Seismic sequence analysis and AUTHORS A. Kusumastuti ϳ Lasmo Companies in reservoir potential of drowned , Ratu Plaza Office Tower, 29th Fl., Jl. Jendral Sudirman 9, Jakarta 10270, Miocene carbonate platforms in Indonesia; [email protected] Arse Kusumastuti graduated with a B.Sc. the Madura Strait, East , degree in geology from Gadjah Mada University (Indonesia) in 1991 and received Indonesia her M.Sc. degree in petroleum geoscience from the University of Brunei Darussalam in A. Kusumastuti, P. Van Rensbergen, and J. K. Warren 1998. From 1991 to 1999 she worked for Huffco/Lapindo Brantas before joining Lasmo Indonesia as a new ventures and exploration geologist. Her professional interests are sequence stratigraphy and petroleum reservoir ABSTRACT evaluation. She is a member of AAPG, IPA Seismic analysis of four Miocene carbonate buildups (the Porong, (Indonesian Petroleum Association), and IAGI KE-11C, KD-11E, and BD buildups) in the Madura Strait region, (Indonesian Geological Association) and Indonesia, shows that they are constructed of four seismic units (1– serves as one of the chairmen for FOSI 4). All the buildups are located atop an east-west–trending Oligo- (Indonesian Sedimentologists Forum). cene fault block, which, through its uplift, favored the deposition P. Van Rensbergen ϳ Renard Centre of and aggradation of shoal-water carbonates. Aggradation, platform Marine Geology, Gent University, Krijgslaan narrowing, and backstepping of leeward reefs into the upper parts 281-S8, 9000 Gent, Belgium; of individual buildups typify the growth profiles. Adjacent off- [email protected] buildup sedimentary rocks, deposited at the same time as the build- Pieter Van Rensbergen is a fellow of the ups, show clear horizontal onlap relationships. In combination, National Fund for Scientific Research at the these observations imply that all the platforms were drowned. The Renard Centre of Marine Geology (Ghent uppermost parts (unit 4) of each of four major drowned buildups University, Belgium). He acquired his doctoral in the trend have been drilled to test for the presence of hydrocar- degree at Ghent University in 1996 and was a bons. An analysis of the drilling results, in combination with our postdoctoral research fellow in the seismic analysis, shows that those buildups that have relatively high Department of Petroleum Geosciences at the levels of hydrocarbons have suitable trap configurations but lack University of Brunei Darussalam from 1997 to seal integrity. For example, compactionally induced strain and grav- 1999. He is experienced in the geological interpretation of two-dimensional and three- itational gliding created leaky collapse-graben faults (extensional dimensional seismic-reflection data in terms of structures) atop some buildups (Porong, KE-11C), which allowed sedimentary processes and structural/ hydrocarbons to drain. Likewise, the updip inclination of some po- stratigraphical evolution. His research interests tential seal beds facilitated leakage (KE-11E). are the stratigraphy of composite sequences In terms of potential targets in similar drowned buildups in the and the study of subsurface sediment region, higher priority should be given to targets that (1) show evi- movement in shale diapirs and mud dence of ongoing platform backstepping and so-called give-up reef volcanoes. growth profiles associated with incipient drowning, (2) contain all ϳ Department of Petroleum four seismic units (units 1–4) in the buildup, (3) lack an extensional J. K. Warren Geosciences, University of Brunei Darussalam, collapse graben over the crest of the drowned structure, and Tungku Link, Bandar Seri Begawan, Brunei; (4) demonstrate bed inclinations in the sealing beds that indicate [email protected] likely closure over the crest of the buildup. J. K. Warren is a faculty member of the Department of Petroleum Geosciences, University Brunei Darussalam (UBD), where Copyright ᭧2002. The American Association of Petroleum Geologists. All rights reserved. he teaches reservoir studies, as well as Manuscript received January 24, 2000; revised manuscript received April 5, 2001; final acceptance June carbonate and evaporite systems. Prior to 26, 2001.

AAPG Bulletin, v. 86, no. 2 (February 2002), pp. 213–232 213 joining UBD he was executive director of JK INTRODUCTION Resources Pty. Ltd (1995–1998). In his last academic appointment before joining UBD he During the Tertiary, the deposition of tropical shallow-water car- was professor of petroleum geology (1993– bonates, including reefs, was extensive in the tectonically complex 1995) in the School of Applied Geology at region of southeast Asia—centered on the islands of Borneo, Su- Curtin University in Perth, Western Australia. lawesi, and Java—where three major tectonic plates and several mi- From 1989 to 1992 he was principal research nor plates have been interacting since the Mesozoic (Fulthorpe and scientist at the National Centre of Petroleum Schlanger, 1989; Wilson, 2000). Carbonate platforms waxed and Geology and Geophysics in Adelaide, waned across midcontinental blocks, around the edges of exten- Australia, and before that spent the 1980s on sional back-arc basins, and in fore-arc and intra-arc settings. the faculty at the University of Texas at Austin. Growth, development, and location of buildups were strongly in- In the early and middle 1970s he worked for fluenced by tectonic subsidence, active faulting, volcanism, ante- the Geological Survey of South Australia and Esso Australia. Warren’s interests are in cedent basement structure, and topography. Of the Tertiary build- characterizing rock matrix properties of ups, Miocene reefs and platforms are especially important in petroleum reservoirs and metalliferous Indonesia, where they are significant hydrocarbon targets both off- systems. shore and onshore (Sun and Esteban, 1994). The first significant hydrocarbon production from Indonesian reefal carbonates began in 1952 (Nayoan et al., 1981). Outcrop equivalents of the reservoir rocks 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 to those under study. Southeast Asian Miocene carbonate buildups typically entrain several stages or cycles of development (largely third-order) sepa- rated by discontinuities in platform growth, with some showing evidence 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 banks to develop, most of which are seismically resolvable (Figure 1B). The buildups under study fit this paradigm. According to Sun and Esteban (1994), the regional development of economic reservoirs is mainly related to relative sea level falls (third-order) and associ- ated meteoric diagenesis. These porosity-enhancing events occurred during an overall aggradation of the platform and are identified in core by solution-enhanced porosity, calcite cementation, and char- acteristic stable isotope signatures. Sun and Esteban (1994) also noted that the basal transgressive carbonates to more localized buildups are mostly tight because of their relatively fine-grained textures, intense compaction, and isolation from meteoric water influence. Best reservoir quality is commonly developed beneath subaerial unconformities atop highstand stages, where effects of meteoric 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 with a terminal exposure surface. Mud is a major component of many of the carbonate platforms of southeast Asia, and substantial parts of total reservoir storage lie within such muds (Longman et al.,

