AUTHORS

Darrin Burton � Bureau of Economic Geol-ogy, John A. and Katherine G. Jackson Schoolof Geosciences, University of Texas at Austin, Austin,Texas 78713-8924; darrinburton@hotmail.com

Darrin Burton received his B.S. degree in geologicalsciences from Brigham Young University in 2007.In 2008, he completed his M.S. degree in geology atthe University of Texas at Austin and is presentlya Ph.D. candidate at the University of Texas.

Seismic geomorphology andtectonostratigraphic fill ofhalf grabens, West NatunaBasin, IndonesiaDarrin Burton and Lesli J. Wood

Lesli J. Wood � Bureau of Economic Geology,John A. and Katherine G. Jackson Schoolof Geosciences, University of Texas at Austin,Austin, Texas 78713-8924,

Lesli Wood is a senior research scientist at the Bu-reau of Economic Geology in the University ofTexas, Jackson School of Geosciences. She receivedher M.S. degree in geology from the University ofArkansas (1988) and her Ph.D. in Earth resourcesfrom Colorado State University (1992). Upon grad-uation, she worked for Amoco for 5 yr and 13 daysand moved to Austin in 1997. She currently runsthe Quantitative Clastics Laboratory IndustrialAssociates program at the University of Texas. Herresearch interests include quantitative seismicgeomorphology, Martian and Earth deltas, shalediapirs, and clastic margin studies.

ABSTRACT

Synrift stratigraphy of the Cenozoic grabens in theWest NatunaBasin (WNB) is commonly poorly constrained because fewdeep wells exist. Stratal slices were produced from high-qualityGabus and Belanak three-dimensional (3-D) seismic surveysto image synrift depositional architectures. Using 3-D seismicgeomorphology, we identified five facies that were used toconstruct a detailed seismic stratigraphic interpretation. Theinterpreted synrift stratigraphy of the WNB shows a strongtectonic control. From this type of analysis, potential hydro-carbon system elements can be identified and mapped. Thus,in petroleum exploration situations in which few well dataexist, 3-D seismic geomorphology can be used to provide keystratigraphic insights.

ACKNOWLEDGEMENTS

Research in quantitative seismic geomorphology ismade possible by the generosity of the membersof the Quantitative Clastics Laboratory IndustrialAssociates research consortium, including BHPBilliton, Statoil, Anadarko, Cairn Energy India, Exxon-Mobil, Marathon, YPF, Woodside Energy, Schlum-berger and Saudi Aramco. The authors would like tothank ConocoPhillips Indonesia Ltd Inc for facili-tating the availability of the data, and thank BPMIGASfor granting a license of usage to COPI and theUniversity of Texas at Austin for purposes of aca-demic evaluation.Ongoing software support of the Bureau of EconomicGeology education through research program isprovided from a Landmark University Grant. This work

INTRODUCTION

Many of Southeast Asia’s largest oil and gas fields (e.g., Minas∼4 × 109 bbl of oil) are found within the Paleogene nonmarinerifts of Indonesia, where production has been declining since1993 (Sladen, 1997a; Doust and Sumner, 2007). Althoughthe prolific nonmarine rift basins will likely be a source of fu-ture reserve additions (Sladen, 1997a), several variables com-plicate exploration success, and fewwells penetrate the deeperparts of the rifts. The depth and complexity of the stratigraphycommonly cause seismic imaging to be low frequency, discon-tinuous, and low amplitude, all of which complicate seismic

was aided greatly by the visualization expertise ofDallas Dunlap and the editorial expertise of LanaDietrich. The authors appreciate the work of GretchenGillis and the AAPG staff, including Paula Sillmanand Frances Whitehurst, who contributed greatlyto the quality of this work. Much thanks from theauthors and the AAPG Editor to the followingreviewers for their work on this paper: Joseph J.Lambiase, Richard J. Moiola and Richard Netherwood.

Copyright ©2010. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received January 8, 2010; provisional acceptance March 26, 2010; revised manuscriptreceived April 7, 2010; final acceptance June 30, 2010.DOI:10.1306/06301010003

AAPG Bulletin, v. 94, no. 11 (November 2010), pp. 1695– 1712 1695

interpretation. In this article, high-quality three-dimensional (3-D) seismic and available well pen-etrations from the West Natuna Basin (WNB),Indonesia, are used to illuminate reservoir, seal,and source characteristics of these importanthydrocarbon-bearing features.

The WNB is an example of an Indonesian riftbasin with potential for commercial quantities ofundiscovered oil and gas (Sladen, 1997b;Maynardet al., 2002). It is located in the center of the SundaShelf of Indonesia, in the South China Sea, and lieswithin a structurally complex region of the worldwhere several tectonic plates collide (Figure 1).The basin exhibits a structural history of extension,compression, and translation related to Cenozoicregional tectonic events.

