seismic sequence analysis and reservoir potential of ... · of the carbonate platforms of southeast...

AAPG Bulletin, v. 86, no. 2 (February 2002), pp. 213–232 213

Seismic sequence analysis andreservoir potential of drownedMiocene carbonate platforms inthe Madura Strait, East Java,IndonesiaA. Kusumastuti, P. Van Rensbergen, and J. K. Warren

ABSTRACT

Seismic analysis of four Miocene carbonate buildups (the Porong,KE-11C, KD-11E, and BD buildups) in the Madura Strait region,Indonesia, shows that they are constructed of four seismic units (1–4). All the buildups are located atop an east-west–trending Oligo-cene fault block, which, through its uplift, favored the depositionand aggradation of shoal-water carbonates. Aggradation, platformnarrowing, and backstepping of leeward reefs into the upper partsof individual buildups typify the growth profiles. Adjacent off-buildup sedimentary rocks, deposited at the same time as the build-ups, show clear horizontal onlap relationships. In combination,these observations imply that all the platforms were drowned. Theuppermost parts (unit 4) of each of four major drowned buildupsin the trend have been drilled to test for the presence of hydrocar-bons. An analysis of the drilling results, in combination with ourseismic analysis, shows that those buildups that have relatively highlevels of hydrocarbons have suitable trap configurations but lackseal integrity. For example, compactionally induced strain and grav-itational gliding created leaky collapse-graben faults (extensionalstructures) atop some buildups (Porong, KE-11C), which allowedhydrocarbons to drain. Likewise, the updip inclination of some po-tential seal beds facilitated leakage (KE-11E).

In terms of potential targets in similar drowned buildups in theregion, higher priority should be given to targets that (1) show evi-dence of ongoing platform backstepping and so-called give-up reefgrowth profiles associated with incipient drowning, (2) contain allfour seismic units (units 1–4) in the buildup, (3) lack an extensionalcollapse graben over the crest of the drowned structure, and(4) demonstrate bed inclinations in the sealing beds that indicatelikely closure over the crest of the buildup.

Copyright �2002. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received January 24, 2000; revised manuscript received April 5, 2001; final acceptance June26, 2001.

AUTHORS

A. Kusumastuti � Lasmo Companies inIndonesia, Ratu Plaza Office Tower, 29th Fl.,Jl. Jendral Sudirman 9, Jakarta 10270,Indonesia; [email protected]

Arse Kusumastuti graduated with a B.Sc.degree in geology from Gadjah MadaUniversity (Indonesia) in 1991 and receivedher M.Sc. degree in petroleum geosciencefrom the University of Brunei Darussalam in1998. From 1991 to 1999 she worked forHuffco/Lapindo Brantas before joining LasmoIndonesia as a new ventures and explorationgeologist. Her professional interests aresequence stratigraphy and petroleum reservoirevaluation. She is a member of AAPG, IPA(Indonesian Petroleum Association), and IAGI(Indonesian Geological Association) andserves as one of the chairmen for FOSI(Indonesian Sedimentologists Forum).

P. Van Rensbergen � Renard Centre ofMarine Geology, Gent University, Krijgslaan281-S8, 9000 Gent, Belgium;[email protected]

Pieter Van Rensbergen is a fellow of theNational Fund for Scientific Research at theRenard Centre of Marine Geology (GhentUniversity, Belgium). He acquired his doctoraldegree at Ghent University in 1996 and was apostdoctoral research fellow in theDepartment of Petroleum Geosciences at theUniversity of Brunei Darussalam from 1997 to1999. He is experienced in the geologicalinterpretation of two-dimensional and three-dimensional seismic-reflection data in terms ofsedimentary processes and structural/stratigraphical evolution. His research interestsare the stratigraphy of composite sequencesand the study of subsurface sedimentmovement in shale diapirs and mudvolcanoes.

J. K. Warren � Department of PetroleumGeosciences, University of Brunei Darussalam,Tungku Link, Bandar Seri Begawan, Brunei;[email protected]

J. K. Warren is a faculty member of theDepartment of Petroleum Geosciences,University Brunei Darussalam (UBD), wherehe teaches reservoir studies, as well ascarbonate and evaporite systems. Prior to

214 Madura Strait Miocene Carbonate Platforms (Indonesia)

INTRODUCTION

During the Tertiary, the deposition of tropical shallow-water car-bonates, including reefs, was extensive in the tectonically complexregion of southeast Asia—centered on the islands of Borneo, Su-lawesi, and Java—where three major tectonic plates and several mi-nor plates have been interacting since the Mesozoic (Fulthorpe andSchlanger, 1989; Wilson, 2000). Carbonate platforms waxed andwaned across midcontinental blocks, around the edges of exten-sional back-arc basins, and in fore-arc and intra-arc settings.Growth, development, and location of buildups were strongly in-fluenced by tectonic subsidence, active faulting, volcanism, ante-cedent basement structure, and topography. Of the Tertiary build-ups, Miocene reefs and platforms are especially important inIndonesia, where they are significant hydrocarbon targets both off-shore and onshore (Sun and Esteban, 1994). The first significanthydrocarbon production from Indonesian reefal carbonates beganin 1952 (Nayoan et al., 1981). Outcrop equivalents of the reservoirrocks occur on many of the major islands of the Indonesian Archi-pelago (Figure 1A) (Doust, 1981; Grainge and Davies, 1985; Wil-son and Moss, 1999). Saller et al. (1993) and Cucci and Clark(1993) have conducted sequence stratigraphic studies on Indone-sian buildups with similar profiles, ages, and eustatic responses tothose under study.