214 Madura Strait Miocene Carbonate Platforms (Indonesia) A. Main biohermal ridge Bioclastic bank debris

30-60 m Periplatform debris apron (with limestone clasts) 1 km Platform interior interreef limestone

B. Platform interior facies Back/interreef Patch reef Back/interreef Skeletal Bank Shelf margin reef Pinnacle Reef Periplatform Periplatform Sea level Fore-reef talus Fore-reef talus Basinal carbonate

Basal transgressive carbonate sands

SHELF SLOPE BASIN

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 of seismic resolution (modified from Sun and Esteban, 1994).

1993). Many reservoirs are stacked within the buildups mentation of the lime mud (Jordan and Abdullah, and indicate several higher order hiatuses or subaerial 1992). Relative sea level changes within all of these exposure episodes created prior to the drowning Indonesian buildups, in combination with exposure episode. episodes and rapid subsidence, have controlled poros- Mayall and Cox (1988) showed the importance ity development. of fractures in linking earlier meteoric biomoldic po- Many Miocene buildups in southeast Asia, includ- rosity in reservoirs within the Miocene buildups of the ing buildups in the region under study, are capped by Senkang basin of Sulawesi. They also noted that the drowning surfaces. This is a typically sharp boundary meteoric corrasion of the micrite was a contributing between the buildup limestone and transgressive factor to the occlusion of many fractures. Likewise, shales, with little or no associated meteoric enhance- much of the economic porosity in Miocene reef res- ment of porosity immediately beneath the drowned ervoirs in the Kasim and Walio fields of the Salawati surface. Drilled examples of this type of bounding sur- basin, Irian Jaya, comes from meteorically induced face include the Miocene Liuhua platform in the solution-enhanced biomoldic units, which have been (Moldovanyi et al., 1995) and the partially occluded by later dolomite and calcite ce- Oligocene buildups of Central Kalimantan (Saller et ments (Gibson-Robinson et al., 1990). al., 1993). The unconformity atop such drowned plat- The buildup that constitutes the Arun field in the forms lacks evidence of exposure; it simply records a North Sumatra Basin experienced similar meteoric en- rapid deepening with indications of winnowing and hancement of its depositional porosity. One-fourth of sediment starvation prior to the onset of shale depo- the field’s total porosity was created by meteoric dis- sition. Schlager (1999a) has suggested that a new solution, but the remaining three-fourths results from term, “type 3 sequence boundary,” be formalized to incompletely cemented lime-mud matrix material, cover this type of boundary: namely, a flooding sur- which was preserved in its present porous state by its face between a highstand tract and an overlying trans- extremely rapid subsidence in a back-arc basin setting. gressive tract. The term defines a situation in which This rapid subsidence left little time for complete ce- the rate of subsidence exceeded even the most rapid

Kusumastuti et al. 215 rate of sea level fall during a particular eustatic duced a regional unconformity atop many highs boundary. throughout the basin (Darman and Sidi, 2000). Given that eustatic controls, interacting with rapid During the late Oligocene–early Miocene, tectonic subsidence, should control much of the porosity dis- activity was relatively calm on the western side of the tribution in the buildups under study, this article uses Basin but active in the central and eastern seismic analysis and sequence stratigraphy in the parts (Darman and Sidi, 2000). The East Java area was drowned carbonate province in the Madura Strait, off- a marine environment except for the Old Andesite vol- shore Indonesia, to (1) better define an exploration canic arc to the south and an emergent area in the model for potential reefal buildups in the region, and present-day to the north (Figure 3). During (2) define those reflector geometries most likely to in- that period, a basinwide transgressive carbonate was dicate potential reservoir buildups. We do use the sys- deposited. This shallow-water carbonate becomes tem stratigraphic nomenclature (as defined by Van quite clean near the top of the interval and is associated Wagoner et al. [1988]) to interpret reflectors and have with the initiation of carbonate mounds and reefs. This related them to Vail’s third-order sea level curve (Vail widespread carbonate sedimentation represents the et al., 1977). We point out, however, that our eustatic first time in the Tertiary that the entire region expe- inferences are highly tentative because of a paucity of rienced marine conditions. Carbonate platforms well data within the buildups under study and the as- formed on some paleobasement highs, whereas deep- sociated lack of chronostratigraphic information below water shale and marls were deposited in the deeper the top of the carbonate system. Therefore, in this ar- areas (Widjonarko, 1990). At that time, the back-arc ticle, we concentrate on the economic implications of basin south of the North Madura platform was subject the various styles of internal reflectors for trap predic- to relatively rapid subsidence and developed into a tion and seal integrity within the region; we feel this is deep-water basin with an influx of volcaniclastic debris the most useful exploration approach in any frontier flows derived from the Old Andesite volcanic arc to province. the south. The top of the early Miocene carbonate is separated by an unconformity from Pliocene– Pleistocene deep-marine volcaniclastic sediment de- GEOLOGICAL EVOLUTION rived from the uplifted volcanic arc in the south (Fig- ure 2B) (Kusumastuti, 1998). The Madura Strait is located in the middle of the East Java Basin between to the north and the present-day volcanic arc to the south. The study area DATA AND METHODOLOGY is located on an east-west–trending, Oligocene fault block (Figures 2A, 3) (Pertamina, 1997). The carbon- Data ate buildups in the area consist of several individual buildups growing above a common elongated plat- Data made available by Lapindo Brantas, Inc. include form. The trend is more than 100 km long and includes a two-dimensional (2-D) seismic data set and well data the Porong-1, KE-11C, KE-11E, and BD-1 buildups (Figure 4). The 2-D seismic data set contains 96 seis- (Figure 3). mic lines, which were interpreted using Landmark’s The first extensional phase of interest occurred SEISWORKS 2-D software. The seismic data cover an km). The 60 ן during the Eocene; it created northeast-southwest– approximate area of 9000 km2 (150 oriented rift basins filled with continental clastics that western part of the Lapindo block is covered by 53 host both source rock and good reservoirs (Figure 2B) seismic lines of the B-91 and B-96 surveys that are gen- (Pertamina, 1997; Darman and Sidi, 2000). Segmen- erally of good quality. In the central part of Kodeco tation of Java into its characteristic longitudinal east- block 29, seismic lines of the KM-89 and KM-90 sur- west–trending zones dates from the second extensional veys were used but are of poor quality. The interpre- phase in the Oligocene and lasted into the Miocene. tation was extended into the adjacent Mobil block us- Ongoing subsidence created a deep-marine environ- ing 14 seismic lines of the MD-85 and MD-91 surveys, ment that was filled with sediments supplied from Ma- where the data quality is poor to fair. dura Island to the north and from Java to the south and Well data used in this article come from wells west. A late Oligocene uplift, probably combined with Porong-1, KE-11C, KE-11E, KE-11G, and BD-1 and the 29.5 Ma late Oligocene sea level lowstand, pro- was provided in the form of hard copies. Well data

