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AUTHORS
Peter Kovac � Fugro Airborne Surveys Pty,Ltd., 435 Scarborough Beach Road, OsbornePark, Western Australia;[email protected]
Peter Kovac AQ1
Sharon Lowe � Fugro Airborne Surveys Pty,Ltd., 435 Scarborough Beach Road, OsbornePark, Western Australia;[email protected]
Sharon Lowe
Tony Rudge � Buru Energy Pty, Ltd., 97William Street, Perth, Western Australia;[email protected]
Tony Rudge
E&P NOTE
Intrasedimentary structureand basement architecturefrom high-resolutiongravity gradiometer, magneticdata, and seismic data,King Sound, Canning Basin,Western Australia
PCarlos Cevallos � Fugro Airborne Surveys Peter Kovac, Sharon Lowe, Tony Rudge,Pty, Ltd., 435 Scarborough Beach Road, Os-borne Park, Western Australia;
Carlos Cevallos, Jurriaan Feijth, and Lynsey Brett
d [email protected]Carlos Cevallos
Jurriaan Feijth � Fugro Airborne SurveysPty, Ltd., 435 Scarborough Beach Road, Os-borne Park, Western Australia;[email protected]
Jurriaan Feijth
Lynsey Brett � Mira Geoscience Asia PacificPty, Ltd., 369 Newcastle Street, Northbridge,Western Australia 6003;[email protected]
Lynsey Brett
ACKNOWLEDGEMENTS
This work was made possible by Buru EnergyPty, Ltd., who gave approval to publish its data.Mira Geoscience Asia Pacific Pty, Ltd., providedthree-dimensional modeling software. Thepaper is published with the permission of FugroAirborne Surveys.
Uncorre
cteABSTRACT
An interpretation of geologic structure at King Sound in theCanning Basin was completed on airborne gravity gradiometer,magnetic data, and seismic data. During the Late Devonianand Mississippian periods, the elevated part of the basementin the north was rimmed by carbonate reefs and redepositedcarbonate debris, whereas in the south, siliciclastic submarinefans and turbidites were deposited along the margin of thebasement in a deep-marine environment.
Three principal lithologic units were identified from thevertical gravity gradient (GDD) in the basin: (1) the FairfieldGroup carbonates of high density are interpreted to be thesource of prominent positive gravity anomalies; (2) forereefdebris and carbonate clastics reworked from carbonates higherup the slope or from the carbonate platform are interpreted tobe the source of medium-density responses; and (3) turbidites,debris flows, and associated clastic basinal sequences of lowdensity are interpreted to be the source of prominent negative
The AAPG Editor thanks the following reviewersfor their work on this paper: Peter G. Purcelland Hongliu Zeng.
Copyright ©2013. The American Association of Petroleum Geologists. All rights reserved.
Manuscript received August 31, 2012; provisional acceptance November 16, 2012; revised manuscriptreceived February 6, 2013; final acceptance March 14, 2013.DOI:10.1306/03141312150
AAPG Bulletin, v. XX, no. XX (XXXX 2013), pp. 1–24 1
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Uncogravity anomalies. Depth slices of GDD indicate
the channelized nature of turbidite flows. In thelower section of the basin, intrasedimentary in-trusives were identified from magnetic, GDD, seis-mic, and well data. Depth to magnetic basement(DTB) calculation indicates that the surface of theArchean to Paleoproterozoic basement ranges from3200 to 130m (10,499–427 ft) below sea level. Thenorthwest- and northeast-oriented, south-dippingfaults cut the basement and propagate upward intothe sediments.
A three-dimensional geologicmodel constructedfor King Sound satisfies all known geologic con-straints and is consistentwith the gravity,magnetic,seismic, and well data.
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INTRODUCTION
This study provides an integrated, three-dimensional(3-D) geologic interpretation of sedimentary basininfill and basement architecture at King Sound inthe Canning Basin inWestern Australia (Figure 1).Airborne gravity gradient (AGG) and magneticdata are integrated with sparse seismic and welldata to produce the geologic interpretation. Thestudy demonstrates how airborne potential fielddata can be used to aid petroleum exploration infrontier basins worldwide, where only limited welland seismic data are available.
King Sound is located on the southern marginof the Lennard shelf, on the northern flank of the
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Figure 1. Location of King Sound survey area (white polygon) overlying the OZ SEEBASE basement topography image (FrOG Tech,2006). The color bar shows basement elevations in meters.
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reCanning Basin (Lehmann, 1986) (Figure 1). TheLennard shelf is an area of relatively shallow base-ment (<4000m [13,123 ft] below surface), boundedto the south by the Fitzroy trough, which is inter-preted as a Paleozoic rift (Cadman et al., 1993).The King Sound area consists of Archean to Pro-terozoic basement overlain by Paleozoic and Me-sozoic sediments. The oldest interpreted sedimen-tary sequence is believed to be a complex of theMiddle to Upper Devonian reefs (Yeates et al.,1984). These developed on the shallow southwest-ern flanks of the fault-controlled Lennard shelf andreached a maximum thickness of 2000 m (6562 ft)(George et al., 1997;Wood, 2000). Contemporaryepisodic collapse of the platform margin alongthe basement fault produced rock falls and clast-rich debris flows that led to the deposition of in-terbedded talus breccias and turbidite grainstones.