TECTONIC SETTING ANDGENERAL STRATIGRAPHY

As in other Southeast Asian basins, stratigraphyof the WNB is tied closely to regional tectonics

1696 Seismic Geomorphology of Half Grabens, Indonesia

(Longley, 1997). Rifting in the WNB might havecommenced as early as theLateCretaceous, butmostprobably occurred in the late Eocene to Oligocenebecause of subduction (slab rollback) to the east anddivergent strike-slip extrusion faults associated withthe India-Asia plate collision (Ginger et al., 1993;Hall, 1996; Tjia and Liew, 1996; Matthews et al.,1997; Morley, 2002). Two major fault trends de-veloped during rifting: southwest-northeast andnorthwest-southeast trends (Ginger et al., 1993).Half grabens in theWNBwere filled with alluvial,lacustrine, deltaic, and fluvial deposits (Figure 2)(Pollock et al., 1984; Ginger et al., 1993; Michaeland Adrian, 1996; Phillips et al., 1997; Maynardet al., 2002; Morley et al., 2003; Doust and Noble,2008). During the early Miocene, the region wasinundated by amarine incursion (Figure 2) as fault-controlled subsidence ended and thermal subsi-dence persisted (Pollock et al., 1984; Tjia andLiew, 1996; Matthews et al., 1997; Phillips et al.,1997; Morley et al., 2003; Doust and Noble,2008). In themiddleMiocene, the island of Borneoexperienced a counterclockwise rotation, creating

Figure 1. Index map showing the locationof the West Natuna Basin (hachured area)within the Sunda Shelf. Major tectonicelements are shown to highlight the struc-tural complexity of the region (modifiedfrom Hall, 1996 and Longley, 1997).

subregional compression and inverting the oldergrabens ofWest Natuna (Hall, 1996; Tjia and Liew,1996; Matthews et al., 1997; Morley, 2002). Fromthe late Miocene to the present, West Natuna hasexperienced relative tectonic quiescence and hascontinued to experience conventional basin sub-sidence (Tjia and Liew, 1996).

Hydrocarbon production in theWNB is closelyassociated with units linked to specific tectonic pe-riods in the basin’s history (Ginger et al., 1993;

Michael and Adrian, 1996; Phillips et al., 1997;Maynard et al., 2002; Prasetyo, 2002; Doust andSumner, 2007; Doust andNoble, 2008). Lacustrineshale and coal of the Benua Formation, depositedduring basin rifting, are the primary source rocks forthe region’s oil and gas (Michael and Adrian, 1996;Phillips et al., 1997; Prasetyo, 2002). Fluvial anddeltaic reservoirs occur in the synrift and postriftGabus Formation, and the postrift Barat Shale formsa significant regional seal (Michael and Adrian,1996; Phillips et al., 1997; Doust and Sumner,2007; Doust and Noble, 2008). Compression dur-ing the middle Miocene inversion of the WNBformed fault propagation anticlines by reactivatingpreexisting normal faults (Ginger et al., 1993). Thedifferent tectonic phases in the basin are thus eachin their turn responsible for some aspect of the re-gion’s working hydrocarbon system. Most produc-tion is from anticlinal structural traps that can beseen in the Gabus and Belanak 3-D seismic survey(Figures 3, 4).

Historically, production in the WNB is re-stricted to inversion anticlines and upper synrift orpostrift reservoirs (Sladen 1997b; Maynard et al.,2002). Consequently, most synrift stratigraphy isinfrequently penetrated, and the data that do existcommonly are reported to be of low quality (MichaelandAdrian, 1996; Phillips et al., 1997; Sladen, 1997b;Maynard et al., 2002; Prasetyo, 2002). The lack ofgood well data in the deepest parts of the rift re-stricts the understanding of rift-related stratigraphyand associated hydrocarbon system elements,making synrift exploration difficult. For example,identification of source rocks can be complicated inlacustrine basins because of source rock faciesvariability (Katz, 1995, 2001; Carrol and Bohacs,2001). Full source rock potential of a lacustrine riftcan be assessed only after each half graben is drilled(Lambiase et al., 1999). This approach is, however,not always economically viable because source rocksin the WNB lie much deeper than most knownreservoirs. A better understanding of synrift stra-tigraphy and associated hydrocarbon systems ofthe basin should lessen exploration risk (Sladen,1997a).