216 Madura Strait Miocene Carbonate Platforms (Indonesia) Figure 2. (A) North-south structural cross section across the Madura Strait. (B) Summary of the geological history of East Java and the Madura Strait (after Kusumastuti, 1998).

consist of composite logs, along with paleontologic and well than in the BD well and has lower porosities. In biostratigraphic data. Strontium isotope data are avail- 1991, Kodeco drilled the KE-11C well located on the able only for the Porong-1 well. Wire-line log suites shallowest culmination of the buildup trend. Faults include gamma-ray, resistivity, and sonic logs for all that crosscut the buildup or thief sands in the overlying studied wells. Checkshot data are available only for the sediments are thought to have facilitated leakage of hy- Porong-1 and BD-1 wells. drocarbons from this structure. In 1993, the Porong-1 Table 1 and the well cross section (Figure 5) sum- well was drilled on a buildup located at the western marize relevant well data. The carbonate buildups in edge of a collapse feature that formed in overlying sed- the region were first tested in 1987 by Mobil Oil using iments. Hydrocarbon indications were poor during the BD-1 well, which encountered oil and gas at the drilling, but sidewall cores returned oil that shows simi- top of a porous Miocene carbonate buildup. In 1990, lar characteristics to nearby surface seeps and is Kodeco drilled the KE-11E well in another buildup to thought to be derived from lacustrine source rocks in the southwest of the BD structure. It was abandoned the underlying Eocene strata (Kusumastuti, 1998; Dar- as a dry hole; the carbonate interval is thinner in this man and Sidi, 2000).

Kusumastuti et al. 217 Generalized early Miocene paleogeography of the East Java Basin, showing the locations of the buildups under study. Figure 3.

218 Madura Strait Miocene Carbonate Platforms (Indonesia) Figure 4. Time-structure map of depth to the top of Miocene carbonate in the study area. Position of various survey lines and buildups also shown.

Methodology BD, KE-11C and KE-11E buildups were also used to help interpret the lithological variation within the vari- This article focuses on sedimentary intervals of ous sequences. Miocene–Pliocene age, which mostly occur between 2.5 and 5.0 s two-way traveltime (TWTT). Three seis- mic horizons are correlated and mapped across the en- SEISMIC STRATIGRAPHY OF PORONG tire study area: top Oligocene, top carbonate, and intra AND OTHER NEARBY BUILDUPS Pliocene marker. Depositional sequences in carbonate buildups are The Porong buildup is located to the southwest of the most commonly defined by unconformities in progra- KE-11C structure, from which it is separated by a dational reflection sets across carbonate platforms (Au- structural saddle (Figure 4). The Porong-1 buildup is bert and Droxler, 1996). In contrast, the drowned car- an early Miocene bioherm buried by Pliocene– bonate buildups in the study area are characterized by Pleistocene sedimentary rocks. Its base is formed by the aggradation and platform narrowing into their upper top Oligocene unconformity, a laterally extensive seis- parts that is indicative of platform retreat and drown- mic marker atop horst blocks in the region (Darman ing, possibly related to episodes of tectonic subsidence and Sidi, 2000). The structure has a maximum thick- ן Erlich et al., 1990; Schlager, 1999a). Sequence strati- ness of 1630 m and covers an area of 220 km2 (22) 4 ן graphic interpretation in this article is therefore based 10 km) at the base and narrows to 68 km2 (17 on seismic facies analysis (variations in reflection char- km) at the top. acter), as well as reflection termination patterns The Porong-1 well was drilled at the western edge (bounding surfaces). The Porong buildup is used as a of a collapse feature that also affected shallower levels. depositional model for the other carbonate buildups Coring of the top of the carbonate established that the along the same trend where, because of poor seismic late Pliocene strata directly overlay the top of the early data quality, it can be difficult to differentiate the in- Miocene buildup, implying a hiatus of at least 10 m.y. ternal seismic reflection character. Well data for the Hydrocarbon indications were poor during drilling,

