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IPA01-G-138

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION

Twenty-Eighth Annual Convention & Exhibition, October 2001

EXPLORATION OF THE NORTH MADURA PLATFORM, OFFSHORE EAST JAVA, INDONESIA

Rachmat Mudjiono* Gadjah Eko Pireno*

ABSTRACT Cities Service initiated exploration of the offshore East Java Basin in the late nineteen-sixties, holding the vast (157,000 sq km) North East Java PSC from 1969 until 1979. Cities shot 46,295 km of seismic data and drilled 48 wells, including three on the North Madura Platform. This feature is a stable high flanked by known hydrocarbon kitchens to the northwest (Central Deep) and south (Madura Basin), and it attracted more drilling by later operators. Like the Cities wells, these wildcats found teasing shows of oil and gas but no commercial accumulations. Exploration continued to focus on shelf edge buildups close to hydrocarbon source areas in adjoining depositional troughs. The wells encountered problems with reservoir quality, and sharp velocity gradients confounded attempts to image buried structures with seismic data. Yet another round of exploration on the North Madura High began when the Ketapang PSC was awarded to Gulf in 1998. Geological and geophysical studies highlighted the prominent, east-west trending Ngimbang (Middle Eocene to Early Oligocene) and Kujung (Late Oligocene to Early Miocene) shelf edge carbonates, but hydrocarbon indicators (HCIs) were also identified in more centrally located areas remote from hydrocarbon kitchens. Laborious correction of velocity artifacts caused by shallow channels revealed subtle, longstanding structural closures associated with these HCIs. _____________________________________________________ * Gulf Indonesia Resource s Ltd.

The new (2001) Bukit Tua and Jenggolo oil and gas discoveries targeted reservoirs in layered Kujung carbonates near the center of the North Madura Platform, 10 to 20 km from the fringing reefs. Porosity development in this area may result from repeated exposure on the crest of the old Madura Platform. Migration pathways connect these reservoirs to distant source kitchens via permeable Kujung I carbonates and near-basement carrier beds such as basal clastics and Ngimbang and Kujung II/III carbonates. Confirmation of porosity and hydrocarbon occurrence in these discoveries opens the stable platform area to exploration. Fringing reefs remain a viable play, as indicated by other recent discoveries in the Ketapang PSC (Bukit Panjang, 2000; Payang, 2001) and nearby West Madura blocks (KE-23B, KE-13, and KE-24, 2000; KE-30, 2001) and Pangkah (Ujung Pangkah, 1998; Sidayu, 2000). EXPLORATION HISTORY The North Madura Platform is a stable high located within the offshore portion of the East Java Basin, an extensive back arc basin that covers an area of approximately 50,000 sq km (Figure 1). Drilling for oil began onshore in the East Java Basin in 1871, and the first oil discovery was made at Kuti Anyer, near Surabaya, in 1888 (Courteney and others, 1989; Soetantri and others, 1973). Early onshore exploration targeted shallow Pliocene and Miocene sandstones, with the largest early discovery at Kawengan Field (cumulative production 160 mmbo from the middle Miocene Ngrayong Sandstone) in 1894.

© IPA, 2006 - 28th Annual Convention Proceedings, 2002

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Cities Service initiated offshore exploration in 1967 on their vast Northeast Java PSC, which covered 157,700 sq km offshore from Java, Madura, and Bali. Cities acquired 46,295 km of seismic data and drilled 48 wells. Most of these wells tested structural features and/or reefal buildups in the top of the Kujung Formation (Figure 2). Three of the Cities wells were classified as oil discoveries, four were gas discoveries, and one was an oil and gas discovery. Of these, Cities produced only Poleng Field (located west of the North Madura Platform, Figure 1) for a few years and then relinquished the block in 1979 without a commercial success. Five of the early Cities wells were drilled in the vicinity of the North Madura Platform, although none were located near the crest of the feature. Cities drilled the JS 3-1 a few km northeast of the platform (Figure 1) in 1970. This well encountered oil shows within the Kujung and a poor oil show in the deeper Ngimbang Formation, but it was not tested. Also in 1970, Cities drilled JS 8-1 in the Kujung reef trend along the southeast flank of the platform. JS 8-1 encountered an oil show in the Ngimbang Formation. Cities drilled the JS 2-1 in 1971 to test a fault closure just off the north edge of the platform. This well tested 4.6 mmscfd from thin carbonates within the Kujung. Two more wells were drilled by Cities Service in the Kujung reef trend along the southern edge of the North Madura Platform before relinquishing the entire North Java PSC in 1979: § the JS 19-1 (1972), near the southwestern corner

of the present Ketapang block, recovered small amounts of oil from tests of the Kujung, upper Ngimbang Carbonate, and lower Ngimbang Clastics, and

§ the JS 44A-1 (1975) tested a low relief closure about 30 km farther east and tested 3.9 mmscfd gas and 95 bbls condensate per day from near the top of a reefal buildup in the Kujung.