We intend to demonstrate the use of 3-D seis-mic geomorphology, in conjunction with seismic

Figure 2. A schematic showing the major tectonic episodes,lithostratigraphy, approximate stratigraphic ages, and regionalhorizons (modified from Doust and Noble, 2008). TSR = top ofthe synrift horizon (horizon T); USR = upper synrift horizon(horizon SR1); LSR = lower synrift horizon (horizon SR2); BAS =horizon at the top of the basement.

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stratigraphy, in extracting details about synrift stra-tigraphy for which few or no well data exist (e.g.,Posamentier, 2004; Posamentier et al., 2007;Wood, 2007). Three-dimensional seismic geomor-phology was defined byWood (2004, p. 713) as the“analysis of the landforms, imaged in 3-D seismicdata, for the purposes of understanding the his-tory, processes, and fill architecture of a basin.”This type of interpretation emphasizes analysis ofhigh-resolution, plan-view, geomorphic elementsthat may not be discernible in vertical seismic sec-tions (Zeng et al., 2001; Zeng and Hentz, 2004).Wood (2007) illustrated how seismic geomorphol-ogy can be used to investigate reservoir characterand architecture. High-quality 3-D seismic dataover the WNB make it possible to image detaileddepositional architecture in planiform through-out the basin fill. Images of depositional systems

1698 Seismic Geomorphology of Half Grabens, Indonesia

can then be used to make detailed seismic strati-graphic interpretations between and below wellpenetrations.

IMAGING ARCHITECTURE IN COMPLEXLYDEFORMED STRATIGRAPHY USINGSTRATAL SLICING

Heterogeneity of fluvial, deltaic, and lacustrine syn-rift deposits of the WNB makes seismic imagingof geomorphic elements difficult. Reflectors com-monly have poor continuity because of sharp de-creases in thickness over short distances. Even thebest synrift reflectors rarely extend beyond individ-ual half grabens. To further complicate seismic in-terpretation, half-graben fills are asymmetric, andtheywerepostdepositionally foldedduring inversion

Figure 3. Index map of the Gabus and Belanak surveys and wells available for this study. Kakap-1, Forel-1, and Forel-2 are the onlywells that penetrate a significant thickness of the synrift fill. Bold gray represents the lines in Figure 4. Dashed line (AA′) marks thelocation of the arbitrary seismic section in Figure 10. The other seismic cross sections are referenced by figure number.

Figure 4. Multiseismic panelfence diagram showing tectono-stratigraphy of the West NatunaBasin (line locations are shownin Figure 3). Stratigraphy ofWest Natuna is commonly sub-divided into four megasequences:synrift, postrift, syninversion,and postinversion. Hydrocarbonproduction is primarily fromupper synrift to postrift reser-voirs in the numerous anticlinalstructures. This study focuseson the lowest megasequence:the synrift.

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urton and Wood 1699

of the basins. As a result, conventional interpreta-tion methods such as horizon interpretation aretime consuming, and attribute imaging using hori-zon slicing commonly yields little information aboutdepositional geometries because of syndepositionalthickness changes. Proportional (stratal-slice) meth-ods yield more stratigraphically consistent informa-tion and provide more meaningful seismic geomor-phic images.

Stratal slicing displays seismic attributes ofan approximate depositional or geologic time sur-face (Zeng et al., 2001) and makes use of refer-ence events picked above and below the intervalof interest as controls for sampling (i.e., slicing) thedata (Zeng et al., 2001). Figure 5, taken from aWNB 3-D seismic survey, illustrates the improvedimaging quality of a stratal slice over other horizon-based attribute-image methods. Along these pro-portional slices, any number of seismic attributescan be extracted for mapping seismic geomorpho-logical features. The 3-D seismic geomorphologieswere used in concert within a framework of seis-mic stratigraphic surfaces to define and reconstructsynrift fill history of individual half grabens. Eachstratal slice was assigned a dominant facies, and oc-casionally, two facies were widely present on oneslice (e.g., delta adjacent to lacustrine facies). Facieswere then mapped onto seismic lines to illustratethe stratigraphy in seismic cross sections (Figure 6).The resulting seismic stratigraphic interpretationhighlights different tectonostratigraphic packages.

Two 3-D seismic data sets, the Gabus andBelanak surveys, were used in this study to imagedown to approximately 8500-m (27,900-ft) depthin the basin (6 s two-way traveltime) with a spatialcoverage of 3154 km2 (1214 mi2) (Figure 3). Fif-teen well-log suites were available within the sur-vey limits (Figure 3). Published data were usedto complement observations on stratigraphy andstructure in the basin.