Kusumastuti et al. 219 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 30Ј 28.392Љ S 07 29Ј 05.43Љ S 07 25Ј 38.05Љ S 07 22Ј 30.291Љ S 112 45Ј 47.266Љ E 112 53Ј 02.31Љ E 13 05Ј 03.22Љ E 113 16Ј 19.487Љ E Line B91-169, SP 1485 Line KM90-9A, SP 250 Line KM89-31, SP 324 Line MD85-208, SP 1090 Spudded August 3, 1993 July 31, 1991 August 10, 1990 July 13, 1987 Completed November 2, 1993 October 30, 1991 December 9, 1990 December 8, 1987 Total Depth 8659 ft 9968 ft 15515 ft 14026 ft Status plugged and abandoned, plugged and plugged and Oil and gas discovery with oil shows abandoned, with gas abandoned shows Remarks Dolomite, limestone, Flowed formation water Drillstem test near top Limestone, mudstone- (carbonate mudstone to packstone. with dissolved gas. carbonate flowed wackestone. Abundant large interval only) Occasional porosity Top of carbonate has formation water. forams, algae, coral near top. (vuggy or moldic). wire-line–estimated Average wire-line– Microfractures, porosity 20– מ 54 ס Trace bright oil porosity around 30%. estimated porosity 32%; permeability fluorescence. Well 5–10%. 1010 md (plug-derived), flowed formation water. passes downward into Wire-line porosity mudstone-wackestone, averages 15%. stylolites. Wire-line–estimated porosity 11–20%, Lower Carbonate Marker near total depth. Total 162 ft gas and 111 ft oil. Oil water contact at 11,316 ft. Drillstem test flowed 4000–5000 BOPD. and on testing, the carbonate interval flowed formation 0.91 m (3 ft) above the top of carbonate, is dated bio- water. Very little core was recovered within the Porong stratigraphically as middle–late Pliocene, about 3 Ma. buildup because of a predominance of lost circulation The overlying Pliocene sediments are largely shaly intervals in the carbonate parts of the well. Paleonto- mudstones and were deposited in an outer neritic or logical determinations were attempted on sidewall deeper water environment (Kusumastuti, 1998). cores, but no age diagnostic fossils were recovered. The only fossils were long-ranging nannofossils, coralline Description of Seismic Data red algae, coral fragments, and traces of encrusting foraminifera. Four seismic facies units are recognized within the Po- Strontium isotopic ages were calculated using a red rong carbonate buildup (Figure 6). These facies units algal fragment from a sidewall core in the Porong-1 are defined primarily on the basis of seismic reflection well at a depth of 2584 m (8487 ft). The result, 87Sr/ patterns, configurations, and the morphologic profiles. indicates an absolute ,0.000036 ע 0.708548 ס 86Sr age of approximately 16 Ma (late early Miocene) Unit 1 within2mofthetopofthebuildup (age from Sr iso- The Oligocene unconformity at its base and an ero- tope curve of Koepnick et al. [1985]). Given a lack of sional unconformity at its top bound unit 1. The unit biostratigraphic definition in the study area, this is the has a uniform thickness of about 250–300 ms TWTT best available estimate of the age of the buildups and and is more or less flat-topped with a northward slope matches isotopically derived early Miocene ages from of approximately 15Њ (Figure 6). The facies are char- many other carbonate buildups in the region (Kusu- acterized by subparallel, continuous seismic reflections mastuti, 1998). A sidewall core at 2584 m (8478 ft), of medium amplitude in the platform area and by

220 Madura Strait Miocene Carbonate Platforms (Indonesia) mean sea level). ס East-west cross section through wells Wunut-1, Porong-1, KE-11C, KE-11G, KE-11C, and BD-1 (MSL Figure 5.

Kusumastuti et al. 221 Figure 6. (A) Uninterpreted seismic line B-91-169 through the early Miocene carbonate buildup. This line displays the asymmetry between the northern and southern flanks. The northern part shows the strong development of bioherms throughout the accumulation of units 1–4 and is inferred to indicate the windward margin. The asymmetry and isolation of the platform developed mostly in unit 4. (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 reflections As well as being visible in the Porong buildup, the along its northern slope. At the northern slope, the re- reflectors bounding unit 1 can be traced laterally flections onlap the regional Oligocene unconformity through the KE-11C buildup to the KE-11E area in (SP 1550 in Figure 6) and downlap on it southward the east and into the BD buildup. Where it was inter- (SP 1390 in Figure 6). At the top of the slope, there sected by the well KE-11E, cuttings and log analysis are some isolated mounds (pinnacles?), but they are show that unit 1 is composed of carbonates, not sili- less common than in units 2–4. ciclastics. Unit 1 also broadens to the southwest, where