Union Oil Company next entered the area in 1981 with a PSC that included the North Madura Platform. Union acquired additional 2D seismic data and drilled Ketapang-1, Montor-1, and Pasian-1 (all north of the Platform) plus Sergang-1 (southeast of the platform). Some of these wells encountered oil shows, but none were tested.

Inpex operated a block covering this area starting in 1990. Inpex acquired a tight grid of seismic data, concentrating on the southern portion of the North Madura Platform. They drilled three wells: Cassiopea-1 to the north of the platform plus Perawan-1 and Waluku-1 along the shelf edge trend to the south (Figure 1). Of these, Perawan reported oil shows and all were plugged and abandoned. Gulf Indonesia signed the Ketapang PSC in 1998 and proceeded to reprocess 5,634 km of existing seismic data on the block. An additional 2,300 km of data were acquired in 1999, filling in gaps to construct a closely spaced grid of 2D seismic lines (Figure 3) across the block. REGIONAL TECTONICS The East Java Basin is situated on the southern margin of the stable Sunda craton and covers an area of approximately 50,000 sq km. The basin formed in the Eocene as a back arc basin associated with a volcanic arc to the south (Figure 4). The Karimunjawa Arch marks the western limit of the East Java Basin, and to the east it is bounded by deep water environments of the oceanic Lombok, Flores, Salayar, and South Makassar basins. The East Java Basin extends northeastward from the volcanic arc offshore across the Java sea and back onshore in the southeastern corner of Kalimantan and the adjoining shelf area in the southern Makassar straits. It is ultimately bounded to the northeast by the Paternoster Platform and the Adang Fault Zone. The northeast-southwest structural grain in the East Java Basin reflects the fabric of underlying basement assemblages (Figure 5), which vary in age from latest Jurassic through Late Cretaceous (Bransden and Matthews, 1992, p. 419). Oceanic crust began subducting northwestward beneath the continental Sunda Plate, part of the Eurasian craton, during the Early Cretaceous. This subduction occurred along the Meratus Suture, which trends southwestwards from the Meratus Ridge in South Kalimantan beneath the Java sea (Figure 4). Pre–Tertiary basement lithologies show a well-defined zonation from cratonic granites and gneisses in the Karimunjawa area of south central Kalimantan into highly metamorphosed subduction mélange sediments within the Meratus.

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This extends southwestward across the Java sea and appears as a band of basic igneous basement to the northwest of the Ketapang PSC (Figure 5). Low grade metasediments surround the band of basic igneous rocks and extend into the northwestern corner of the Ketapang block. Southeast of the metasedimentary basement are northeast-southwest trending bands of acidic and intermediate igneous rocks, indicating presence of a continental fragment within the suture complex. With continuing compression, the East Java Basin underwent uplift and peneplanation during the early Tertiary. A series of northeast–southwest trending ridges and grabens then developed along the southeastern edge of Sunda shield, perhaps in response to back arc spreading. From west to the east, the major features in this extensional complex are the Karimunjawa Arch, the Muriah Trough, the Bawean Arch, and the Tuban-North Madura High (Figure 4). These features extend onshore, bending to the west into an east northeast-west southwest alignment. The rift basins filled initially with a thick sequence of non-marine clastics, including lacustrine shales, starting in the Middle Eocene (Figure 2). A sag phase followed the rift episode, and basin fill in the grabens changed upward into a fully marine Late Eocene to Early Oligocene sequence of shale with local carbonate development. Renewed uplift terminated this depositional episode in the Early Oligocene, resulting in widespread erosion. This was followed by a period of relative tectonic quiescence and carbonate accumulation which extended into the Early Miocene. The latest and most pronounced period of compressional tectonism began in Late Miocene time and continued episodically through the Pleistocene and up to the present. Pre-existing graben faults were reactivated with a reversal in throw, resulting in large inversion structures. The North Madura Platform remained a stable high area, periodically inundated by regional subsidence or eustacy and bounded by inverted former basins to the north (Central Deep) and south (Madura). NGIMBANG AND KUJUNG STRATIGRAPHY Stratigraphy of the East Java Basin consists of a