Southeast Asian Miocene carbonate buildups typically entrainseveral stages or cycles of development (largely third-order) sepa-rated by discontinuities in platform growth, with some showingevidence of episodic subaerial exposure. Sun and Esteban (1994),in their study of Miocene reefs and buildups in southeast Asia, doc-umented a warming trend, paralleled by a generally rising second-order sea level, which allowed thick coral reefs and skeletal banksto develop, most of which are seismically resolvable (Figure 1B).The buildups under study fit this paradigm. According to Sun andEsteban (1994), the regional development of economic reservoirsis mainly related to relative sea level falls (third-order) and associ-ated meteoric diagenesis. These porosity-enhancing events occurredduring an overall aggradation of the platform and are identified incore by solution-enhanced porosity, calcite cementation, and char-acteristic stable isotope signatures. Sun and Esteban (1994) alsonoted that the basal transgressive carbonates to more localizedbuildups are mostly tight because of their relatively fine-grainedtextures, intense compaction, and isolation from meteoric waterinfluence. Best reservoir quality is commonly developed beneathsubaerial unconformities atop highstand stages, where effects ofmeteoric water leaching and karstification are most intense.

We are aware, however, that porosity distribution in many rap-idly subsiding southeast Asian Tertiary reefal reservoirs is not sim-ply related to meteoric exposure of clean lime sands associated witha terminal exposure surface. Mud is a major component of manyof the carbonate platforms of southeast Asia, and substantial partsof total reservoir storage lie within such muds (Longman et al.,

joining UBD he was executive director of JKResources Pty. Ltd (1995–1998). In his lastacademic appointment before joining UBD hewas professor of petroleum geology (1993–1995) in the School of Applied Geology atCurtin University in Perth, Western Australia.From 1989 to 1992 he was principal researchscientist at the National Centre of PetroleumGeology and Geophysics in Adelaide,Australia, and before that spent the 1980s onthe faculty at the University of Texas at Austin.In the early and middle 1970s he worked forthe Geological Survey of South Australia andEsso Australia. Warren’s interests are incharacterizing rock matrix properties ofpetroleum reservoirs and metalliferoussystems.

Kusumastuti et al. 215

A.

B.

SLOPE

Patch reefSkeletal BankBack/interreef Back/interreef

Periplatform Periplatform

Basal transgressivecarbonate sands

SHELF BASIN

Shelf margin reef

Fore-reef talus

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Fore-reef talusBasinal carbonate

Sea level

Platform interior facies

Platform interior interreef limestone

Bioclastic bank debrisMain biohermal ridge

Periplatformdebris apron (withlimestone clasts)

1 km

30-60 m

Figure 1. (A) Depositional topography of Miocene reefal buildups based on the Kampung Baru field, Senkang Basin, Indonesia(modified from Mayall and Cox, 1988). (B) Facies relationships of Miocene platform carbonates in southeast Asia at the scale ofseismic resolution (modified from Sun and Esteban, 1994).

1993). Many reservoirs are stacked within the buildupsand indicate several higher order hiatuses or subaerialexposure episodes created prior to the drowningepisode.

Mayall and Cox (1988) showed the importanceof fractures in linking earlier meteoric biomoldic po-rosity in reservoirs within the Miocene buildups of theSenkang basin of Sulawesi. They also noted that themeteoric corrasion of the micrite was a contributingfactor to the occlusion of many fractures. Likewise,much of the economic porosity in Miocene reef res-ervoirs in the Kasim and Walio fields of the Salawatibasin, Irian Jaya, comes from meteorically inducedsolution-enhanced biomoldic units, which have beenpartially occluded by later dolomite and calcite ce-ments (Gibson-Robinson et al., 1990).

The buildup that constitutes the Arun field in theNorth Sumatra Basin experienced similar meteoric en-hancement of its depositional porosity. One-fourth ofthe field’s total porosity was created by meteoric dis-solution, but the remaining three-fourths results fromincompletely cemented lime-mud matrix material,which was preserved in its present porous state by itsextremely rapid subsidence in a back-arc basin setting.This rapid subsidence left little time for complete ce-

mentation of the lime mud (Jordan and Abdullah,1992). Relative sea level changes within all of theseIndonesian buildups, in combination with exposureepisodes and rapid subsidence, have controlled poros-ity development.

Many Miocene buildups in southeast Asia, includ-ing buildups in the region under study, are capped bydrowning surfaces. This is a typically sharp boundarybetween the buildup limestone and transgressiveshales, with little or no associated meteoric enhance-ment of porosity immediately beneath the drownedsurface. Drilled examples of this type of bounding sur-face include the Miocene Liuhua platform in theSouth China Sea (Moldovanyi et al., 1995) and theOligocene buildups of Central Kalimantan (Saller etal., 1993). The unconformity atop such drowned plat-forms lacks evidence of exposure; it simply records arapid deepening with indications of winnowing andsediment starvation prior to the onset of shale depo-sition. Schlager (1999a) has suggested that a newterm, “type 3 sequence boundary,” be formalized tocover this type of boundary: namely, a flooding sur-face between a highstand tract and an overlying trans-gressive tract. The term defines a situation in whichthe rate of subsidence exceeded even the most rapid


rate of sea level fall during a particular eustaticboundary.

Given that eustatic controls, interacting with rapidsubsidence, should control much of the porosity dis-tribution in the buildups under study, this article usesseismic analysis and sequence stratigraphy in thedrowned carbonate province in the Madura Strait, off-shore Indonesia, to (1) better define an explorationmodel for potential reefal buildups in the region, and(2) define those reflector geometries most likely to in-dicate potential reservoir buildups. We do use the sys-tem stratigraphic nomenclature (as defined by VanWagoner et al. [1988]) to interpret reflectors and haverelated them to Vail’s third-order sea level curve (Vailet al., 1977). We point out, however, that our eustaticinferences are highly tentative because of a paucity ofwell data within the buildups under study and the as-sociated lack of chronostratigraphic information belowthe top of the carbonate system. Therefore, in this ar-ticle, we concentrate on the economic implications ofthe various styles of internal reflectors for trap predic-tion and seal integrity within the region; we feel this isthe most useful exploration approach in any frontierprovince.