HORIZONS

Four regional horizons were mapped: a top of thesynrift horizon (horizon T), an upper synrift hori-zon (horizon SR1), a lower synrift horizon (hori-

1700 Seismic Geomorphology of Half Grabens, Indonesia

zon SR2), and a horizon at the top of the basement(basement) (Figure 7). Horizon T was picked justbelow the Barat Shale (Figure 2). The strong im-pedance contrast at the base of the Barat Shale cre-ates a strong negative amplitude that is mappableacross the entire survey (Figure 7). The Barat is alsoeasily recognized in well logs by its high gamma-ray response. Horizon SR1 is a positive-amplitudereflector that has variable reflection strengths acrossthe survey (Figure 7). This horizon is less contin-uous, being continuous in the center of the grabensand losing continuity at the margins of the basin.The third horizon, horizon SR2 is a high-amplitudecontinuous reflector in deep grabens, which losescontinuity and decreases in amplitude in the south-eastern half of the survey (Figure 7). This hori-zon was picked on a strong negative reflector thatoverlies an interval of discontinuous reflectors. Thebasement horizon shows up as a strong positiveamplitude reflector above a deep interval of chaoticreflectors (Figure 7). These mapped horizons wereused to generate interval isochore and structuremaps, which were in turn used to delineate originalhalf-graben architecture of the basin and present-day structural configuration of the rifts. Careful ex-amination of the fill growth within these basinshelped define the timing of movement on the ma-jor faults versus episodes of sediment filling.

In addition to the four regional horizons, sevenhorizons (horizons 1–7) were picked about every300 ms (Figure 8) within the basin’s major halfgraben. These seven surfaces provided a frameworkof control horizons for stratal slicing the amplitudedata volume, thereby improving the ability to im-age time-synchronous seismic geomorphologicalbodies. In all, 150 stratal slices covering an approxi-mately 2-s data interval were produced and inter-preted. Horizon and interval attributes were alsoused to analyze these mapped intervals.

SEISMIC GEOMORPHIC ENVIRONMENTS

Seismic geomorphic analysis revealed five primarydepositional environments in theGabus andBelanaksurveys. Although well-log motif analysis was usedto support some of the interpreted depositional

facies, many of the deeper facies were interpretedusing only seismic geomorphic formand amplitude.Hundreds of stratal slices allowed 20- to 40-m (65–130 ft) vertical resolution of the temporal history

of depositional environments in the rift basins. Thefive primary depositional systems found were flu-vial, deltaic, alluvial-fan, shallow lacustrine, anddeeplacustrine systems.

Figure 5. Comparison of stratal, horizon, and time slice through an amplitude volume. Stratal slicing is a quick method of creating timesynchronous surfaces. TWT = two-way traveltime.

Burton and Wood 1701

Fluvial

Fluvial systems are interpreted from stratal slicesbearing channel forms (Figure 9A, B). Most synriftstratal slices contained channels, all of which variedwidely in their seismic amplitude. Fluvial morphol-ogies at different stratigraphic levels exhibited differ-ent morphometrics and orientations, but they do notshow a distributive pattern (see subsequent discus-sion). Channels in the lower synrift are small (<100m[328 ft] wide) and fairly straight, and they par-allel the major fault trends (northeast-southwest).In the upper synrift, channels can exceed 500 m(1640 ft) wide, and they demonstrate a variety of

1702 Seismic Geomorphology of Half Grabens, Indonesia

forms: straight, meandering, and slightly anasto-mosing.Nobraided channel formswere interpretedin stratal slices from the synrift.

Deltaic

Like stratal slices of fluvial environments, deltaicintervals contain channels displaying highly variableplan-view character and amplitude. Stratal slicesfrom deltaic depositional intervals show channelsexhibiting a distributive pattern (Figure 9C, D).These patterns occur in the middle to upper partof the synrift fill, albeit they are not frequent. Oc-casionally, ambiguous channel forms, particularly

Figure 6. Seismic geomorphicfacies interpreted seismic geo-morphology of 150 stratal slices.Seismic line flattened on hori-zon T. Three distinct stratigraphicpackages are identified: a lowerfluvial/alluvial unit, a middlelacustrine and deltaic unit, andan upper fluvial unit. Note thebright-amplitude alluvial-fan fa-cies filling the lower parts of thehalf-graben fills. TWT = two-waytraveltime.

anastomosing channels, made it impossible to dis-tinguish between fluvial and deltaic facies. In suchinstances, sedimentary units were classfied as fluvial/deltaic.

Alluvial Fans

Alluvial fans are known to be common in rift ba-sins and were imaged in the Gabus and Belanaksurveys as high-amplitude reflections with a lobateshape (Figure 9E). They are areally limited withabrupt terminations, and they are found most com-monly immediately overlying basement rock (i.e.,in the earliest history of the basin fill). They com-monly occur adjacent to the faulted margins of halfgrabens, but they also appear to be derived fromflexural margins and fault-transfer zones.