Carbonate to fine-grained siliciclastic sequences ofthe overlying FairfieldGroupwere deposited in theshallow-marine, restricted platform, lagoonal, andfluvial environments. Overlying Paleozoic to Me-sozoic sequences consist of siliciclastic products ofshelf, fluvial deltaic, and nonmarine deposits.
The Canning Basin is an emerging oil-producingregion in Western Australia (Brown et al., 1984;Kennard et al., 1984; Goldstein, 1989; Shaw et al.,1994). Seismic surveys are routinely acquired forpetroleum exploration to locate possible oil playsby mapping fault structures and favorable basementarchitecture. Two-dimensional (2-D) seismic sur-veys suffer from a lack of data between the linesand ambiguity in dip information. To overcomethis problem, 3-D seismic surveys can be acquired,but the data acquisition andprocessing is expensiveand time-consuming. Airborne gravity gradient
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Figure 2. Schematic diagramshowing pseudodepth slices ofthe vertical gravity gradient(GDD), magnetic data (total mag-netic density), and seismic hor-izons used to constrain the geo-logic model. The derivation ofeach data set is discussed in thetext. (1) Bathymetry. (2) Pseu-dodepth slice of GDD data (370 m[1214 ft]). (3) Blina Shale seismichorizon. (4) Liveringa Groupseismic horizon. (5) Poole Sand-stone seismic horizon. (6) Pseudo-depth slice of GDD data (1140 m[3740 ft]). (7) Intra-AndersonSandstone seismic horizon. (8)Laurel Formation seismic hor-izon. (9) Pseudodepth slice ofGDD data (2570 m [8432 ft]). (10)Pseudodepth slice of magneticdata (2750 m [9022 ft]). (11)Pseudodepth slice of magneticdata (6000 m [19,685 ft]). Thewhite line outlines the surveyboundary. Figure 3 provides thelocation of the seismic lines andwells.
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Undata maps lateral density variations in the subsur-face. Large areas can be covered relatively quicklyand cheaply compared to seismic acquisition costs,which makes AGG surveys an excellent comple-mentary data set to standard seismic acquisitionfor petroleum exploration.
In the King Sound study area, interpretation ofintrasedimentary structure and basement architec-ture was completed using an integration of high-resolution AGG andmagnetic data, integrated with
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2-D seismic and well data (Figures 2, 3). Relevantpublications and open-source information includ-ing previous mapping and tectonic sketches werealso used (Towner, 1981; Craig et al., 1984; Kingsley,2009, personal communication).
Ground gravity surveys measure vertical accel-eration caused by gravity (gD). Airborne measure-ments of gD are influenced by the own accelerationsof the aircraft and cannot be made with the sameaccuracy and spatial resolution as ground gravity
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Figure 3. (A) Vertical gravity gradient (GDD). The color bar gives the data range in eotvos units (E), where 1 E = 1 × 10–9 s–2 (B) Totalmagnetic intensity image. The color bar shows the data range in nanoTeslas (nT). Seismic lines and petroleum exploration wells areoverlain on the magnetic image. Flight line 1000810, used as a control for the depth to basement interpretation, is also shown.
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Figure 4. Pseudodepth slices of gravity gradient data vertical gravity gradient (GDD) at 370 m (1214 ft) (A), 1140 m (3740 ft) (B), and2570 m (8432 ft) (C).
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measurements. Airborne gravity gradiometry is adifferential measurement and removes the effectsof the aircraft motion, thereby delivering gravitydata of a spatial resolution and sensitivity compa-rable with ground gravity data (Dransfield, 2013,personal communication). The FALCON gradiom-eter instrument acquires two curvature gradients.Because these curvature gradients cannot easilyand intuitively be related to the causative geology,they are transformed into vertical gravity gradient(GDD) and vertical gravity (gD) data sets. The ver-tical gravity gradient, GDD, is more sensitive tosmall or shallow sources and has greater spatialresolution than gD. Vertical gravity, gD, is measuredin units of milligals (mGals), whereas gravity gra-dient units are termed eotvos (E), where 1 E = 1 ×10–9 s–2. Airborne gravity gradiometer surveysare traditionally used for mapping lateral densitychanges and structural offsets in sediments andbasement (Dransfield and Lee, 2004; Hinks et al.,2004; Murphy, 2004).
To constrain the vertical distribution of litho-facies, pseudodepth slices of the vertical gravitygradient (GDD) and magnetic data were calculated(Figure 4). Pseudodepth sections are derived fromthe analysis of the log radially averaged power spec-trum of theGDD data in the Fourier domain. Thisis a curve representing the amount of power in eachwave number (Spector andGrant, 1970) andwhichcan be broken up into a series of segments that ap-proximate straight lines. Each of these segmentscan be related to the potential field signature ofbodies at a particular depth, with the depth beingproportional to the slope of the line. This techniqueworks for bothmagnetic andGDD data because bothdata sets have a similar response in terms of theirvariation with distance to the source body. ForGDD
data, it is useful to strip off the small and shallowshort-wavelength features to enhance the gravityresponse of the larger deeper sources.