222 Madura Strait Miocene Carbonate Platforms (Indonesia) it cannot be clearly resolved beneath a thickening cover form drowning (see following section). At the margins, of younger volcanic sediment. hummocky reflections occur in an aggradational pat- tern at the northern margin and backstepping at the Unit 2 southern margin. The unit ends at the top in a steep- Unit 2, which is geometrically similar to unit 1 is rela- sided cone. A regional collapse zone lies above the crest tively flat-topped but has a steeper slope of about 30Њ of unit 4 in some parts of the Porong buildup and ex- along the northern side and a gentler slope to the south tends out to KE-11C, casting an acoustic shadow over as compared to unit 1 (Figure 6). Again, some isolated the underlying crest. mounds occur at the top of the slope. The unit broad- ens to the southwest of the main buildup but is lost Interpretation beneath chaotic diffractions of younger volcaniclastic sedimentary rocks. The top of the unit is marked by a The Porong-1 buildup is an alternation of parallel- clear facies change to unit 3. Where equivalents to unit stratified facies (units 1 and 3) with poorly stratified, 2 are intersected by the well BD-1, cuttings and log low-amplitude facies (units 2 and 4). The continuous, analysis show that it is composed of carbonates, not parallel, high-amplitude reflections that characterize siliciclastics. facies units 1 and 3 are typical of fine-grained platform This unit is differentiated from unit 1 based on or bank interior carbonates (e.g., Sun and Esteban, differences in seismic facies. It is characterized by ir- 1994). The continuous reflections in units 1 and 3 are regular, discontinuous, low-amplitude, subparallel to likely caused by alternating successions of deeper la- hummocky reflections. These reflections occur in a goonal (wackestones or mudstones) and better ce- near random pattern marked by nonsystematic reflec- mented (denser), shallower platform sediments (bio- tion terminations and splits. At the base, the oldest clastic packstones). The lateral extent of unit 1 implies sediment fills erosional depressions at the top of unit that it may be equivalent to the unit described as basal 1. Again, onlapping reflection sets climb up the north- transgressive carbonate sand by Sun and Esteban ern slope. Within the unit, the vague outline of a (1994) (Figure 1B). buildup can be seen (between SP 1380 and SP 1530), Similar laterally extensive platform interior car- with discontinuous reflections onlapping it at the sides bonates offshore Vietnam were tested and found to (Figure 6). have low porosity (Ͻ15%) and permeability due to pervasive compaction (Mayall et al., 1997). Likewise Unit 3 in China, the Miocene bank interior carbonates Unit 3 is constrained in areal extent to regions above showed low porosities on drowned platforms, which the top of unit 2. The unit is bounded at the top by an had not experienced subaerial exposure prior to erosional unconformity (Figure 6). The seismic facies, drowning (Erlich et al., 1990). Continuous reflections similar to unit 1, is characterized by parallel, high- in units 1 and 3 within the bank interior are clearly amplitude, continuous reflections that are truncated at eroded and onlapped by the basal reflections of over- the platform margins. At the northern side, mound- lying units (Figure 6). Based on the published litera- shaped buildups occur at the top of the unit (SP 1410– ture, these erosional surfaces may indicate higher order 1430 in Figure 6). Few onlapping reflections occur at lowstand responses (fourth-order or fifth-order) (e.g., the base of the unit at the northern slope or in depres- Kendall and Schlager, 1981) sions at the top of unit 2. Discontinuous, low-amplitude facies (units 2 and 4) are interpreted as cleaner carbonates. Based on a Unit 4 comparison with constituents of asymmetric facies in The areal extent of unit 4 is reduced compared to unit the Holocene buildups in the East Java Sea and Pulau 3, and the topography shows a stronger asymmetry Seribu (Roberts, 1987; Longman et al., 1993), the (Figure 6). The northern slope is very steep, up to 55Њ; northern side of the Porong buildup is probably dom- the southern slope is about 30Њ. The seismic facies are inated by a mixture of coralgal patch reefs and bio- characterized by low-amplitude, subparallel discontin- clastic sands (possibly with minor oolites), which pass uous reflections with increasing amplitude and conti- into fine-grained intertidal and lagoonal sediments far- nuity toward the top. Typically, a high-amplitude re- ther into the platform interior. At the base of units 2 flector defines the top of this unit; it is perhaps and 4, climbing onlap at the margins (especially the indicative of a condensed section associated with plat- northern windward margin) can be interpreted either

Kusumastuti et al. 223 224 Madura Strait Miocene Carbonate Platforms (Indonesia) as late lowstand or early transgressive deeper water car- rimmed platforms (Handford and Loucks, 1993). Prior bonate encroaching on the platform crest. to drowning, backstepping clearly occurred at the lee- Along the northern edge of the Porong buildup, ward margin of the Porong buildup, whereas the units 1–3 include high-relief mounded features toward healthier windward-facing bioherms maintained up- their upper parts, all within the same geographic po- ward (aggradational) growth and attempted to catch sitions, that is, mounded intervals tend to sit atop older up to sea level for a longer time than their more mounded intervals (shaded gray in Figure 6). Mounds nutrient-starved leeward equivalents. Hence, the seem to occur on top of each other, perhaps slightly growth asymmetry of the buildup is clearly seen in pro- shifted toward the center in the upper part of unit 3. files (Figures 6; 7A, B) and the time-structure maps of The mounded facies also forms a single high-relief, the Porong buildup (Figure 8). elongated buildup province in unit 4 of the Porong Platform interior facies change several times from (Figures 6; 7A, B; 8). This inheritance in the stacking low-amplitude discontinuous reflections to continu- pattern suggests that the mounds are depositional ous, high-amplitude reflections within the Porong and rather than erosional in origin. The fact that all the other buildups under study (Figure 6; 3000–4400 ms buildups in the study area (the Porong-1, KE-11C, KE- TWTT). This is interpreted as indicating fining-upward 11E, and BD-1 buildups) are isolated platforms and are trends whereby at times the platform interior lagged not joined together at any time during the deposition behind the faster growth at the platform margins. As of units 2–4 suggests that buildups were always sepa- the waters of the platform interior deepened, the bot- rated by deeper water areas (Figure 4). The strong tom was increasingly shielded from waves and tides asymmetry in the mounded growth profile in unit 4 (Kendall and Schlager, 1981; Schlager, 1999a). This suggests a windward northern side and a leeward cycle of incipient interior drowning was repeated sev- southern side to the buildup, much like the asymmetry eral times during deposition of units 2–4; each time seen in Holocene buildups in the East Java Sea and the areal extent of the platform lessened until finally Pulau Seribu (Roberts, 1987; Longman et al., 1993). the volume of sediment generated by the carbonate The profile of unit 4 in all platforms where it is factory could not keep pace with sea level rise and the present is that of a drowned system in which, under an buildup drowned. The onlapping geometries of the overall rising relative sea level, the areas of the various post–unit 4 sedimentary rocks (SP 1300–1450 in Fig- carbonate factories shrank until they gave up and were ure 6) suggest that an increasing detrital influx from drowned. Drowned buildups show reflector geome- the south may have abetted long-term platform retreat tries and characters that can be used to distinguish due to detrital poisoning. The drowning of the Porong them from reefs whose growth was stopped by a low- buildup occurred more than 10 m.y. prior to the arrival ering of sea level and associated subaerial exposure of the Pliocene sediment wedge; its arrival is indicated (Figure 9A) (Erlich et al., 1990). As a reef complex by reflectors that are the distal toes of basinal downlaps drowns, it typically evolves a narrowing-upward pro- or even coastal onlaps. file as the faster growing parts of the reef complex at- After drowning, and prior to the arrival of the Pli- tempt to catch up to a rapid rise in relative sea level. ocene wedge, the carbonates of the Porong area were A condensed section that is subsequently covered by covered by a veneer of sediment, a condensed or sediments characterizes the uppermost part of any sediment-starved sequence, which also defines the drowned buildup. maximum flooding surface of the subsequent Pliocene Until their drowning, all of the buildups under succession (Figure 6). This surface atop unit 4 tends to study possessed profiles typical of detached, reef- form 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, and there is horizontal onlap of adjacent sediment, all features indicative of a drowned platform. (B) Seismic line B-91-129 showing late stage growth reef buildup in the Porong platform. (C) Seismic line KM-90-07 shows the typically poor quality seismic data across the KE-11C carbonate buildup. Above the buildup is a collapse feature that continues along strike to the Porong buildup. (D) Seismic line MD-85-202 across the KE-11E carbonate buildup. This buildup is the deepest in the study area and shows less vertical development than others along the trend. (E) Seismic line MD-85-208 shows the BD carbonate buildup, which is the only hydrocarbon discovery in 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.