GEOLOGICAL EVOLUTION

The Madura Strait is located in the middle of the EastJava Basin between Madura Island to the north and thepresent-day volcanic arc to the south. The study areais located on an east-west–trending, Oligocene faultblock (Figures 2A, 3) (Pertamina, 1997). The carbon-ate buildups in the area consist of several individualbuildups growing above a common elongated plat-form. The trend is more than 100 km long and includesthe Porong-1, KE-11C, KE-11E, and BD-1 buildups(Figure 3).

The first extensional phase of interest occurredduring the Eocene; it created northeast-southwest–oriented rift basins filled with continental clastics thathost both source rock and good reservoirs (Figure 2B)(Pertamina, 1997; Darman and Sidi, 2000). Segmen-tation of Java into its characteristic longitudinal east-west–trending zones dates from the second extensionalphase in the Oligocene and lasted into the Miocene.Ongoing subsidence created a deep-marine environ-ment that was filled with sediments supplied from Ma-dura Island to the north and from Java to the south andwest. A late Oligocene uplift, probably combined withthe 29.5 Ma late Oligocene sea level lowstand, pro-

duced a regional unconformity atop many highsthroughout the basin (Darman and Sidi, 2000).

During the late Oligocene–early Miocene, tectonicactivity was relatively calm on the western side of theEast Java Basin but active in the central and easternparts (Darman and Sidi, 2000). The East Java area wasa marine environment except for the Old Andesite vol-canic arc to the south and an emergent area in thepresent-day Java Sea to the north (Figure 3). Duringthat period, a basinwide transgressive carbonate wasdeposited. This shallow-water carbonate becomesquite clean near the top of the interval and is associatedwith the initiation of carbonate mounds and reefs. Thiswidespread carbonate sedimentation represents thefirst time in the Tertiary that the entire region expe-rienced marine conditions. Carbonate platformsformed on some paleobasement highs, whereas deep-water shale and marls were deposited in the deeperareas (Widjonarko, 1990). At that time, the back-arcbasin south of the North Madura platform was subjectto relatively rapid subsidence and developed into adeep-water basin with an influx of volcaniclastic debrisflows derived from the Old Andesite volcanic arc tothe south. The top of the early Miocene carbonateis separated by an unconformity from Pliocene–Pleistocene deep-marine volcaniclastic sediment de-rived from the uplifted volcanic arc in the south (Fig-ure 2B) (Kusumastuti, 1998).

DATA AND METHODOLOGY

Data

Data made available by Lapindo Brantas, Inc. includea two-dimensional (2-D) seismic data set and well data(Figure 4). The 2-D seismic data set contains 96 seis-mic lines, which were interpreted using Landmark’sSEISWORKS 2-D software. The seismic data cover anapproximate area of 9000 km2 (150 � 60 km). Thewestern part of the Lapindo block is covered by 53seismic lines of the B-91 and B-96 surveys that are gen-erally of good quality. In the central part of Kodecoblock 29, seismic lines of the KM-89 and KM-90 sur-veys were used but are of poor quality. The interpre-tation was extended into the adjacent Mobil block us-ing 14 seismic lines of the MD-85 and MD-91 surveys,where the data quality is poor to fair.

Well data used in this article come from wellsPorong-1, KE-11C, KE-11E, KE-11G, and BD-1 andwas provided in the form of hard copies. Well data


Figure 2. (A) North-south structural cross section across the Madura Strait. (B) Summary of the geological history of East Java andthe Madura Strait (after Kusumastuti, 1998).

consist of composite logs, along with paleontologic andbiostratigraphic data. Strontium isotope data are avail-able only for the Porong-1 well. Wire-line log suitesinclude gamma-ray, resistivity, and sonic logs for allstudied wells. Checkshot data are available only for thePorong-1 and BD-1 wells.

Table 1 and the well cross section (Figure 5) sum-marize relevant well data. The carbonate buildups inthe region were first tested in 1987 by Mobil Oil usingthe BD-1 well, which encountered oil and gas at thetop of a porous Miocene carbonate buildup. In 1990,Kodeco drilled the KE-11E well in another buildup tothe southwest of the BD structure. It was abandonedas a dry hole; the carbonate interval is thinner in this

well than in the BD well and has lower porosities. In1991, Kodeco drilled the KE-11C well located on theshallowest culmination of the buildup trend. Faultsthat crosscut the buildup or thief sands in the overlyingsediments are thought to have facilitated leakage of hy-drocarbons from this structure. In 1993, the Porong-1well was drilled on a buildup located at the westernedge of a collapse feature that formed in overlying sed-iments. Hydrocarbon indications were poor duringdrilling, but sidewall cores returned oil that shows simi-lar characteristics to nearby surface seeps and isthought to be derived from lacustrine source rocks inthe underlying Eocene strata (Kusumastuti, 1998; Dar-man and Sidi, 2000).


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Figure 4. Time-structure map of depth to the top of Miocene carbonate in the study area. Position of various survey lines andbuildups also shown.

Methodology

This article focuses on sedimentary intervals ofMiocene–Pliocene age, which mostly occur between2.5 and 5.0 s two-way traveltime (TWTT). Three seis-mic horizons are correlated and mapped across the en-tire study area: top Oligocene, top carbonate, and intraPliocene marker.

Depositional sequences in carbonate buildups aremost commonly defined by unconformities in progra-dational reflection sets across carbonate platforms (Au-bert and Droxler, 1996). In contrast, the drowned car-bonate buildups in the study area are characterized byaggradation and platform narrowing into their upperparts that is indicative of platform retreat and drown-ing, possibly related to episodes of tectonic subsidence(Erlich et al., 1990; Schlager, 1999a). Sequence strati-graphic interpretation in this article is therefore basedon seismic facies analysis (variations in reflection char-acter), as well as reflection termination patterns(bounding surfaces). The Porong buildup is used as adepositional model for the other carbonate buildupsalong the same trend where, because of poor seismicdata quality, it can be difficult to differentiate the in-ternal seismic reflection character. Well data for the

BD, KE-11C and KE-11E buildups were also used tohelp interpret the lithological variation within the vari-ous sequences.