The magnetic data was used to outline the ex-tent of intrasedimentary intrusives and to identifybasement architecture. A DTB model was derivedfrom the magnetic data, based on a combination oftraditional magnetic depth estimation techniques,such as Euler and Werner deconvolution and inter-pretation and extrapolation of all known geologic
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and geophysical information of the King Soundstudy area and surrounds. The depth modeling wascalibrated using the 1 Padilpa Well log.
To understand the magnetic and gravity re-sponse of geologic units, in particular, their dis-tribution and depth, synthetic models similar tothe geologic configurations found at King Soundwere produced.
Forward modeling of the gravity response alongan existing seismic line was done to define the 2-Dgeometry of geologic formations along this line andto relate the gravity response to the seismic data.Lithologic and density data from existing wells wereincorporated into the models.
Lastly, a 3-D geologic model was constructedfor King Sound, and the densities of the modeledlithologies were calculated using a 3-D constrainedinversion algorithm.
ctedAIRBORNE DATA
Gravity gradient and magnetic data were acquiredby the FALCON AGG system with an averageflight height of 100 m (328 ft), along flight linesspaced 400 m (1312 ft) apart and oriented north–south. The digital terrain model (DTM) was de-rived from the onboard high-resolution laser scannersystem of FALCON. The measured componentsof the gravity gradiometer were corrected for ter-rain effects using the following method:
• A density of 1.00 g/cm3 was applied using an on-shore DTM model and the sea surface offshore.
• A second correction was applied using a densityof 1.67 g/cm3 and a DTM combined with oceanbathymetry data.
The terrain-corrected, measured curvature gra-dients were converted into vertical gravity (gD) andvertical gravity gradient (GDD) data sets. The totalamplitude range of theGDD data are 46 E, and theaverage system noise was 1.50 E for this survey.
Themagnetic data were acquired concurrentlywith the gravity gradient data. The magnetic datawere diurnally corrected and leveled using tie linedata. The total amplitude range of the magnetic
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Figure 5. Lithostratigraphic table of theLennard shelf sedimentary sequence(Playford, 1984; Middleton, 1990; Crostella,1998). Timescale from Gradstein and Ogg(1996).
Kovac et al. 7
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data is 72 nanoTeslas (nT) and has an averagesystem noise of 0.2 nT.
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GEOLOGIC SUMMARY
The Lennard shelf consists of Archean igneousrocks, Paleoproterozoic metamorphic rocks, andgranite overlain by Mesoproterozoic and Neopro-terozoic sedimentary sequences (Towner, 1981).The sedimentary sequence of the King Sound arearanges in age from Ordovician(?) to Early Creta-ceous, with the total thickness of the Phanerozoicsediments exceeding 5500m (18,045 ft) (Figure 5).The sedimentary sequences, mostly marginal basinsediments, show a gentle dip to the southwest. Thelowermost geologic unit drilled by an explorationwell is theUpper Devonian basinal clastic turbiditesequence, deposited south of the active, northwest-southeast fault-controlled margin of the Lennardshelf (Huebner,1980, personal communication).Overlying uppermost Devonian to lower Missis-sippian sandstone submarine fan deposits containfossil wood, quartzite, garnet, and allochthonouslimestone clasts. The presence of limestone clastssuggests the existence of shallow-marine UpperDevonian(?) carbonates, overlying the elevatedpart of the Precambrian basement of the Kimber-ley Block (Huebner,1980, personal communica-tion). The presence of fossil wood and terrigenoussediments may indicate the proximity of a land-mass. In the Mississippian, subsidence of the de-pression and/or possible uplift of the KimberleyBlock to the north leads to a thick sandstone se-quence infilling the depocenter in the south. Fault-controlled subsidence probably slowed or stoppedin the uppermost Visean, resumed in the Pennsyl-vanian (Towner, 1981; Cadman et al., 1993). Then,extensional faulting slowed or ceased, as indicatedby Pennsylvanian to Permian Grant Group sand-stones, which do not thicken markedly into thetrough (Huebner, 1980, personal communication).The renewed sagging in the Early Permian is doc-umented by the thickening of the calcareous mud-stone and limestone of the Noonkonbah Formationtoward the south (Huebner, personal communica-
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tion, 1980; Cadman et al., 1993). Thick deposits ofthe fine-grained clastics of the Blina Shale suggestthat the trough and terrace areas at the edge of theLennard shelf subsided, controlled by extensionaltectonics in the Early Triassic (Huebner,1980,personal communication). In the Middle Triassic,marine regression prevailed, and progradation offluviodeltaic deposits occurred on the Lennardshelf. TheLate Triassicwas characterized by uplift,erosion, and strike-slip tectonics, which was fol-lowed by Gondwana breakup–related subsidence.