Shallow Lacustrine

Lacustrine intervals were identified by their seis-mically featureless, homogeneous, low-amplitudeseismic response in stratal slices (Figure 9F but withlower amplitude). The continuity and homogene-ity of amplitude in these intervals are interpretedto represent fine-grained deposits (Sladen, 1997b;Maynard et al., 2002). Shallow lacustrine intervalshave lower amplitude values than deep lacustrineintervals. This facies dominates the middle part ofthe largest and deepest half grabens, and it is al-most completely absent in the smaller half grabens.Such graben-to-graben differences in the occur-rence of this facies are interpreted to show a pro-pensity for lakes to have become established in largegrabens. This will be discussed in following sections.

Figure 7. Vertical seismic section showing four regional horizons mapped, including the basement (BAS), a lower synrift horizon(horizon SR2 [LSR]), an upper synrift horizon (horizon SR1 [USR]), and the top of the synrift (horizon T [TSR]). These horizons were usedto produce isochore maps of synrift stratigraphy.

Burton and Wood 1703

Deep Lacustrine

Deep lacustrine intervals were identified by theirseismic homogeneity and high amplitude in stratalslices (Figure 9F). Like shallow lacustrine intervals,amplitude homogeneity and continuity of deep la-custrine stratal slices indicate fine-grained deposi-tion (Sladen, 1997b; Maynard et al, 2002). Thehigher amplitude of deep lacustrine shales in theWNB as compared with the shallow lacustrine fa-cies is thought to result from the high organic con-tent of these deep lacustrine shales, which givesthem a lower density (Ginger et al., 1993; Sladen,1997b; Maynard et al., 2002). The deep lacustrinefacies is also confined to the largest half grabens.

1704 Seismic Geomorphology of Half Grabens, Indonesia

This facies is found exclusively in the middle syn-rift section underlying the shallow lacustrine fa-cies (Figure 6).

TECTONICALLY CONTROLLEDSYNRIFT STRATIGRAPHY

Several models for interpreting the fill of conti-nental rifts have been developed fromwell-knownanalogs (Schlische andOlsen, 1990; Prosser, 1993;Schlische, 1993; Lambiase et al., 1999; Gawthorpeand Leeder, 2000). Many of these studies focus onthe relationship between faulting style and sedi-mentary basin-fill character. The synrift stratigraphy

Figure 8. Vertical seismic section showing seven control horizons (in white) used to produce the 150 stratal slices used in this study.TWT = two-way traveltime.

of the WNB, as determined from the interpretedseismic facies and depositional systems previouslydiscussed, fits well within the conceptual frameworkof several models of continental rift stratigraphy.Graben geometry, fill character (facies presenceand stacking), isochore maps, and limited well-logsuites, all show a classic rift-related stratigraphy intheWNB, similar to the framework documented inother rift basins (Schlische andOlsen, 1990; Prosser,1993; Schlische, 1993; Lambiase and Bosworth,1995;Lambiase et al., 1999;Gawthorpe andLeeder,

2000). The stratigraphy between the T and base-ment horizons can be subdivided into four majortectonostratigraphic packages, here referred to as(1) lower synrift, (2) middle synrift, (3) uppersynrift, and (4) lower postrift (Figure 10).

Lower Synrift Description and Interpretation

The lower synrift tectonostratigraphic unit wasdeposited during the early stages of rifting, whenapproximately 12 small asymmetric half grabens

Figure 9. Seismic geomorphic facies shown in various amplitude stratal slices. (A) Small fluvial channels in lower postrift section,(B) point bar in upper synrift section, (C) bird-foot delta at the base of the postrift section, (D) distributary channel forms in middle synriftsection, (E) image interpreted to be an alluvial fan, and (F) deep lacustrine facies imaged in middle synrift stratal slices.

Burton and Wood 1705

(Figure 11) developed. Two seismic geomorphicfacies dominate the lower synrift: alluvial-fan andfluvial facies (Figure 12). Alluvial fans were de-

1706 Seismic Geomorphology of Half Grabens, Indonesia

posited directly on the basement during initiationof faulting. A thick succession of the fluvial seis-mic geomorphic facies sits atop and adjacent to

Figure 10. Arbitrary seismic line, flattened on horizon T through three deeper wells (with gamma-ray logs displayed) in the survey,highlighting rift-related tectonostratigraphic units. Note isolated small grabens in the lower synrift. Only the largest faults extend into theupper synrift. No wells penetrate the lower synrift, and the middle synrift in deeper parts is infrequently penetrated. Location of sectionon Figure 3. TWT = two-way traveltime.

the alluvial fans (Figure 6). Channels in the earlysynrift phase of fill parallel major fault trends, in-dicating strong tectonic control on sediment trans-port. In Figures 5 and 10, alluvial fans can be seen ashigh-amplitude reflectors directly above the base-ment. Fluvial units in the lower synrift appear seis-mically transparent with mostly chaotic reflections.An isochore of this interval highlights the isolatedasymmetry of early synrift half grabens (Figure 11).Lower synrift fill onlaps the flexural margin of halfgrabens and terminates against the faulted margins(Figure 10).