SEISMIC STRATIGRAPHY OF PORONGAND OTHER NEARBY BUILDUPS

The Porong buildup is located to the southwest of theKE-11C structure, from which it is separated by astructural saddle (Figure 4). The Porong-1 buildup isan early Miocene bioherm buried by Pliocene–Pleistocene sedimentary rocks. Its base is formed by thetop Oligocene unconformity, a laterally extensive seis-mic marker atop horst blocks in the region (Darmanand Sidi, 2000). The structure has a maximum thick-ness of 1630 m and covers an area of 220 km2 (22 �

10 km) at the base and narrows to 68 km2 (17 � 4km) at the top.

The Porong-1 well was drilled at the western edgeof a collapse feature that also affected shallower levels.Coring of the top of the carbonate established that thelate Pliocene strata directly overlay the top of the earlyMiocene buildup, implying a hiatus of at least 10 m.y.Hydrocarbon indications were poor during drilling,


Table 1. Well and Fluid Data Summary

PORONG-1 KE-11C KE-11E BD-1

Operator Lapindo Brantas, Inc. Kodeco Energy Co. Ltd. Kodeco Energy Co. Ltd. Mobil Oil Indonesia, Inc.Location 07 30� 28.392� S

112 45� 47.266� ELine B91-169, SP 1485

07 29� 05.43� S112 53� 02.31� ELine KM90-9A, SP 250

07 25� 38.05� S13 05� 03.22� ELine KM89-31, SP 324

07 22� 30.291� S113 16� 19.487� ELine MD85-208, SP 1090

Spudded August 3, 1993 July 31, 1991 August 10, 1990 July 13, 1987Completed November 2, 1993 October 30, 1991 December 9, 1990 December 8, 1987Total Depth 8659 ft 9968 ft 15515 ft 14026 ftStatus plugged and abandoned,

with oil showsplugged and

abandoned, with gasshows

plugged andabandoned

Oil and gas discovery

Remarks(carbonateinterval only)

Dolomite, limestone,mudstone to packstone.Occasional porosity(vuggy or moldic).Trace bright oilfluorescence. Wellflowed formation water.Wire-line porosityaverages 15%.

Flowed formation waterwith dissolved gas.Top of carbonate haswire-line–estimatedporosity around 30%.

Drillstem test near topcarbonate flowedformation water.Average wire-line–estimated porosity5–10%.

Limestone, mudstone-wackestone. Abundant largeforams, algae, coral near top.Microfractures, porosity 20–32%; permeability � 54 �

1010 md (plug-derived),passes downward intomudstone-wackestone,stylolites. Wire-line–estimatedporosity 11–20%, LowerCarbonate Marker near totaldepth. Total 162 ft gas and111 ft oil. Oil water contact at11,316 ft. Drillstem test flowed4000–5000 BOPD.

and on testing, the carbonate interval flowed formationwater. Very little core was recovered within the Porongbuildup because of a predominance of lost circulationintervals in the carbonate parts of the well. Paleonto-logical determinations were attempted on sidewallcores, but no age diagnostic fossils were recovered. Theonly fossils were long-ranging nannofossils, corallinered algae, coral fragments, and traces of encrustingforaminifera.

Strontium isotopic ages were calculated using a redalgal fragment from a sidewall core in the Porong-1well at a depth of 2584 m (8487 ft). The result, 87Sr/86Sr � 0.708548 � 0.000036, indicates an absoluteage of approximately 16 Ma (late early Miocene)within 2 m of the top of the buildup (age from Sr iso-tope curve of Koepnick et al. [1985]). Given a lack ofbiostratigraphic definition in the study area, this is thebest available estimate of the age of the buildups andmatches isotopically derived early Miocene ages frommany other carbonate buildups in the region (Kusu-mastuti, 1998). A sidewall core at 2584 m (8478 ft),

0.91 m (3 ft) above the top of carbonate, is dated bio-stratigraphically as middle–late Pliocene, about 3 Ma.The overlying Pliocene sediments are largely shalymudstones and were deposited in an outer neritic ordeeper water environment (Kusumastuti, 1998).

Description of Seismic Data

Four seismic facies units are recognized within the Po-rong carbonate buildup (Figure 6). These facies unitsare defined primarily on the basis of seismic reflectionpatterns, configurations, and the morphologic profiles.

Unit 1The Oligocene unconformity at its base and an ero-sional unconformity at its top bound unit 1. The unithas a uniform thickness of about 250–300 ms TWTTand is more or less flat-topped with a northward slopeof approximately 15� (Figure 6). The facies are char-acterized by subparallel, continuous seismic reflectionsof medium amplitude in the platform area and by


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Figure 6. (A) Uninterpreted seismic line B-91-169 through the early Miocene carbonate buildup. This line displays the asymmetrybetween the northern and southern flanks. The northern part shows the strong development of bioherms throughout the accumulationof units 1–4 and is inferred to indicate the windward margin. The asymmetry and isolation of the platform developed mostly in unit4. (B) Interpreted line drawing of carbonate sequences and systems tract. A large collapse structure occurs over the top of the buildup.

disrupted, discontinuous, lower amplitude reflectionsalong its northern slope. At the northern slope, the re-flections onlap the regional Oligocene unconformity(SP 1550 in Figure 6) and downlap on it southward(SP 1390 in Figure 6). At the top of the slope, thereare some isolated mounds (pinnacles?), but they areless common than in units 2–4.

As well as being visible in the Porong buildup, thereflectors bounding unit 1 can be traced laterallythrough the KE-11C buildup to the KE-11E area inthe east and into the BD buildup. Where it was inter-sected by the well KE-11E, cuttings and log analysisshow that unit 1 is composed of carbonates, not sili-ciclastics. Unit 1 also broadens to the southwest, where


it cannot be clearly resolved beneath a thickening coverof younger volcanic sediment.