Two petroleum exploration wells exist at KingSound: Esso, 1 Puratte; andCommand Petroleum, 1Padilpa (Figure 3). 1 Puratte reached basinal faciesof the Upper Devonian to Mississippian FairfieldGroup at the top of an Upper Devonian basinalclastic turbidite sequence. 1 Padilpa drilled shallow-marine sequences of the southern margin of theLennard shelf. The well penetrated two layers ofMississippian gabbroic rocks, interbedded withmetamorphosed sediments. The sediments belowthe upper layer may be correlated to the Missis-sippian Laurel Formation (a part of the FairfieldGroup, Figure 5) (Owad-Jones,1988, personal com-munication). If this assumption is correct, thenthe Padilpa gabbroic complex may mark an earlyphase of the pull-apart tectonics, contemporary tothe Mississippian regional base Laurel unconfor-mity. An alternate interpretation suggests thatthese metasediments are Proterozoic in age andthat the Padilpa gabbro was emplaced within theProterozoic basement rocks during theMississippianuplift and subsequent rifting (Owad-Jones, 1988,personal communication). Seismic surveys furtherindicate that several intrusive and/or volcanic bodiesin the King Sound area may have existed. Some ofthese resemble reefs on seismic data, and successivephases of extrusive flows appear to have movedseaward, becoming interbedded with sediments(Owad-Jones, 1988, personal communication).
A major tectonic feature in the King Soundstudy area is a northwest-trending fault system,which separates the Lennard shelf from the Fitzroytrough. The polyphase activity indicates a dynamictectonic evolution, including Devonian to Carbon-iferous extension, followed by Pennsylvanian andEarly Triassic compression events (Towner, 1981).
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rrecThe major extensional northwest-trending faultsof the Late Paleozoic were reactivated during theMesozoic as dextral transfer faults, whereas thenortheast-trending Paleozoic extensional faultsbecame Mesozoic extensional faults (Craig et al.,1984). A conjugate set of faults cuts the shelf andsets up tilted fault blocks and terraces that stepdown toward the trough (Kingsley, 2009, personalcommunication).
oteSEISMIC INTERPRETATION
Seismic lines F80B-210 and F81A-454 have beenused to constrain the geologic interpretation of thepotential field data (Figure 3). Five seismic ho-rizons were interpreted, representing (1) the baseof the Blina Shale (Lower Triassic shales); (2) thebase of the Liveringa Group (Kungurian marine,fluvial, and deltaic rocks); (3) the top of the Poole
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Figure 6. Seismic line F81A-454 used to constrain the depth of a magnetic source interpreted as a gabbroid intrusion drilled by the 1Padilpa well. For the location of the seismic line, see Figure 3.
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Figure 7. Seismic profile F80B-210 indicates the presence of a thick Upper Devonian reef complex overlain by a complex depositionalsystem of the Upper Devonian to Mississippian Fairfield Group. For the location of the seismic line, see Figure 3.
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Sandstone (Artinskian sandstone); (4) an intra-Anderson Sandstone horizon (Mississippian fine-grained sandstones and carbonates); and (5) thetop of the upper Laurel Formation (Tournaisiancarbonates and sandstones) (Figures 6, 7). On lineF81A-454, these horizons were constrained at 1Padilpa. Away from this well, the interpretationrelies on the correlation of seismic character ofreflectors. The upper part of the basin infill at KingSound was deposited in a shelf environment, andseismic reflectors are generally parallel. The un-derlying Fairfield Group of rocks indicates shelf-margin and slope environments and is charac-terized by changes in amplitude, frequency, andcontinuity of reflectors. The relief of the top of theupper Laurel carbonate is typical for a shelf mar-gin (Figure 7). Geometric relationships and seis-mic character suggest, from south to north, a shelfslope, carbonate buildups around the shelf break,and a lagoon to the north. In the forereef area, talusslopes can be interpreted. Two relatively small andone large carbonate buildups have been identifiedon line F80B-210 (Figure 7). The steep boundariesof the individual buildups suggest that they havebeen mostly stationary during their growth. Depthand relative position of the buildups suggest severalperiods of limited relative sea level rise separated bymore pronounced transgressional events. This re-sulted in a stepwise landward migration of sites ofreef growth. The internal geometry and lateral fa-cies changes within the Fairfield Group suggest re-latively large lateral density variations.
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UncDEPTH TO MAGNETIC BASEMENT
ADTBmodel for King Sound was generated usingFugro Gravity and Magnetic Services MAGPROBEmagnetic depth analysis software. The softwarederives magnetic source depth solutions based on(1) manual methods such as the half-slope (Peters,1949) and straight-slope (Vacquier et al., 1951)techniques, and (2) automated methods such asWerner and extended Euler deconvolution tech-niques. The Euler and Werner window sizes wereranged between 1000 and 10,000 m (3281 and32,808 ft) and contact-fault solution sets (with a
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structural index of 0) were created to obtain mag-netic source solutions generated by geologic con-tacts only. The manually picked final depth solu-tions are based on their correlation of (1) magneticunits logged in the 1 Padilpa well log, (2) solutionclustering criteria, and (3) depth solutions gener-ated by the MAGPROBE magnetic depth analysissoftware. A basement surface was interpreted fromthe positions of the final depth solutions.