During initiation of rift-basin development,grabens are typically small, isolated, and fault andflexure bound (Gupta et al., 1998; Gawthorpe andLeeder, 2000). During this early stage of fault-ing, alluvial fans were shed off the margins of thehalf grabens, and regional drainage was deflectedinto the topographic lows. In the WNB, this allu-vial/fluvial unit is known as the Lama Formation(Pollock et al., 1984; Ginger et al., 1993; Michaeland Adrian, 1996; Phillips et al., 1997; Maynardet al., 2002;Morley et al., 2003; Doust andNoble,2008).

Figure 11. Isochores of tectonostratigraphic units in survey areas. (A) Lower synrift section characterized by numerous small grabens.(B) Middle synrift has fewer active faults. (C) By late synrift, only the largest fault is active. (D) Entire synrift fill reaches 3-s two-waytraveltime (∼5 km [∼3.1 mi]).

Burton and Wood 1707

Middle Synrift Description and Interpretation

Themiddle synrift marks a dramatic change in half-graben geometry and fill character, with three seis-mic geomorphic facies being dominant (Figure 6).Deep lacustrine is the main facies imaged in thebase of the middle synrift. A thick succession ofshallow lacustrine facies occupies most of the mid-dle synrift and is capped by the deltaic facies.Deep lacustrine shales form high-amplitude con-tinuous reflectors. Shallow lacustrine shales havesimilar seismic character but are of lower ampli-tude. Discontinuous and variable-amplitude seis-

1708 Seismic Geomorphology of Half Grabens, Indonesia

mic reflections indicate deltaic units imaged in thetop of the middle synrift. A few wells penetrate themiddle synrift and gamma-ray logs upward depo-sitional shallowing. In the Kakap-1 and the Forel-2wells (Figure 10), the middle synrift stratigraphicinterval is characterized by an upward decrease ingamma-ray response. An isochore of the middlesynrift indicates that subsidence during this phasewas concentrated along a fewmajor through-goingfaults (Figure 11).

During middle synrift times, subsidence wasconcentrated along major half-graben–boundingfaults, and many minor faults appear no longer

Figure 12. Interpreted depositional environments in the early (A), middle (B), and late synrift (C). Channelized features illustrated inFigure 9 are noted.

active. Rift models suggest that as the major faultsbegin to link, intrabasinal highs were establishedbetween fault segments (Schlische andOlsen, 1990;Prosser, 1993; Gupta et al., 1998; Lambiase et al.,1999; Gawthorpe and Leeder, 2000). In addition,basin subsidence increased along these longer faultsegments (Schlische and Olsen, 1990; Gupta et al.,1998; Gawthorpe and Leeder, 2000). As a result,subsidence outpaced sedimentation, and lakes wereestablished in the active half grabens. Deep lacus-trine units occur at the base of the middle synrift.Lake fills indicate shallowing, and themiddle synriftinterval is capped by deltaic deposits. This upwardshallowing occured as major basin-bounding faultsbecame linked, reducing the topography of intra-basinal highs and allowing progradation of fluvial/deltaic systems at the basin margin and filling thehalf-graben lakes (Figure 12). Lacustrine shales inthe middle synrift are known as the Benua or BelutFormation, and lacustrine deltaics belong to thelower Gabus Formation (Pollock et al., 1984; Gingeret al., 1993; Michael and Adrian, 1996; Phillipset al., 1997; Maynard et al., 2002; Morley et al.,2003; Doust and Noble, 2008).

Upper Synrift Description and Interpretation

Channel forms are abundant in the upper synriftfill of the WNB, being imaged on 92% of the am-plitude stratal slices of this interval. The fluvial faciesis by far the most prevalent seismic geomorphicfacies in the upper synrift interval (Figure 12). Inplaces, deltaic and fluvial channel forms were in-distinguishable, and these units aremarked as fluvial/deltaic in Figure 6. Channels generally trend north-east to southwest, parallel or slightly oblique tomajor fault trends. Fluvial/deltaic channel seismicreflectors are discontinuous and have highly vari-able amplitudes. The upper synrift and lower post-rift isochore show thickening into the largest graben-bounding fault (Figure 11).