Unit 2Unit 2, which is geometrically similar to unit 1 is rela-tively flat-topped but has a steeper slope of about 30�along the northern side and a gentler slope to the southas compared to unit 1 (Figure 6). Again, some isolatedmounds occur at the top of the slope. The unit broad-ens to the southwest of the main buildup but is lostbeneath chaotic diffractions of younger volcaniclasticsedimentary rocks. The top of the unit is marked by aclear facies change to unit 3. Where equivalents to unit2 are intersected by the well BD-1, cuttings and loganalysis show that it is composed of carbonates, notsiliciclastics.

This unit is differentiated from unit 1 based ondifferences in seismic facies. It is characterized by ir-regular, discontinuous, low-amplitude, subparallel tohummocky reflections. These reflections occur in anear random pattern marked by nonsystematic reflec-tion terminations and splits. At the base, the oldestsediment fills erosional depressions at the top of unit1. Again, onlapping reflection sets climb up the north-ern slope. Within the unit, the vague outline of abuildup can be seen (between SP 1380 and SP 1530),with discontinuous reflections onlapping it at the sides(Figure 6).

Unit 3Unit 3 is constrained in areal extent to regions abovethe top of unit 2. The unit is bounded at the top by anerosional unconformity (Figure 6). The seismic facies,similar to unit 1, is characterized by parallel, high-amplitude, continuous reflections that are truncated atthe platform margins. At the northern side, mound-shaped buildups occur at the top of the unit (SP 1410–1430 in Figure 6). Few onlapping reflections occur atthe base of the unit at the northern slope or in depres-sions at the top of unit 2.

Unit 4The areal extent of unit 4 is reduced compared to unit3, and the topography shows a stronger asymmetry(Figure 6). The northern slope is very steep, up to 55�;the southern slope is about 30�. The seismic facies arecharacterized by low-amplitude, subparallel discontin-uous reflections with increasing amplitude and conti-nuity toward the top. Typically, a high-amplitude re-flector defines the top of this unit; it is perhapsindicative of a condensed section associated with plat-

form drowning (see following section). At the margins,hummocky reflections occur in an aggradational pat-tern at the northern margin and backstepping at thesouthern margin. The unit ends at the top in a steep-sided cone. A regional collapse zone lies above the crestof unit 4 in some parts of the Porong buildup and ex-tends out to KE-11C, casting an acoustic shadow overthe underlying crest.

Interpretation

The Porong-1 buildup is an alternation of parallel-stratified facies (units 1 and 3) with poorly stratified,low-amplitude facies (units 2 and 4). The continuous,parallel, high-amplitude reflections that characterizefacies units 1 and 3 are typical of fine-grained platformor bank interior carbonates (e.g., Sun and Esteban,1994). The continuous reflections in units 1 and 3 arelikely caused by alternating successions of deeper la-goonal (wackestones or mudstones) and better ce-mented (denser), shallower platform sediments (bio-clastic packstones). The lateral extent of unit 1 impliesthat it may be equivalent to the unit described as basaltransgressive carbonate sand by Sun and Esteban(1994) (Figure 1B).

Similar laterally extensive platform interior car-bonates offshore Vietnam were tested and found tohave low porosity (�15%) and permeability due topervasive compaction (Mayall et al., 1997). Likewisein China, the Miocene bank interior carbonatesshowed low porosities on drowned platforms, whichhad not experienced subaerial exposure prior todrowning (Erlich et al., 1990). Continuous reflectionsin units 1 and 3 within the bank interior are clearlyeroded and onlapped by the basal reflections of over-lying units (Figure 6). Based on the published litera-ture, these erosional surfaces may indicate higher orderlowstand responses (fourth-order or fifth-order) (e.g.,Kendall and Schlager, 1981)

Discontinuous, low-amplitude facies (units 2 and4) are interpreted as cleaner carbonates. Based on acomparison with constituents of asymmetric facies inthe Holocene buildups in the East Java Sea and PulauSeribu (Roberts, 1987; Longman et al., 1993), thenorthern side of the Porong buildup is probably dom-inated by a mixture of coralgal patch reefs and bio-clastic sands (possibly with minor oolites), which passinto fine-grained intertidal and lagoonal sediments far-ther into the platform interior. At the base of units 2and 4, climbing onlap at the margins (especially thenorthern windward margin) can be interpreted either


as late lowstand or early transgressive deeper water car-bonate encroaching on the platform crest.

Along the northern edge of the Porong buildup,units 1–3 include high-relief mounded features towardtheir upper parts, all within the same geographic po-sitions, that is, mounded intervals tend to sit atop oldermounded intervals (shaded gray in Figure 6). Moundsseem to occur on top of each other, perhaps slightlyshifted toward the center in the upper part of unit 3.The mounded facies also forms a single high-relief,elongated buildup province in unit 4 of the Porong(Figures 6; 7A, B; 8). This inheritance in the stackingpattern suggests that the mounds are depositionalrather than erosional in origin. The fact that all thebuildups in the study area (the Porong-1, KE-11C, KE-11E, and BD-1 buildups) are isolated platforms and arenot joined together at any time during the depositionof units 2–4 suggests that buildups were always sepa-rated by deeper water areas (Figure 4). The strongasymmetry in the mounded growth profile in unit 4suggests a windward northern side and a leewardsouthern side to the buildup, much like the asymmetryseen in Holocene buildups in the East Java Sea andPulau Seribu (Roberts, 1987; Longman et al., 1993).

The profile of unit 4 in all platforms where it ispresent is that of a drowned system in which, under anoverall rising relative sea level, the areas of the variouscarbonate factories shrank until they gave up and weredrowned. Drowned buildups show reflector geome-tries and characters that can be used to distinguishthem from reefs whose growth was stopped by a low-ering of sea level and associated subaerial exposure(Figure 9A) (Erlich et al., 1990). As a reef complexdrowns, it typically evolves a narrowing-upward pro-file as the faster growing parts of the reef complex at-tempt to catch up to a rapid rise in relative sea level.A condensed section that is subsequently covered bysediments characterizes the uppermost part of anydrowned buildup.