Kusumastuti et al. 225 Time-structure maps of units 1–4 of the Porong-1 buildup. Note the narrowing and increased asymmetry of the buildup to the top. Figure 8.

226 Madura Strait Miocene Carbonate Platforms (Indonesia) Asymmetric reef-rimmed platform (Unit 4) platform Carb. with parallel reflectors (Unit 1) Carbonate with platform parallel reflectors (Unit 3) Carb. platform Carb. with discontinuous reflectors (Unit 2) 16 17 18 Ma 1–4 to the sea level e Porong-1 buildup with 111 ft oil 111 162 ft gas e) divergent basinal onlap patterns e) divergent Subaerial carbonate profile seismic sequence a) unconformable b) karsted/eroded surface boundaries; and discontinuous); (often hummocky c) weathering creates data attenuation multiples; intervals and/or shallow d) shelf-to-basin reflector continuity; submarine due to lowstand possibly fans Lowstand fan Lowstand Shelf to basin reflector continuity 3.31 sec 3.50 sec 2.81 sec* 3.78 sec surface Karsted/eroded KE-11C KE-11E BD-1 Data loss OLIGOCENE Multiples OP T Unconformable Type 1 or 2 Sequence Boundary Type Unconformable Porong-1 TUM DA *time is based on Porong buildup seismic markers correlation seismic markers buildup *time is based on Porong 0 Feet C. 4000 3000 2000 1000 5000 .7090 Sr 86

Drowned carbonate profile Drowned seismic sequence a)conformable b)good internal boundaries; c) horizontal to sub- reflectors; horizontal basinal marine onlap; reefs at or near shelf d)late growth capping e) hardground margin; Sr/ 87

Type 3 boundary Type .7080 Ma

15 20 25

(temp.)

Warming Warming Cooling Horizontal onlap 2.3 2.4 2.2 2.1 1.5 1.4 1.3 3rd order 3rd

Later progradation 2nd Sea level Type 1 boundary Type RISE FALL

Unit 4 Unit 2 Unit 1

Unit 3

Conformable

Early Miocene Early (A) Characteristics of drowned vs. subaerially exposed carbonate platforms (modified from Erlich et al., 1990). (B) Possible relationships of units Continuous data Oligocene Type 3 Sequence Boundary Type sequence boundaries Ma ?Facies change ?Facies 15 20 25 A. B. Figure 9. curve of Haq et al.early (1987) Miocene and third-order tied sea to level the fluctuations. marine Sequences strontium are isotope based curve on of well Koepnick intersections et (shown al. as (1985). white (C) rectangles). Correlation of the four sequences identified in th