In survey areas, only one major through-goingfault appears to have been active during late synrifttimes. As in other rift basins, intrabasinal highswere nonexistent at this tectonic stage, and fluvialsystems fill the once lacustrine-dominated grabens(Schlische andOlsen, 1990; Lambiase et al., 1999;

Gawthorpe and Leeder, 2000). Increased gamma-ray and decreased seismic amplitude in the top ofthe upper synrift may indicate a change in fluvialcharacter. As fault movement decreases duringthe later stages of rifting, degradation of the sourcearea results in a decrease in the potential energy offluvial systems (Ravnås and Steel, 1998; Lee et al.,2001)—a possible explanation for observed upwardfining of fluvial sandstones at the top of the syn-rift. Fluvial/deltaic sandstones in the upper synriftare considered to belong to the Gabus Formation(Pollock et al., 1984; Ginger et al., 1993; Michaeland Adrian, 1996; Phillips et al., 1997; Maynardet al., 2002;Morley et al., 2003; Doust andNoble,2008).

Lower Postrift Description and Interpretation

The lower postrift marks the transition from fault-controlled subsidence to thermal subsidence in theWNB. A major unconformity at the margin of thesurveymarks the base of the lower postrift (Figure 6).The fluvial and deltaic seismic geomorphic faciesare simultaneously present on some lower postriftstratal slices, and channels trend oblique or normalto the northeast-southwest fault trend. The T ho-rizon is picked at the base of the Barat Shale andis the topmost horizon used for stratal slicing(Figure 7). The Barat Formation is marine shalethroughout most of the WNB (Michael and Adrian,1996; Phillips et al., 1997; Doust and Sumner, 2007;Doust and Noble, 2008).

Lower postrift stratigraphy is quite variable.The central part of the basin (north and west inthe Gabus and Belanak surveys) is more lacustrineor marine in character. At the margins of the basin(south and east), fluvial channels and deltas areimaged. The lower postrift is penetrated by almostall wells in the survey area. Downhole gamma-raylogs through this interval appear sandier (lowergamma count in the Belanak-5 well) at the basinmargin and show increasing silt and clay (highergamma count in the Kakap-1 well) in the basincenter (Figure 10).

As the basin transitioned from fault controlledto thermal subsidence, the locus of subsidencemoved to the basin center (Katz, 1995; Lee et al.,

Burton and Wood 1709

2001). This change in subsidence distribution flexedthe basinmargin upward, and this flexural highwaseroded, creating a major unconformity (Sladen,1997b). The central part of the basin was inun-dated with marine waters. At the edge of the ba-sin, rivers draining the flexed basinmargin emptiedinto this body of water. Thus, fluvial and deltaicenvironments are commonly imaged in lower post-rift stratal slices. Brackish mudstones of the postriftare known as the Barat Formation, and associatedsandstones are called either the upper Gabus orUdang Formation (Ginger et al., 1993; Michaeland Adrian, 1996; Phillips et al., 1997; Maynardet al., 2002; Prasetyo, 2002; Doust and Noble,2008).

The previous discussion integrating seismic geo-morphology and stratigraphy documents the clas-sic continental half-graben-fill character duringthe early development history of theWNB. Faciesdistributions and proportions of fill vary; however,it is important to analyze each set of half grabensindividually.

IMPLICATIONS FOR EXPLORATION IN THEWEST NATUNA BASIN

Seismic geomorphology is a valuable analytical ap-proach for interpreting the depositional history, pro-cesses, and fill architecture of a basin (Posamentier,2004;Wood,2004,Davies et al., 2006;Wood,2007).Even in theWNB,where basin fills are asymmetric,folded, and difficult to image, stratal slicing is aneffective data analysis technique for imaging basin-fill architectural elements. In frontier or early pe-troleum exploration conditions, fewwell data existto interpret reservoir, seal, and source facies. In suchcircumstances, and as is demonstrated by this study,seismic geomorphology can provide informationconcerning the occurrence and distribution of hy-drocarbon system components (Posamentier, 2004).

Exploration in most Southeast Asian rift ba-sins is hindered by difficulty identifying reservoirsand reservoir quality (Sladen, 1997b). The prov-enance for synrift clastic reservoirs in the WNB istypically uplifted metamorphic and granitic horstblocks (Sudarmono and Eza, 1997). Synrift sand-

1710 Seismic Geomorphology of Half Grabens, Indonesia

stones tend to be immature because of their lowclose proximity to source and a feldspathic prov-enance (Sladen, 1997b; Sudarmono andEza, 1997).Burial compaction and immaturity increase withdepth in rifts, further limiting reservoir potential(Sudarmono and Eza, 1997). However, in some in-stances, diagenesis of immature sediments contrib-utes to porosity and permeability of sandstones inWest Natuna (Pollock et al., 1984). No attempt ismade here to reconstruct the diagenetic history ofreservoirs, but the possibility of secondary porosityand permeability development may increase theprospectivity of synrift reservoirs. In general, syn-rift reservoirs in theWNBare relatively unexplored(Sudarmono and Eza, 1997; Lambiase et al., 1999).The quality of Belanak and Gabus 3-D seismic sur-veys allows us to identify potential reservoirs fromstratal slices within the synrift section.