Until their drowning, all of the buildups understudy possessed profiles typical of detached, reef-

rimmed platforms (Handford and Loucks, 1993). Priorto drowning, backstepping clearly occurred at the lee-ward margin of the Porong buildup, whereas thehealthier windward-facing bioherms maintained up-ward (aggradational) growth and attempted to catchup to sea level for a longer time than their morenutrient-starved leeward equivalents. Hence, thegrowth asymmetry of the buildup is clearly seen in pro-files (Figures 6; 7A, B) and the time-structure maps ofthe Porong buildup (Figure 8).

Platform interior facies change several times fromlow-amplitude discontinuous reflections to continu-ous, high-amplitude reflections within the Porong andother buildups under study (Figure 6; 3000–4400 msTWTT). This is interpreted as indicating fining-upwardtrends whereby at times the platform interior laggedbehind the faster growth at the platform margins. Asthe waters of the platform interior deepened, the bot-tom was increasingly shielded from waves and tides(Kendall and Schlager, 1981; Schlager, 1999a). Thiscycle of incipient interior drowning was repeated sev-eral times during deposition of units 2–4; each timethe areal extent of the platform lessened until finallythe volume of sediment generated by the carbonatefactory could not keep pace with sea level rise and thebuildup drowned. The onlapping geometries of thepost–unit 4 sedimentary rocks (SP 1300–1450 in Fig-ure 6) suggest that an increasing detrital influx fromthe south may have abetted long-term platform retreatdue to detrital poisoning. The drowning of the Porongbuildup occurred more than 10 m.y. prior to the arrivalof the Pliocene sediment wedge; its arrival is indicatedby reflectors that are the distal toes of basinal downlapsor even coastal onlaps.

After drowning, and prior to the arrival of the Pli-ocene wedge, the carbonates of the Porong area werecovered by a veneer of sediment, a condensed orsediment-starved sequence, which also defines themaximum flooding surface of the subsequent Pliocenesuccession (Figure 6). This surface atop unit 4 tends toform a characteristic high-amplitude reflector seen in

Figure 7. Representative lines through the various buildups. (A) Seismic line B-91-171 across the eastern part of the Porong buildup.Note that units 1–4 are present, there is strong asymmetry to the buildup, there is a late growth reef on the windward reef rim, andthere is horizontal onlap of adjacent sediment, all features indicative of a drowned platform. (B) Seismic line B-91-129 showing latestage growth reef buildup in the Porong platform. (C) Seismic line KM-90-07 shows the typically poor quality seismic data across theKE-11C carbonate buildup. Above the buildup is a collapse feature that continues along strike to the Porong buildup. (D) Seismic lineMD-85-202 across the KE-11E carbonate buildup. This buildup is the deepest in the study area and shows less vertical developmentthan others along the trend. (E) Seismic line MD-85-208 shows the BD carbonate buildup, which is the only hydrocarbon discoveryin the carbonate trend. Note the more favorable seal configuration immediately above the carbonate and the lack of a crestal graben.Line interpretations of these or similar nearby lines are given in Figure 10.


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lines crossing the Porong and other nearby buildups.Similar features define the top of most drowned plat-forms (Figure 9A) (Erlich et al., 1990; Schlager,1999a). The predominance of coralline red algae incore recovered near the top of the Porong-1 buildupmay indicate deepening during growth mounding andherald the drowning of the buildup. These red algaeprobably grew in several tens of meters of water depth,still in the euphotic zone but in water that was toodeep for prolific coral production (James and Mount-joy, 1983; Ehrlich et al., 1990).

The steep flanks of the windward flank of thePorong-1 buildup are not directly related to largedeeper faults but are depositional in origin. A majoroverlying extensional fault also exists, along with anassociated collapse graben whose position was con-trolled by compactional hinging and the strength con-trast between the well-cemented sediments of the un-derlying carbonate buildup and the depositionallylater, but adjacent, deeper water siliciclastic muds (Fig-ure 6B). This collapse feature extends farther along thecarbonate trend and can also be seen atop the KE-11Cbuildup (Figure 7C).

In summary, the Porong-1 buildup is interpretedas an asymmetric, at times reef-rimmed, carbonateplatform with a classic drowning profile (Figure 9A).It sits on the Oligocene unconformity (29.5 Ma); thetop of the buildup is dated at about 16 Ma (Sr date;see previous discussion). Second-order sea level rosecontinually during the time it was deposited, creatingongoing accommodation space for carbonate deposi-tion (Figure 9B; early Miocene). Transgressive systemtracts dominate the buildup, stacking patterns are ag-gradational, and highstand spillover and progradationalgeometries fed from the platform margin are missing,whereas lowstand wedges are small or missing. Thelack of these features in the Porong-1 buildup and theother buildups along the trend implies that the car-bonate sequences formed during accelerated rates oflong-term accommodation within a rapidly subsidingbasin.

Influence of Sea Level ChangesThe bounding facies pattern of units 1–4 suggests thatthe Porong platform experienced several episodes oflittle or no sedimentation. These times were typifiedby flooding surfaces, possibly preceded by times of ei-ther lowered or raised sea level (fourth-order or fifth-order) when the crests of the platform were subaeriallyexposed or covered by waters too deep for prolific car-bonate aggradation. The lower rimming parts of each

unit, where mounded facies are not easily discerned,are interpreted as deeper water platform facies or“start-up” phases, whereas the remainder of each unitis interpreted as a “catch-up” phase (terminology ofKendall and Schlager, 1981). The maintenance ofmound relief after the deposition of each unit ceasedand the lack of an adjacent prograding deeper waterprism suggest that aggrading or catch-up styles domi-nated through the deposition of each unit. The Porongplatform and others in the region uncommonly, if ever,attained a “keep-up” style of deposition (terminologyof Kendall and Schlager, 1981). In sequence strati-graphic terms, each of the four units are transgressivesystems tracts (Ehrlich et al., 1990).