series of depositional cycles. This study is limited to the basal two cycles, the Ngimbang and Kujung, which constitute petroleum systems now being explored in and around the North Madura Platform. Pre-Tertiary basement underlying these units in the vicinity of the Ketapang block consists of low grade metasediments and acidic to intermediate igneous rocks (Figure 5). Ngimbang Cycle The earliest sediments deposited on pre-Tertiary basement were largely controlled by the underlying basement architecture. These comprise the Ngimbang Formation, which accumulated within the grabens from the Middle to Late Eocene through the Lower Oligocene (Figure 2). The lower Ngimbang consists of a series of interbedded quartz sandstones and gray to brown carbonaceous shales with minor coals, which accumulated in brackish to lacustrine environments. Lacustrine environments probably developed in the graben depocenters during Middle Eocene time, making the Lower Ngimbang a likely hydrocarbon source rock in the East Java Basin. Basinal sag resulted in widespread incursion of marine environments during the Late Eocene, and marine sediments dominate the upper Ngimbang Formation. These onlap the southern flank of the North Madura Platform, as shown by the paleogeographic reconstruction in Figure 6. Water depths increased substantially away from the Platform, forming deep sea conditions over the Madura Basin. Shelf edge carbonates developed along northern edge of the Madura deep sea in the upper Ngimbang (Figure 6). Kujung Cycle A major compressive event linked to both eustatic and tectonic causes initiated a new regressive phase with deposition in the Mid–Oligocene. Cities Service subdivided this section from youngest to the eldest as members I, II, and III of the Kujung Formation (Ardhana, 1993, p.417). The basal Kujung III is a clastic-rich regressive interval ranging from few tens of feet in the KE-17 well near the southwest extremity of the North Madura Platform (Figure 7) to 2,000 ft of conglomeratic sandstones in the Tuban Trough area (Figure 4) 100 km to the west.

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The overlying Kujung II reflects a change to quiescent tectonic conditions accompanied by regional subsidence. These deposits consist of a transgressive sequence of shallow water carbonates and calcareous shales. Localized carbonate build-ups developed on stable high areas. Irregular and discontinuous seismic events characterize the Kujung II and Kujung III intervals in the North Madura Platform area, indicating abundant lateral variation in sedimentary facies. Deposition of these units was controlled to large extent by the pre-existing northeast-southwest trending structural pattern. Both Kujung II and III units are important reservoir targets for current exploration in the North Madura Platform area. The Kujung I interval is a thick (1300 ft) carbonate section that lacks the significant shale intercalations of the underlying Kujung II. Figure 7 shows the interpreted paleogeography for the Kujung I. Buildups occur as linear reefs fringing uplifts and isolated patch reefs on stable platforms. A prominent shelf-edge reef tract developed along an east-west trend parallel with a present day Madura and Java northern coast line. Overburden Four additional depositional cycles followed the Kujung in the North Madura Platform area. Each follows the pattern of starting with clastic sedimentation and ending with at least local development of marine carbonates: 1. Tuban (Early Miocene), renewed clastic

sedimentation following the pervasive carbonates of the Kujung I. The Tuban Formation consists of a thick (1,500-8,000 ft) interval of deep water carbonaceous shales with thin interbedded sandstones and marls. This cycle closed with reefal limestones (Rancak Member) accumulating on structural highs, sometimes directly overlying the Kujung I.

2. Ngrayong (Middle Miocene), widespread

accumulation of quartz sandstone overlying the shales and limestones of the Tuban. Marine sandstones of the Ngrayong constitute an important reservoir onshore and in the Muriah

Trough area along the western edge of the offshore East Java Basin (Figure 4). Although quite shallow (3,000 ft) over the North Madura Platform, the Ngrayong remains a secondary objective in this area.