The magnetic data indicate the presence oftwo magnetically definable surfaces in the surveyarea: the first occurs at the top of intrabasinal in-trusives, as seen on seismic line F81A-454 (Figure 6);the second is the magnetic basement itself, inter-preted to be coincident with, or very close to, thetop of the Archean to Paleoproterozoic basement.The existence of two genetically different mag-netic sources makes the DTB model of the KingSound study area extremely complex. Additionally,the low variation of the totalmagnetic intensity field(72 nT) across the survey area resulted in a limitednumber of reliable magnetic depth solutions. There-fore, the basement surface could only be calculatedfor approximately one-third of the project area.In regions where no depth solutions existed, thebasement surface was linearly extrapolated usinginformation obtained from the seismic interpreta-tion andwell logs. Basement depth ranges between130 and 3200 m (427 and 10,499 ft) below sealevel. Figure 8 gives a perspective view of the twomagnetic horizons interpreted from the magneticdata, as described above.
SYNTHETIC MODELING
Synthetic models have been produced to investi-gate themagnetic and gravity responses of relevantgeologic units in the study area. These syntheticmodels represent the geophysical response of typ-ical sedimentary basin margins such as the KingSound study area.
Magnetic, vertical gravity (gD), and verticalgravity gradient (GDD) responses have been calcu-lated at a simulated flight height of 80 m (262 ft)over a series of listric faults that displace modeledbasement and overlying sedimentary rocks by a total
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recvertical distance of 1 km (0.62mi) over a horizontal
distance of 5 km (3 mi). The calculated total mag-netic intensity field has been reduced to the poleto account for latitude effects. Two scenarios havebeen considered: (1) a shallow basin where thedepth to the top of the basement ranges between2 and 3 km (1 and 2mi) (Figure 9), and (2) a deepbasin where the basement is at 4- to 5-km (2- to3-mi) depth (Figure 10).
Physical properties assigned to the sedimentaryunits are based on downhole densities measured inthe 1 Puratte well. The average density value foreach stratigraphic unit was determined and as-signed to the modeled units. The sediments wereassumed to be nonmagnetic for this model. Base-ment density (2.8 g/cm3) and magnetic suscepti-bility (0.02 SI) were determined from averagesquoted for gneiss (Hunt et al., 1995). In this model,limestones of the Upper Devonian and Mississip-pian (the Fairfield Group) and Upper Devonianreef systems (Figure 5) have a higher density thanthe overlying clastic sediments.
For the shallow basin scenario (Figure 9), boththe magnetic and gD responses slowly decrease over
tethe deepening basement and carbonate package,with a total variation of 14mGals for gD and 40 nTfor the reduced-to-the-pole magnetic response. Ingeneral, neither the location of the faults nor in-ternal fault-block geometries are discernible fromthese responses. The gravity gradient response(GDD) shows the largest variation caused by theunderlying displacement of the basement and sed-imentary rocks when compared to themagnetic andvertical gravity responses. A relative low is formedin theGDD response over the downthrown side ofeach fault contact, where the sedimentary packageis thickest relative to the hanging wall. Becausethe clastic sediments have a lower density than thebasement and carbonate package, the deeper base-ment and thicker sediments result in a reducedmass in the downthrown block, producing a low intheGDD response. Because the fault block is tilted,the basement and carbonate units become shal-lower away from the fault, accompanied by an in-crease in the GDD response until a high is ob-tained over the hanging wall before the next faultcreates another deeper fault block to the south,accompanied by another relative low in GDD.
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Figure 8. Depth to magnetic basement interpretation for King Sound. Magnetic data indicate the presence of two magnetically definablesurfaces in the survey area. The first one is the top of intrabasinal intrusives (green). The second one is themagnetic basement itself (yellow),which is interpreted to be identical or very close to the top of the Archean to Paleoproterozoic basement. Viewed from the southeast.
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UnThe total variation in amplitude over fault block 1 is30E (Figure 9). A lower amplitude anomaly (8 E) isobtained across fault trace 3 where the basementbecomes deeper and where less horizontal distanceseparates successive faults.
For the deep basin scenario (Figure 10), thebasement is at 4- to 5-km (2- to 3-mi) depth. ThegD response is very smooth, decreasing gently overthe deepening basement from north–south tosouth–east, with a total variation of 10 mGals.The reduced-to-the-pole magnetic response is
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very similar to the gD response being very smoothand decreasing from left to right over the deepen-ing magnetic basement with a total variation of28 nT across the profile. No fault structures arediscernable from the magnetic response. TheGDD
response is also very smooth. A slight local mini-mum of approximately 3 E is obtained over faultblock 1, but this amplitude is within the noise en-velope and cannot be interpreted with confidencein a real survey. Therefore, this deep basin modelrepresents the limit of detection of the airborne
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Figure 9. Shallow basin synthetic model showing calculated magnetic response that has been reduced to the pole (TMI_RTP) in units ofnanoTeslas (nT) (top panel), vertical gravity gradient (GDD) response in units of eotvos (E) (middle panel), and vertical gravity (gD)response in units of milligals (mGals) (bottom panel).