Units 1 and 2 and units 3 and 4 likely are groupedresponses to longer term fluctuations of third-order sealevel cycles. If so, they indicate shallowing of units de-posited at times of high sea level, dominated in theirlower parts by deeper water sediment. This cyclic re-sponse creates layered muddy platform facies (unit 1or 3) that pass up into cleaner, shallower platform fa-cies deposited during times of lower relative sea levelrise (unit 2 or 4). Fitting this interpretation to thethird-order sea level curve of the early Miocene (Haqet al., 1987) means the erosion surfaces at the top ofunit 1 and unit 3 may correspond, respectively, to the21 and 16.5 Ma sequence boundaries, but this has yetto be confirmed by coring (Figure 9B).

Although biostratigraphic data are lacking to con-trol seismic stratigraphic interpretation of the internalreef geometries in the buildups, this same repetitivepattern allows us to extrapolate the Porong interpre-tation of shallowing, terminated by drowning, to theother buildups in the trend and so predict reservoirporosity based on sequence stratigraphic interpretationof seismic facies changes (Figure 9C). A likely se-quence stratigraphic interpretation can be made if oneassumes that all the buildups were subject to linearincreasing subsidence and that this effect can be addedto the Haq world sea level curve for the early Miocene(Figure 9B).

From the lower to the middle Miocene, third-order sea level is rising, sometimes quickly, sometimesslowly, punctuated with rapid high-amplitude falls at21 and 16.5 Ma, followed by rapid rises (Figure 9B)(Neogene transgressive cycle of Fulthorpe and Schlan-ger [1989]). The rapid high-amplitude rise around16.5 Ma corresponds to the drowning of the Porong,KE-11C, and BD-1 buildups (Figure 9B, C). It definesa type 3 surface atop these buildups. The preceding sealevel falls are interpreted as defining type 1 sequence


boundaries at the tops of units 3 and 4. The internallychaotic facies of units 2 and 4 are interpreted to indi-cate start-up/catch-up growth styles associated withearly transgression. The more laterally extensive inter-nal reflectors in the platform interior of units 1 and 3correspond to keep-up styles of platform sedimenta-tion associated with much slower rates of sea levelchange that typified the deposition of units 1 and 3.The small mounds at the top of unit 3 may correspondto the onset of more rapid sea level rise immediatelyprior to a major fall (Figure 9B). In terms of predictingreservoir quality using this model, drowned buildupsbeneath a type 3 boundary or platform buildups be-neath a type 2 boundary should both be high prioritytargets.

The ongoing rapid rise of the third-order sea levelprobably did not drown or terminate growth in thebuildups. Carbonate accumulation can typically keepup with rapid rates of third-order sea level rise (Kendalland Schlager, 1981; Schlager, 1999b). Terminaldrowning is more likely caused by higher order sealevel fluctuations that first exposed the carbonate plat-form and then flooded it abruptly before carbonateproduction could start up. The early Miocene is situ-ated at the transition from greenhouse to icehouse con-ditions, and the widespread drowning may reflect theincreasing amplitude, frequency, and rate of sea levelrise brought on by the onset of glacial-interglacial sealevel cycles. Alternatively, the terminal drowning mayreflect proximity to the Old Andesite volcanic arc inthe south, which periodically shed ash and other debristhat made the East Java basin inimical to carbonategrowth. Increased volcaniclastic sediment supply, de-creased light intensity, changed water chemistry due toeruptions, and increased tectonic subsidence are alter-natives that singularly or in combination may havecaused drowning of the buildups. Possible tuffs havenot yet been recognized, however, in wire-line logcurves run within the uppermost part of the carbonatesequence.

The seismically defined four-part stratigraphicsubdivision of the Porong-1 buildup can be extrapo-lated to the other buildups along the trend to generatea tentative correlation between the Porong-1, KE-11C,KE-11E, and BD-1 buildups (Figure 9C). Well dataindicate that the tops of Porong-1, KE-11C, and BD-1are porous, with average porosities of 20–32%. Theseuppermost intervals are interpreted to be equivalentsof unit 4. In the BD-1 well, the porosity decreases to11–20% below the oil-water contact. In contrast to theother wells, the KE-11E well sampled a tight formation

in the carbonate interval, with an average porosity of8% (Table 1). Based on its deep structural position, theKE-11E well is most likely made up of the unit 1 sed-imentary rocks (Figure 9). This is also seen in seismicdata across the buildup, which do not show the differ-entiation into units 1–4 that typifies other buildups;instead, the data show internal character typical of unit1 only (Figure 7D). The KE-11E structure appears tohave drowned earlier than the other buildups in theregion, so sequences 2–4 never formed.

Today, the top of the BD carbonate sequence isdeeper than the Porong and KE-11C buildups (Figures4, 9). This relates to the regional history of the lateMiocene, when the southern basin was subsiding intoa deep-water environment. The rates of subsidencewere not the same across the basin; the central andeastern parts were tectonically more active and so sub-sided faster than the western part. Hence, the BDbuildup has a better seal configuration and is overlainby thick shale (or other fine-grained sedimentaryrocks); it was the only well where drilling encounterednoticeable hydrocarbons (Table 1).

EXPLORATION IMPLICATIONS

Worldwide, 43% of productive hydrocarbon reservesin carbonate buildups occurs in Miocene strata (Green-lee and Lehmann, 1993). Drowned carbonate buildupshave long been attractive targets in both frontier andmature basin settings; they tend to be porous large-scale structures with good four-way closure. Becausethey are commonly encased in shales, such buildupstend to be one of the more recognizable carbonatestructures in seismic reflection data and are typicallyamong the first carbonate prospects to be identifiedand tested in a new basin.

In any frontier hydrocarbon region, the major riskfactors are source, migration, reservoir, trap, and seal.Early rift-related sediment fills (Eocene) in continentalgrabens underlying the buildups in the Madura Straitand East Java regions are thought to contain organic-rich lacustrine shales (Darman and Sidi, 2000). This issupported by the hydrocarbon discovery in the BD car-bonate buildup and hydrocarbon seeps around the Po-rong area, demonstrating that hydrocarbon maturationand migration has occurred in the study area. Hydro-carbon source, generation, and migration are consid-ered relatively minor geological risks in the study areacompared to reservoir, trap, and most importantly, sealintegrity.