3. Wonocolo (Middle to Late Miocene), widespread

deep marine shales and marls with minor sands capped in offshore areas by major accumulations of platform and reefal carbonates. These were exposed in the Late Miocene and incised by a series of channels, some cutting entirely through the carbonate complex. Velocity contrasts between slow channel fill deposits and fast carbonates and the resultant sag in deeper seismic events causes difficulty in interpreting depth structure. These are discussed in the following section on Depth Conversion.

4. Late Miocene to Recent deposition started with

renewed transgression and accumulation of highly varied sedimentary facies. These include marls and reworked sandstone, globigerinoid chalks and reefal limestones, and a large influx of volcaniclastic sandstone from the volcanic arc to the south starting in the latest Pliocene. Local tectonic effects became more severe with time and have the greatest control on sedimentary facies. Volcaniclastics and globigerinoid chalks are major reservoirs in the early onshore discoveries, and they have remaining potential in some parts of the East Java Basin.

PETROLEUM SYSTEMS Source Rock Occurrence The widespread distribution of effective hydrocarbon source rocks is indicated by occurrence of oil and gas throughout much of the East Java Basin. Potential source rocks include the Ngimbang, Kujung, and Tuban formations. Total Organic Carbon (TOC) values in each of these units range from 1–4% in shales and carbonaceous limestones, and TOC content exceeds 40% within the coals. A study of the East Java Basin published by Pertamina BPPKA (1996, p. 77) stated that stratigraphic equivalents of these three formations all are potential hydrocarbon

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sources, but the study noted that the Eocene Ngimbang is believed to have sourced 95% of the oil and gas in the East Java Basin. Studies of particular areas within the basin by Phillips and others (1991, Kangean Island area) and by Manur and Barraclough (1994, Bawean area) also concluded that the Eocene Ngimbang is the source for oil in those regions. Source Rock Maturity Corrected logging temperatures from wells drilled throughout the East Java Basin indicate an overall range in geothermal gradients from 1.4 to 2.4 deg F/100 feet. In the vicinity of the Ketapang Block, the average geothermal gradient is 1.95 deg F/100 feet. Maturation models were generated using local log and seismic data to examine the history of hydrocarbon generation for two areas flanking the North Madura Platform: the Central Deep to the northwest and the Madura Basin to the south (Figure 4). The model for the southern part of the Central deep (Figure 8) uses a geothermal gradient of 1.8 deg. F/100 feet, based on local well control. It shows that lacustrine and alluvial source rocks in the lower Ngimbang sequence, with kerogen types I and II, entered the window for oil generation (Ro 0.7-1.0) during Oligocene and Miocene time between 34-11 million years BP. Maximum generation (Ro 1.0-1.3) occurred over a broad span of time from Late Oligocene to Pleistocene. The basal part of this unit first entered the gas window (Ro 1.3-2.6) in the Early Miocene (23 million years BP) and remains there today. The marine upper Ngimbang with kerogen types II and III has been in the window of maximum oil generation from Late Miocene (10 million years BP) to the present. The overlying Kujung remains in the early oil generation phase, and shallower strata are thermally immature. The Madura Basin has a slightly lower geothermal gradient, and the maturation model for this area (Figure 9) indicates that onset of oil generation in the lower Ngimbang extended from Early Miocene to Late Pliocene time (22 to 3 million years BP), reaching maximum oil generation during the Late Miocene to Pleistocene (6 to 1 million years BP). The lower Ngimbang entered the gas window in Late

Pliocene and remains there now (Figure 9). The upper Ngimbang started maximum oil generation about 6 million years BP and entered the gas window only about 1.5 million years ago. Kujung and shallower strata are still in early oil generation or thermally immature. Results from the maturation modeling for the deep basins adjoining the North Madura Platform indicate that lacustrine and alluvial facies in the lower Ngimbang are likely sources for oil and gas in the North Madura Platform area, but that marine kerogen in the upper Ngimbang may also contribute. The long history of oil generation and predominance of kerogen types I and II in the lower Ngimbang bode well for oil accumulation on early structures connected by faults or carrier beds to the basinal source kitchens. Late forming traps and those connected to current gas generating areas may be more prone to accumulated gas. Reservoirs, Traps and Seals Reservoir targets in the North Madura Platform area include: 1. lower Ngimbang clastics developed on the flanks

of the Platform, 2. upper Ngimbang carbonates, except for crestal

areas where this unit may be absent due to nondeposition or erosion,

3. Kujung reef tracts in an east-west trending band of carbonate buildups along the southern flank of the Platform. Although characterized as Kujung I, this area typically develops massive carbonate buildups that extend throughout the undifferentiated Kujung section, and

4. Kujung I, II and III units developed over the crest of the basement high as platform carbonates with some isolated Kujung I reefs. On the flanks of this area, exploration targets include basal Ngimbang Clastics plus high-energy shelf edge carbonates of the upper Ngimbang and Kujung Formations.