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Umagnetic and gravity methods for the particularmodel geometry and physical property contrastsused in this synthetic study. The only informationthat can be deduced from the observed data over thedeep basin scenario is that the basement deepens tothe right along this profile. No detailed basementarchitecture is discernable.GRAVITY MODELING OF SEISMICLINE F80B-210
In the northern part of the survey, along seismicline F80B-210, detailed gravity modeling hasbeen performed to define the 2-D geometry ofgeologic formations. Density data from 1 Puratte
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Figure 10. Deep basin synthetic model showing calculated magnetic response that has been reduced to the pole (TMI_RTP) in units ofnanoTeslas (nT) (top panel), vertical gravity gradient (GDD) response in units of eotvos (E) (middle panel), and vertical gravity (gD)response in units of milligals (mGals) (bottom panel).
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reexploration well have been incorporated into the2-D model.
Seismic profile F80B-210 indicates the pres-ence of a thick Upper Devonian reef complexoverlain by a complex depositional system of theFairfield Group (Figure 7), which includes (fromnorth to south) a lagoon, a carbonate reef, andslope deposits. A thin intrusive body, concordantwith the bedding, can be an alternative interpre-tation of the strong reflectors, which have beeninterpreted as slope debris on the southern partof the line. Above the Fairfield Group, flat-lyingyounger sequences consist mostly of siliciclasticdeposits. The location of different rocks matcheson both gravity and seismic data (Figure 11). Theseismic interpretation supports the gravity model(Figure 12). Because of the high density of thecarbonate with respect to the overlying sediments,the topography of the upper surface of the car-bonate unit has a big influence on the gravity re-sponse. The carbonate mound in the center of thesection produces the long-wavelength gravity high
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cobserved in the northern half of the gravity profile.This unit produces broad (long-wavelength) grav-ity anomalies because it is located at 2000-m(6562-ft) depth. The short-wavelength (∼800 m[2625 ft] wide) anomalies observed in the gravitygradiometry data result from much shallower den-sity changes and were not modeled here: they arepresumed to be a combination of density varia-tions within shallow formations and instrumentand/or system noise. Based on the sediment dis-tribution inferred from seismic facies and the rel-ative position on the shelf with respect to the car-bonate mound, the body north of the mound hasbeen interpreted as lagoonal sediments (Figure 12).Similarly, the unit south of the mound representspossible slope debris in a forereef position. Theunit at the southern end of the profile located at anapproximate depth of 1600m (5249 ft) and with ahigh density (2.85 g/cm3) may represent an in-trusive sill that produces a positive anomaly on thesouthern edge of the GDD profile. The upper sed-imentary units in the model have similar densities
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Figure 11. Schematic diagramshowing the vertical gravity gra-dient (GDD) image, sun shadedfrom the northeast and seismicline F80B-210. The location ofdifferent lithotypes matches onboth gravity gradient and seismicdata.
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GEOLOGIC INTERPRETATION
The intrasedimentary structure and basement ar-chitecture at King Sound in the Canning Basin inWestern Australia have been interpreted from high-resolution AGG, magnetic, and seismic data.
TheGDD anomalies represent the total responseof multiple sources, with varying densities, occur-ring at different depths. Several geologic units, in-cluding carbonates, redeposited carbonate debris,siliciclastics, and intrasedimentary intrusives all con-tribute to the observed GDD response. The averagedensity of the siliciclastic sequences is approxi-mately 2.4 g/cm3 (Telford et al., 1990). The den-sity of the turbidites and associated debris-flowbreccias contrasts with the density of carbonatereefs, the latter averaging 2.8 g/cm3(Telford et al.,1990). Synthetic modeling showed that GDD datareflect mostly shallow geologic features.
Three principal lithologic units have been iden-tified from the vertical gravity gradient data (GDD)in the basin: (1) the Fairfield Group carbonates ofhigh density are interpreted to be the source ofprominent positive gravity anomalies; (2) forereefdebris and carbonate clastics reworked from car-bonates higher up the slope or from the carbonateplatform are interpreted to be the source of me-dium-density responses; (3) turbidites, debrisflows, and associated clastic basinal sequences oflow density are interpreted to be the source ofprominent negative gravity anomalies.
The position of carbonate buildups, as iden-tified in the seismic data (Figure 7), is expressed inGDD data as anomalous highs (Figure 13). Thethickness and position of backreef lagoonal sedi-ments is given by the isopach of the interval be-tween the top of the upper Laurel carbonate andthe interpreted intra-Anderson Sandstone seismichorizon (Figures 6, 7).
Depth slices of GDD data indicate the highlychannelized nature of low-density turbidite flows
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(Figure 13). The gravitational response of some ofthe sedimentary sequences is overprinted by thepresence of high-density intrasedimentary intru-sive rocks, as in the western part of the study area,where more than 200 m (656 ft) of gabbro hasbeen intersected by 1 Padilpa (Huebner, 1980,personal communication).