Lacustrine deltas form potentially good res-ervoirs in lower middle synrift rocks, particularly,flexural margin deltas. Hydrocarbons in rifts gen-erally migrate outward from the “kitchen” towardthe flexural margin (Lambiase et al., 1999). In ad-dition, flexural margins are less likely to be affectedby both rift and inversion faulting. As a result, flex-ural margin deltaic reservoirs have the highest po-tential of receiving a charge and maintaining sealintegrity. Limiting factors for productivity in thiscase are reservoir size and lithologic composition.Channelized features, associated with the flexuralmargin (Figure 9D) and found in the lower middlesynrift fill, are interpreted as deltas and indicate thepresence of clastic sediments accumulating in ba-sin margins. High-amplitude deep lacustrine shalesare thought to be the dominant source rock in theWNB (Sladen, 1997b; Maynard et al., 2002; Doustand Sumner, 2007; Doust and Noble, 2008). Theproximity of these deltas to source facies providesa high probability of success with charge. Thesurrounding thick succession of lacustrine shalesshould provide excellent seals for synrift reservoirs.No wells in the available data set penetrate theseflexural margin deltas, making them unexploredplays. Significant questions associated with middle-synrift deltaic sandstones are reservoir quality (i.e.,immature sandstone) and relative timing of hydro-carbon generation and trap formation.

CONCLUSIONS

The WNB is an inverted Cenozoic rift in the south-ern South China Sea. Hydrocarbons potentiallyexist in the relatively unexplored synrift section(Sladen, 1997a, b; Maynard et al., 2002). Inade-quate well data increase the exploration risks be-cause little is known about the synrift petroleumsystems in much of the basin. Three-dimensionalseismic data provide insight into the nature of thesynrift fill, but the discontinuity of seismic reflec-tors in fluvial, deltaic, and lacustrine synrift fillsmakes the imaging and mapping of continuoussurfaces difficult. In addition, the half-graben fill isasymmetric and was folded during the middleMiocene inversion of the basin. As a result, con-ventional interpretation methods, such as horizoninterpretation and slicing, do not produce imagesof adequate quality for seismic geomorphologicalanalysis. Stratal slicing was used and found to be aneffective method of producing surfaces that yieldinterpretable attribute extractions of synrift de-positional architectures.

Seismic geomorphic analysis was conductedon 150 stratal slices. Five principal facies were iden-tified. Channelized intervals were interpreted asfluvial (single trunk) or deltaic distributive (exhibit-ing multiple trunks in a distributary pattern). Stratalslices showing a homogeneous amplitude were in-terpreted as representing fine-grained, deep (high-amplitude), or shallow (low-amplitude) lacustrineintervals. High-amplitude lobate bodies imaged inthe basal synriftwere interpreted to be alluvial fans.An interpretation of each of the 150 stratal-sliceimages provided for a robust seismic stratigraphicinterpretation of rift-related stratigraphy.

Rift stratigraphy across the study area showssignificant tectonic control. Rift fill can be dividedinto four tectonostratigraphic units: lower synrift,middle synrift, upper synrift, and lower postrift.The basal lower synrift is composed of alluvial fansand fluvial facies that were deposited when fault-ing was initiated and small isolated grabens de-veloped. During the middle synrift stage, majorbasin-bounding faults commenced to be linked.Interbasinal highs and increased subsidence createdunderfilled basins, and deep lakes became estab-

lished. As through-going faults developed, graben-axial rivers filled the rift lakes with sediment. Flu-vial processes dominated during the late synrift,and only the largest faults were active. As the basintransitioned from fault- to thermal-controlled sub-sidence, the center of the basin became inundated.The basinmarginwas upwarped, as indicated by anunconformity at the basin margins.

Seismic geomorphic analysis was used to re-construct the stratigraphic-fill history of a set ofgrabens in the WNB. Integrated seismic geomor-phic and stratigraphic study can also be used toidentify and characterize potential hydrocarbon sys-tem elements. Lower synrift lacustrine rift-margindeltas encased in lacustrine shales are thought topresent the best opportunity for play developmentin the study area. This study demonstrates the use of3-D seismic geomorphology, in which well data arelimited, for exploring deep rift-basin opportunities.

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