature, underpressured systems, thepressures most likely evolved from an originally over-pressured system due to gas loss, and gas volume con-traction associated with uplift, erosion, and cooling.

There is a strong association of abnormal pressuresand conventional and unconventional hydrocarbonaccumulations. A general decrease in the size and fre-quency of oil and gas fields with increasing pressure iscommon, with the bulk of production coming fromreservoirs with pressure gradients less than 0.75 psi/ft(17.4 kPa/m). The threshold for economic oil and gasproduction in conventionally trapped accumulations isapproximately 0.85 psi/ft (19.6 kPa/m). Unconven-tional gas accumulations are commonly associatedwith abnormally high or low reservoir pressures.

ACKNOWLEDGEMENTS The authors gratefully ack-nowledge F. Meissner and V.I Slavin for assisting in the taskof providing locations of some abnormally pressured sys-tems. Illustrations for this manuscript, as well as severalother manuscripts in this book, were graciously prepared byCarol Holtgrewe. The manuscript benefited greatly from thecomments of T.D. Dyman, R.C. Johnson, M.D. Lewan, andL.C. Price.

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