Kusumastuti et al. 227 lines crossing the Porong and other nearby buildups. unit, where mounded facies are not easily discerned, Similar features define the top of most drowned plat- are interpreted as deeper water platform facies or forms (Figure 9A) (Erlich et al., 1990; Schlager, “start-up” phases, whereas the remainder of each unit 1999a). The predominance of coralline red algae in is interpreted as a “catch-up” phase (terminology of core recovered near the top of the Porong-1 buildup Kendall and Schlager, 1981). The maintenance of may indicate deepening during growth mounding and mound relief after the deposition of each unit ceased herald the drowning of the buildup. These red algae and the lack of an adjacent prograding deeper water probably grew in several tens of meters of water depth, prism suggest that aggrading or catch-up styles domi- still in the euphotic zone but in water that was too nated through the deposition of each unit. The Porong deep for prolific coral production (James and Mount- platform and others in the region uncommonly, if ever, joy, 1983; Ehrlich et al., 1990). attained a “keep-up” style of deposition (terminology The steep flanks of the windward flank of the of Kendall and Schlager, 1981). In sequence strati- Porong-1 buildup are not directly related to large graphic terms, each of the four units are transgressive deeper faults but are depositional in origin. A major systems tracts (Ehrlich et al., 1990). overlying extensional fault also exists, along with an Units 1 and 2 and units 3 and 4 likely are grouped associated collapse graben whose position was con- responses to longer term fluctuations of third-order sea trolled by compactional hinging and the strength con- level cycles. If so, they indicate shallowing of units de- trast between the well-cemented sediments of the un- posited at times of high sea level, dominated in their derlying carbonate buildup and the depositionally lower parts by deeper water sediment. This cyclic re- later, but adjacent, deeper water siliciclastic muds (Fig- sponse creates layered muddy platform facies (unit 1 ure 6B). This collapse feature extends farther along the or 3) that pass up into cleaner, shallower platform fa- carbonate trend and can also be seen atop the KE-11C cies deposited during times of lower relative sea level buildup (Figure 7C). rise (unit 2 or 4). Fitting this interpretation to the In summary, the Porong-1 buildup is interpreted third-order sea level curve of the early Miocene (Haq as an asymmetric, at times reef-rimmed, carbonate et al., 1987) means the erosion surfaces at the top of platform with a classic drowning profile (Figure 9A). unit 1 and unit 3 may correspond, respectively, to the It sits on the Oligocene unconformity (29.5 Ma); the 21 and 16.5 Ma sequence boundaries, but this has yet top of the buildup is dated at about 16 Ma (Sr date; to be confirmed by coring (Figure 9B). see previous discussion). Second-order sea level rose Although biostratigraphic data are lacking to con- continually during the time it was deposited, creating trol seismic stratigraphic interpretation of the internal ongoing accommodation space for carbonate deposi- reef geometries in the buildups, this same repetitive tion (Figure 9B; early Miocene). Transgressive system pattern allows us to extrapolate the Porong interpre- tracts dominate the buildup, stacking patterns are ag- tation of shallowing, terminated by drowning, to the gradational, and highstand spillover and progradational other buildups in the trend and so predict reservoir geometries fed from the platform margin are missing, porosity based on sequence stratigraphic interpretation whereas lowstand wedges are small or missing. The of seismic facies changes (Figure 9C). A likely se- lack of these features in the Porong-1 buildup and the quence stratigraphic interpretation can be made if one other buildups along the trend implies that the car- assumes that all the buildups were subject to linear bonate sequences formed during accelerated rates of increasing subsidence and that this effect can be added long-term accommodation within a rapidly subsiding to the Haq world sea level curve for the early Miocene basin. (Figure 9B). From the lower to the middle Miocene, third- Influence of Sea Level Changes order sea level is rising, sometimes quickly, sometimes The bounding facies pattern of units 1–4 suggests that slowly, punctuated with rapid high-amplitude falls at the Porong platform experienced several episodes of 21 and 16.5 Ma, followed by rapid rises (Figure 9B) little or no sedimentation. These times were typified (Neogene transgressive cycle of Fulthorpe and Schlan- by flooding surfaces, possibly preceded by times of ei- ger [1989]). The rapid high-amplitude rise around ther lowered or raised sea level (fourth-order or fifth- 16.5 Ma corresponds to the drowning of the Porong, order) when the crests of the platform were subaerially KE-11C, and BD-1 buildups (Figure 9B, C). It defines exposed or covered by waters too deep for prolific car- a type 3 surface atop these buildups. The preceding sea bonate aggradation. The lower rimming parts of each level falls are interpreted as defining type 1 sequence

228 Madura Strait Miocene Carbonate Platforms (Indonesia) boundaries at the tops of units 3 and 4. The internally in the carbonate interval, with an average porosity of chaotic facies of units 2 and 4 are interpreted to indi- 8% (Table 1). Based on its deep structural position, the cate start-up/catch-up growth styles associated with KE-11E well is most likely made up of the unit 1 sed- early transgression. The more laterally extensive inter- imentary rocks (Figure 9). This is also seen in seismic nal reflectors in the platform interior of units 1 and 3 data across the buildup, which do not show the differ- correspond to keep-up styles of platform sedimenta- entiation into units 1–4 that typifies other buildups; tion associated with much slower rates of sea level instead, the data show internal character typical of unit change that typified the deposition of units 1 and 3. 1 only (Figure 7D). The KE-11E structure appears to The small mounds at the top of unit 3 may correspond have drowned earlier than the other buildups in the to the onset of more rapid sea level rise immediately region, so sequences 2–4 never formed. prior to a major fall (Figure 9B). In terms of predicting Today, the top of the BD carbonate sequence is reservoir quality using this model, drowned buildups deeper than the Porong and KE-11C buildups (Figures beneath a type 3 boundary or platform buildups be- 4, 9). This relates to the regional history of the late neath a type 2 boundary should both be high priority Miocene, when the southern basin was subsiding into targets. a deep-water environment. The rates of subsidence The ongoing rapid rise of the third-order sea level were not the same across the basin; the central and probably did not drown or terminate growth in the eastern parts were tectonically more active and so sub- buildups. Carbonate accumulation can typically keep sided faster than the western part. Hence, the BD up with rapid rates of third-order sea level rise (Kendall buildup has a better seal configuration and is overlain and Schlager, 1981; Schlager, 1999b). Terminal by thick shale (or other fine-grained sedimentary drowning is more likely caused by higher order sea rocks); it was the only well where drilling encountered level fluctuations that first exposed the carbonate plat- noticeable hydrocarbons (Table 1). form and then flooded it abruptly before carbonate production could start up. The early Miocene is situ- ated at the transition from greenhouse to icehouse con- EXPLORATION IMPLICATIONS ditions, and the widespread drowning may reflect the increasing amplitude, frequency, and rate of sea level Worldwide, 43% of productive hydrocarbon reserves rise brought on by the onset of glacial-interglacial sea in carbonate buildups occurs in Miocene strata (Green- level cycles. Alternatively, the terminal drowning may lee and Lehmann, 1993). Drowned carbonate buildups reflect proximity to the Old Andesite volcanic arc in have long been attractive targets in both frontier and the south, which periodically shed ash and other debris mature basin settings; they tend to be porous large- that made the East Java basin inimical to carbonate scale structures with good four-way closure. Because growth. Increased volcaniclastic sediment supply, de- they are commonly encased in shales, such buildups creased light intensity, changed water chemistry due to tend to be one of the more recognizable carbonate eruptions, and increased tectonic subsidence are alter- structures in seismic reflection data and are typically natives that singularly or in combination may have among the first carbonate prospects to be identified caused drowning of the buildups. Possible tuffs have and tested in a new basin. not yet been recognized, however, in wire-line log In any frontier hydrocarbon region, the major risk curves run within the uppermost part of the carbonate factors are source, migration, reservoir, trap, and seal. sequence. Early rift-related sediment fills (Eocene) in continental The seismically defined four-part stratigraphic grabens underlying the buildups in the Madura Strait subdivision of the Porong-1 buildup can be extrapo- and East Java regions are thought to contain organic- lated to the other buildups along the trend to generate rich lacustrine shales (Darman and Sidi, 2000). This is a tentative correlation between the Porong-1, KE-11C, supported by the hydrocarbon discovery in the BD car- KE-11E, and BD-1 buildups (Figure 9C). Well data bonate buildup and hydrocarbon seeps around the Po- indicate that the tops of Porong-1, KE-11C, and BD-1 rong area, demonstrating that hydrocarbon maturation are porous, with average porosities of 20–32%. These and migration has occurred in the study area. Hydro- uppermost intervals are interpreted to be equivalents carbon source, generation, and migration are consid- of unit 4. In the BD-1 well, the porosity decreases to ered relatively minor geological risks in the study area 11–20% below the oil-water contact. In contrast to the compared to reservoir, trap, and most importantly, seal other wells, the KE-11E well sampled a tight formation integrity.