Geologic interpretation of magnetic data (Fig-ure 14) shows the basement architecture becausesedimentary sequences at King Sound are generallynonmagnetic. Intrusive bodies that have been em-placedwithin the sediments as seen in 1 Padilpa alsoproduce magnetic highs. Two major fault trends,oriented northwest and northeast, respectively, in-tersect the basement. They are consistent with thetrend of major regional structures that controlledthe development of the Fitzroy trough and itsnorthern margin, the Lennard shelf. In the south-west, the northwest-trending fault bounds theelevated basement in the north from the down-thrown basement in the south and can be corre-lated with the Puratte fault (Figure 14). East of thePuratte fault, several major faults separate a horst-like basement elevation indicating right lateral offset(Figure 14). The southwest-trending King Soundfault in the central study area (Figure 14) is inter-preted as a major offset of the basement, which canbe correlated to large-scale tectonic features onregional gravity and magnetic maps (Nakamura,2011).
Integrated interpretation of magnetic andGDD
data shows the complex geologic structure of themarginal parts of the Lennard shelf, bounded bymajor basement fault systems adjacent to the deep-marine environment of the Fitzroy trough. ThePrecambrian basement interpreted from magneticdata is overlain by carbonate reefs, carbonate brec-cias, siliciclastic submarine fan deposits, and turbi-dite flows, which are identifiable on GDD and seis-mic data. Intrasedimentary intrusives are emplacedin the lower parts of the Phanerozoic sedimen-tary fill of the basin and can be seen on magnetic,GDD, and seismic data and drilled by 1 Padilpawell. Conjugate northwest- and northeast-orientedsoutherly dipping basement faults propagate up-ward into the sedimentary fill of the basin and havebeen interpreted from magnetic and GDD data.
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THREE-DIMENSIONAL GEOLOGIC MODEL
During the past decade, 3-D geologicmodeling hasbeen widely used in the oil industry to define sed-imentary basin architecture (Clarke et al., 2006;Ainsworth, 2006), and an ongoing interest exists inbuilding complete 3-D geological settings to pro-vide a realistic solution to exploration problems.The 3-D geologic model for King Sound was con-structed from interpreted geologic interfaces andfaults, considering all the available data and currentgeologic understanding; it can be improved as ad-ditional new information becomes available.
Model construction started with the produc-tion of the basement surface, as described in thesection “Depth to Magnetic Basement.” Intrusiveunits were also interpreted from the magnetic andwell data, and surfaces were created to representthem. The top surfaces of the siliciclastic formationsin the upper section of the basin were constrainedfrom the available 2-D seismic data and input in the3-Dmodel. The lithostratigraphic column (Figure 5)was used to define geologic relationships. Lateralvariations of the lower parts of the basinal sedi-mentary sequence (Devonian reef system and Fair-field Group of rocks), which directly overlie thebasement, were interpreted from theGDD and mag-netic data. Depth slices of the GDD data and 2-Dseismic data were used to constrain the depths ofthese older sedimentary units (Figure 2). Geologicknowledge of the area, gained from the study orpertinent articles and published maps, was used toconstrain the time sequences of fault systems andtheir kinematics, and to estimate fault throws. Onceall the surfaces were constructed, a 3-D voxel modelwas generated and classified by lithologic type(Figure 15). This voxel model was the primary in-put for the quantitative modeling phase. A densitywas assigned to each lithology in the model classi-fication, based on downhole densities measured in
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the wells and values obtained from literature (Huntet al., 1995). A forward model was computed andcompared to the observed GDD data, using a 3-Dpotential fields inversion program, VPmg (Fullagarand Pears, 2007). Based on the misfit between thecomputed and observed data, the densities wereadjusted, and an upper and lower bound was as-signed to control the range of densities allowed foreach lithologic type so that they remain geologicallyrealistic. A homogeneous property inversion wascomputed in VPmg to calculate a single-densityvalue per lithology that best fits the observed data.This was followed by a heterogeneous propertyinversion where the density of three of the litho-logic types in the model was allowed to vary withincertain bounds, producing a density distribution forthose lithologies instead of a single density for therock type. A good fit between the calculated andobserved data was obtained (Figure 16). The finalgeologic model and density distribution is pre-sented in Figure 17. The 3-D geologic model andassociated density distribution provide a good fit tothe observed data whereas maintaining the correctrelationships between geologic units. It provides aninvaluable tool to aid in exploration planning suchas siting well locations and seismic investigations.
CONCLUSIONS
The geologic interpretation of basin infill and base-ment architecture of King Sound in the CanningBasin in Western Australia was completed on high-resolution AGG, magnetic, and seismic data aidedby well logs. Pseudodepth slices of GDD and mag-netic data together with depth-converted seismichorizons and 2-D gravity forward models of se-lected seismic lines were used to constrain the ver-tical distribution of geologic units.