Many trap types are available. The northern and northwestern margins of the North Madura Platform have endured multiple stages of faulting and

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inversion, and upthrown and downthrown closures occur in association with major and minor faults in this structurally complex area. Kujung gas tests in JS 2-1 and the recent Payang-1 discovery show that some fault traps work in this area, but the risk of fault timing and seal remains a potential dry hole hazard. Less vexing four way dip closures are recognized as low relief structures on the undulatory surface of the Platform and as structures formed by drape over reefal buildups in the Kujung. Unfortunately, significant velocity gradients pose a major challenge to depth conversion of structural interpretations on seismic data. Imaging these subtle closures requires accurate depth mapping, and proper depth conversion of seismic time maps is a key factor in recognizing prospect size and selecting well locations. Methods used in siting the recent Ketapang discoveries are described below. The flanks of the North Madura Platform also have potential for stratigraphic traps formed by onlap of basal clastics in the Ngimbang. Several wells flanking the Platform recorded oil shows in this unit, but as yet there have been no commercial tests. The onlap edge remains difficult to identify with confidence, but this will likely improve with acquisition of 3D seismic data and additional well control. Shale in the Tuban Formation provides a regionally extensive seal for Kujung fault traps and dip closures. Intraformational shales in both the Kujung and Ngimbang Formations provide local seals for multiple objectives within these units. Migration Lateral migration of hydrocarbons into the North Madura Platform area following expulsion from the source rocks relies on both carrier beds and faults. Hydrocarbon shows in basement fractures and lower Ngimbang clastics indicate that these zones provide a mechanism for migration up the dip slope from the south (Figure 10). Porous zones within the upper Ngimbang and Kujung carbonates also provide potential migration paths. Faults are effective in connecting Kujung reservoirs to deeper source rocks in basinal areas flanking the North Madura Platform, and the bounding fault zone to the north and west may source nearby accumulations on the Platform such as

Payang (Figure 10). Faults may also be important in plumbing source kitchens to carrier beds, which then transport oil and gas updip into traps. Play Concepts Lack of success over 30 years of exploration has not downgraded the exploration potential of the North Madura Platform. Good oil and gas shows in several early wells indicates that hydrocarbons have migrated into the block. On the north side of the Platform, Kujung oil and gas shows encountered in the JS 2-1 and JS 3-1 wells proved hydrocarbon migration into this area from the Central Deep. The southern margin of the Platform has oil and gas shows in Kujung and Ngimbang formations in the JS 19-1, JS 44A-1 JS 8-1 and Perawan-1 wells. This documents hydrocarbon migrating from the southern kitchen area in the Madura Basin. Recent study indicated that the previous wells were not located in optimum locations, due in part to problems with velocity gradients and seismic depth conversion. The best areas appeared as yet untested. Play concepts for this area are illustrated with a regional northwest–southeast cross section from Camar field to the Island of Madura, constructed from both well and seismic data (Figure 11). Figure 11-A shows generalized structure, stratigraphy, and source rock maturity at the present. Figure 11-B reconstructs the configuration following deposition of the Tuban Formation, the regional seal for the underlying Kujung Formation. As shown, the North Madura Platform occupies the crest of a half graben that extends up a dip slope from one mature hydrocarbon kitchen (the Madura Basin) and is faulted against another (the Central Deep). Both kitchen areas have good source potential in the Ngimbang Formation. The Platform existed as a stable high since the Late Eocene, and oil would have migrated into this area starting with earliest generation in the Oligocene. Dip closures on the crest of the Platform were available for this early charge and appear to have remained intact through later deformational events. Fault traps were reconfigured by rejuvenation and inversion during the Middle Miocene tectonism and subsequently reactivated by Plio-Pleistocene tectonics.