Kusumastuti et al. 229 Work to date has shown that unit 4 in the early bility of any collapse-zone faults above the buildup, Miocene carbonate buildups in the Madura Strait is the and (3) the structural configuration of any potential most attractive target for exploration. Where present, seal to the buildup. Drilling has shown that the pre- the upper part of unit 4 shows strong evidence of dominant seal in the study area is thick Pliocene shale growth-induced closure via drowning followed by en- with minor silts and tight shaly sands. Although thick, casement in fine-grained deeper water sediments. The it still may lack integrity where it is faulted or it con- fact that its upper surface is a type 3 boundary implies tains laterally extensive thin sands. In advance of drill- that porosity is likely to be retained by rapid subsidence ing, predicting the sealing capability of the faults above and associated exclusion of meteoric waters, which the carbonate buildups is still difficult, so any buildups would otherwise redistribute or even occlude porosity. that have one or more of features 1–3 should not be The porosity potential of the upper part of unit 4 is given priority as potential targets. supported by the hydrocarbon discovery in the BD car- For example, the Porong buildup (Figure 10A) ex- bonate buildup. perienced a hydrocarbon charge during the time that Seal integrity appears to be a major geological risk it was capped with a late Pliocene seal, as indicated by associated with the carbonate buildups (Downey, hydrocarbons noted during drilling of this interval. 1994), including those in the study area. Sealing is de- Much of this charge, however, was probably lost via pendent upon (1) the lithology of the strata immedi- extensional fault breaches created during the episode ately above the buildup, (2) the seal-breaching capa- of 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 reflecting a lack of seal integrity because of the extensional collapse graben over the crest and the updip drainage configuration in the sealing beds. (B) The KE-11C buildup is dry for reasons similar to those for the Porong structure; note the potential for leakage indicated by crestal breaching of the seal by the collapse zone. (C) The KE-11E buildup is dry because of a reservoir quality problem (tight-muddy facies on unit 1 dominates); it also has an updip drainage configuration in the seal. (D) The BD discovery is the only buildup that trapped hydrocarbons; it lacks a crestal graben and seal beds are in a trapping configuration.

230 Madura Strait Miocene Carbonate Platforms (Indonesia) tic of the overlying fault-defined collapse zone (around CONCLUSIONS SP 1450–1500 in Figure 10A). The updip drainage configuration in potential seal beds atop the Porong Detailed seismic analysis of the Porong and other early structure also lessened the likelihood of seal integrity, Miocene buildups in the region of the Madura Strait as it facilitated southerly updip hydrocarbon leakage has shown a well-defined buildup trend in which there and migration. A similar collapse-graben breach occurs are four recognizable seismic units. Analysis of the atop the KE-11C buildup (Figure 10B). As in Porong- buildup geometries in combination with well data 1, these collapse-zone faults on top of the KE-11C shows that buildups are most likely to retain a hydro- buildup probably acted as conduits to secondary updip carbon charge when all four seismic units are present. migration (see the arrow direction on top of buildup This is perhaps a water-depth–related effect. Those in Figure 10). Both buildups failed because they lacked buildups that do not show mound-related rimming a prospective seal configuration. structures immediately beneath a type 1 surface may Reservoir potential of the KE-11E type of play is not have built up to sea level and so were not subject thought to be low because of a lack of a suitable res- to the vagaries of meteoric exposure associated with ervoir unit (i.e., no unit 2 or 4). The KE-11E carbonate the higher order low-amplitude sea level oscillations. buildup failed because it shows less vertical develop- Also, if the upper part of a backstepping buildup is not ment of facies than the other buildups in the region. It capped by a type 3 surface, the likelihood of the reten- lacks the repetitive four-tier internal reflector structure tion of primary framework porosity is lessened. of the other structures (most likely only the lower po- In terms of priority for potential targets in buildups rosity unit 1 is present). Seal integrity in the KE-11E in the region, higher priority should be given to targets buildup is also poor, as potential seal beds show an that updip drainage configuration. Any hydrocarbons prob- ably leaked away in a northerly direction (see arrow in 1. Show evidence of ongoing platform backstepping Figure 10C). Relatively thick thief sands, which can be and ultimately last stage reef growth associated with seen in the wire-line curves from the KE-11E well, per- incipient drowning haps aided leakage. Chances for success in Porong and 2. Entrain all four sequences (units 1–4) in the buildup KE-11C styles of drowned play or the incompletely 3. Lack an extensional collapse graben over the crest developed KE-11E style of structure are interpreted as of the drowned structure low. 4. Demonstrate bed inclinations in the sealing beds Hence, the best play in this area is the BD type, that indicate likely closure over the crest of the where all four seismic units occur in the buildup. The buildup structure is drowned; it lacks a crestal collapse graben and has a thick Pliocene seal facies that also shows a suitably closed configuration in the region directly REFERENCES CITED overlying the buildup (Figure 7E). That is, the BD buildup has good top seal integrity and seal configu- Aubert, O., and A. W. 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232 Madura Strait Miocene Carbonate Platforms (Indonesia)