Figure 12. Gravity forward model of seismic line F80B-210. The bottom three panels show the gravity model compared to the in-terpreted seismic profile and a legend providing the densities used in the model. The densities were derived from measurements quotedin the 1 Puratte well. The observed vertical gravity, gD (dashed line), and calculated gD response (solid line), are provided in the top panelin units of milligals (mGals) and the observed vertical gradient, GDD (dashed line), and calculated GDD response (solid line), is presentedbelow gD in units of eotvos (E). AGG = airborne gravity gradient; Fm = Formation; Gp = Group.
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.Geologicinterpretationofverticalgradient(G D
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otethechannelized
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.Geologicinterpretationofmagnetic
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faulttrends.
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ofFigure 15. Perspective viewfrom the southeast of the three-dimensional geologic model forKing Sound, which was con-structed from interpreted geo-logic interfaces and faults. (A)Visualization of the crystallinebasement (yellow), intrased-imentary intrusive rocks (green),and faults derived from magneticdata (violet). (B) Visualization ofthe Fairfield carbonates (blue)and faults derived from the ver-tical gravity gradient (GDD) (red).The surfaces were converted to athree-dimensional geologic voxelmodel (C) with colors: yellow =basement; brown = basinal clas-tic rocks; olive green = carbonatedebris; blue = carbonate rocks;dark green = intrusive rocks. Theupper clastic section of the basinalfill is not shown.
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UncoThree principal lithologic units in the sedi-
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survey area because sedimentary sequences at KingSound are nonmagnetic. The basement is inter-sected by conjugate northwest- and northeast-oriented south-dipping basement faults that prop-agate upward into the sediments. Two intrusivebodies, based on their elevated magnetic responseand constrained by 1 Padilpa exploration well, havebeen identified in the lower section of the basin.
Selected depth-converted seismic horizons andseismic lines indicate the presence of a thick UpperDevonian reef complex overlain by a complex de-positional system of the Upper Devonian to Mis-sissippian Fairfield Group, which includes a lagoon,a carbonate reef, and slope deposits. A gabbroid
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Figure 17. Perspective view from the southeast of the geologic model for King Sound. The color bar represents density derived fromthe inversion process and ranges from –0.15 to 0.05 g/cm3. These densities are relative to the terrain correction density of 2.67 g/cm3.3-D = three-dimensional.
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intrusion drilled by 1 Padilpa has also been identi-fied in seismic data. The location of the main lith-ologic units matches on gravity, magnetic, and seis-mic data.
The 3-D geologic model for King Sound wasconstructed from interpreted geologic interfacesand faults, and honors all the available data. Themodel indicates an elevated, fault-controlled Ar-chean to Proterozoic basement of the Lennardshelf in the north, rimmed by Upper Devonianand Mississippian carbonate reefs and carbonatebreccias, whereas, in the south, siliciclastic sub-marine fan deposits and turbidites have been de-posited along the fault-controlled margin of thebasement in the deep-marine environment of theFitzroy trough. Depth slices of GDD data indicatethe highly channelized nature of low-density tur-bidite flows. Constrained inversion of GDD dataproduced a calculated GDD response that was agood match to the observed data. This provides astrong quantitative measure of the validity of the3-D rock distribution in the geologic model.
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AUTHOR QUERIES
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AQ1 = Please provide the author’s vitae of Peter Kovac, Sharon Lowe, Tony Rudge, Carlos Cevallos, JurriaanFeijth, and Lynsey Brett.AQ2 = The citation ’Kennard et al., 1994’ has been changed to ’Kennard et al., 1984’ to match the author name/date in the reference list. Please check here and correct if necessary.AQ3 = Please provide the initial(s) of the first name of Huebner.AQ4 = Please provide the initial(s) of the first name of Huebner.AQ5 = Please provide the initial(s) of the first name of Huebner.AQ6 = Please provide the initial(s) of the first name of Huebner.AQ7 = Please provide the initial(s) of the first name of Huebner.AQ8 = Please provide the initial(s) of the first name of Owad-Jones.AQ9 = This sentence has been slightly modified for clarity. Please check if appropriate.AQ10 = Please provide the initial(s) of the first name of Owad-Jones.AQ11 = Please provide the initial(s) of the first name of Owad-Jones.AQ12 = Please provide the initial(s) of the first name of Kingsley.AQ13 = The slash in ’contact/fault’ has been changed to a hyphen. Please check if appropriate.AQ14 = Please provide the expanded form of ’MAG SUS’.AQ15 = Please provide the expanded form of ’MAG SUS’.AQ16 = The slash in ’contact/fault’ has been changed to a hyphen. Please check if appropriate.AQ17 = Please provide the initial(s) of the first name of Huebner.AQ18 = The citation ’Fullagar and Peard, 2007’ has been changed to ’Fullagar and Pears, 2007’ to match theauthor name/date in the reference list. Please check here and correct if necessary.AQ19 = Please provide the page range or number of pages for this reference.AQ20 = Please provide the page range or number of pages for this reference.AQ21 = Please provide the editors (if any) for this reference.AQ22 = Please provide the publisher’s location and the number of pages for this reference.AQ23 = Please provide the number of pages for this reference.AQ24 = Please check if the changes incorporated in this reference are appropriate.AQ25 = Please provide the volume number for this reference.
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