Work to date has shown that unit 4 in the earlyMiocene carbonate buildups in the Madura Strait is themost attractive target for exploration. Where present,the upper part of unit 4 shows strong evidence ofgrowth-induced closure via drowning followed by en-casement in fine-grained deeper water sediments. Thefact that its upper surface is a type 3 boundary impliesthat porosity is likely to be retained by rapid subsidenceand associated exclusion of meteoric waters, whichwould otherwise redistribute or even occlude porosity.The porosity potential of the upper part of unit 4 issupported by the hydrocarbon discovery in the BD car-bonate buildup.

Seal integrity appears to be a major geological riskassociated with the carbonate buildups (Downey,1994), including those in the study area. Sealing is de-pendent upon (1) the lithology of the strata immedi-ately above the buildup, (2) the seal-breaching capa-

bility of any collapse-zone faults above the buildup,and (3) the structural configuration of any potentialseal to the buildup. Drilling has shown that the pre-dominant seal in the study area is thick Pliocene shalewith minor silts and tight shaly sands. Although thick,it still may lack integrity where it is faulted or it con-tains laterally extensive thin sands. In advance of drill-ing, predicting the sealing capability of the faults abovethe carbonate buildups is still difficult, so any buildupsthat have one or more of features 1–3 should not begiven priority as potential targets.

For example, the Porong buildup (Figure 10A) ex-perienced a hydrocarbon charge during the time thatit was capped with a late Pliocene seal, as indicated byhydrocarbons noted during drilling of this interval.Much of this charge, however, was probably lost viaextensional fault breaches created during the episodeof compaction draping and gravity gliding characteris-

Figure 10. Seismic traces showing potential for trapping in the various buildups. (A) The Porong buildup is dry, perhaps reflectinga lack of seal integrity because of the extensional collapse graben over the crest and the updip drainage configuration in the sealingbeds. (B) The KE-11C buildup is dry for reasons similar to those for the Porong structure; note the potential for leakage indicated bycrestal breaching of the seal by the collapse zone. (C) The KE-11E buildup is dry because of a reservoir quality problem (tight-muddyfacies on unit 1 dominates); it also has an updip drainage configuration in the seal. (D) The BD discovery is the only buildup thattrapped hydrocarbons; it lacks a crestal graben and seal beds are in a trapping configuration.


tic of the overlying fault-defined collapse zone (aroundSP 1450–1500 in Figure 10A). The updip drainageconfiguration in potential seal beds atop the Porongstructure also lessened the likelihood of seal integrity,as it facilitated southerly updip hydrocarbon leakageand migration. A similar collapse-graben breach occursatop the KE-11C buildup (Figure 10B). As in Porong-1, these collapse-zone faults on top of the KE-11Cbuildup probably acted as conduits to secondary updipmigration (see the arrow direction on top of buildupin Figure 10). Both buildups failed because they lackeda prospective seal configuration.

Reservoir potential of the KE-11E type of play isthought to be low because of a lack of a suitable res-ervoir unit (i.e., no unit 2 or 4). The KE-11E carbonatebuildup failed because it shows less vertical develop-ment of facies than the other buildups in the region. Itlacks the repetitive four-tier internal reflector structureof the other structures (most likely only the lower po-rosity unit 1 is present). Seal integrity in the KE-11Ebuildup is also poor, as potential seal beds show anupdip drainage configuration. Any hydrocarbons prob-ably leaked away in a northerly direction (see arrow inFigure 10C). Relatively thick thief sands, which can beseen in the wire-line curves from the KE-11E well, per-haps aided leakage. Chances for success in Porong andKE-11C styles of drowned play or the incompletelydeveloped KE-11E style of structure are interpreted aslow.

Hence, the best play in this area is the BD type,where all four seismic units occur in the buildup. Thestructure is drowned; it lacks a crestal collapse grabenand has a thick Pliocene seal facies that also shows asuitably closed configuration in the region directlyoverlying the buildup (Figure 7E). That is, the BDbuildup has good top seal integrity and seal configu-ration, contains units 1–4, and is encased in more than106.7 m (350 ft) of Pliocene shale (Figure 10D). TheBD-1 well proved the presence of porosity and hydro-carbons (oil and gas) in this buildup, although the vol-ume of hydrocarbons was subeconomic.

In addition to the BD style of play, it is likely thathydrocarbons have leaked from Porong and KE-11Cstructures and so migrated laterally or updip, perhapsfocused into higher level traps by the fault structures.Hydrocarbons may well be trapped in structuresabove the early Miocene carbonates, but theseyounger Pliocene and Pleistocene prospects have yetto be tested. The presence of hydrocarbons in Plio-cene sands in the nearby Wanut field also supportsthis conjecture.

CONCLUSIONS

Detailed seismic analysis of the Porong and other earlyMiocene buildups in the region of the Madura Straithas shown a well-defined buildup trend in which thereare four recognizable seismic units. Analysis of thebuildup geometries in combination with well datashows that buildups are most likely to retain a hydro-carbon charge when all four seismic units are present.This is perhaps a water-depth–related effect. Thosebuildups that do not show mound-related rimmingstructures immediately beneath a type 1 surface maynot have built up to sea level and so were not subjectto the vagaries of meteoric exposure associated withthe higher order low-amplitude sea level oscillations.Also, if the upper part of a backstepping buildup is notcapped by a type 3 surface, the likelihood of the reten-tion of primary framework porosity is lessened.

In terms of priority for potential targets in buildupsin the region, higher priority should be given to targetsthat

1. Show evidence of ongoing platform backsteppingand ultimately last stage reef growth associated withincipient drowning

2. Entrain all four sequences (units 1–4) in the buildup3. Lack an extensional collapse graben over the crest

of the drowned structure4. Demonstrate bed inclinations in the sealing beds

that indicate likely closure over the crest of thebuildup

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