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Presence of gas in and around the North Madura Platform was previously recognized from gas shows in dry holes and from abundant hydrocarbon indicators on seismic data. Velocity gradients, however, masked the significance of the gas shows and confounded attempts to determine whether the wells were optimally located. Recent detailed velocity studies and new well control help address those questions, and indications of gas reservoirs in all four recent wildcats in the Ketapang PSC indicate the widespread occurrence of Kujung gas in this area. Acquisition of 3D seismic data and more drilling are still needed to quantify the size of these discoveries and indicate remaining exploration potential for the North Madura Platform. Oil potential for this area was recognized by maturation modeling following analysis of source rock type and quality, geothermal gradients, and regional structure and stratigraphy. This study showed migration pathways providing access from Central Deep and Madura Basin kitchen areas to potential oil reservoirs in the Ngimbang and Kujung. Timing for old dip closures on the crest of the Madura Platform is ideal to receive hydrocarbons starting with the onset of hydrocarbon generation. Fault traps need careful study, but most appear to have formed in time to capture oil generated during much of the period of peak oil generation. Several wells drilled in the vicinity of the North Madura Platform lack thick sections with high porosity and permeability in the Ngimbang and Kujung intervals, and reservoir thickness and quality remains one of the main risks for the hydrocarbon exploration in this area. Recent study indicates that more favorable locations may be available for both reservoir development and hydrocarbon occurrence. This was confirmed by the 2000-2001 drilling campaign, which found both oil and gas on the crest of the Platform (Bukit Tua-1 and Jenggolo-1), gas in a fault bounded trap on the northern margin (Payang-1), and gas in the reef track flanking the platform to the south (Bukit Panjang-1). These discoveries also dispel doubts about lateral migration into this area over significant distances (25 km). DEPTH CONVERSION Accurate imaging of depth structure poses a major

challenge to exploration success in the North Madura Platform area. Problems with depth conversion of seismic maps result from two types of velocity gradients: abrupt lateral changes in velocity caused by channels in the shallow Wonocolo carbonate section, and much less severe velocity gradients associated with lithologic or thickness changes in the underlying section. The Wonocolo channels cause the biggest problems. Prominent north-south and northwest-southeast channels show clearly on the Top Wonocolo time map (Figure 12). These features are about 2 km wide, and they incise deeply (250 ms, approximately 750 ft) into platform carbonates of the Wonocolo. The channel fill is much slower in seismic velocity than the surrounding carbonates, causing a significant sag in seismic events beneath the channels (Figure 13). This sag indents deeper horizons such as the inter-Tuban marker (Figure 14) beneath the shallow channels. Similar effects occur at the prospective Ngrayong, Kujung, and Ngimbang horizons, and these shallow artifacts must be removed to properly image structural closures. To remove effects of the shallow channels, time maps for reservoir horizons were corrected prior to depth conversion. Horizons between the Wonocolo and Kujung formations are quite uniform in thickness throughout the North Madura Platform area, indicating relatively uniform sedimentation without major tectonic events. Seismic sections were flattened on the top of the Ngrayong marker, the closest consistent seismic event beneath the Wonocolo. Phantom horizons were then extended through the channels. The amount of sag between these flattened phantom horizons and the actual markers was mapped and used to calculate the amount of sag. This correction was then applied to the reservoir time maps, and the time maps were converted to depth using velocity gradients established by well control. Recent processing tests indicate that pre-stack depth migration may be even more effective in removing sag associated with both the Wonocolo channels and with faults that bound the North Madura Platform to the north and west. Regional velocity gradients also occur, and recent exploration wells drilled out as much as 150 ft off prognosis although they tie quite well with the

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seismic data. Average velocities generally increase southward toward the shelf edge reef complex, reflecting the change from interbedded limestone and shale in the platform area to massive carbonate in the shelf edge reef tracts. Average velocity maps were constructed and used for conversion of the seismic time for the final depth maps. CONCLUSIONS 1. The Central Deep and Madura Basin constitute

effective hydrocarbon kitchen areas for the North Madura Platform. Hydrocarbon shows in wells along the southern flank document oil and gas charging from the Madura Basin. The JS 2-1, which tested 4 mmscfd, indicates hydrocarbon migration into the north Platform area from the adjoining Central Deep. Gas tested in the Kujung at Payang is closest to the Central Deep, but sourcing from either kitchen area remains possible.

2. Long distance (25 km) migration along carrier

beds in the Ngimbang and/or Kujung charged the central part of the North Madura Platform, as indicated by oil and gas discoveries at Bukit Tua-1 and Jenggolo-1.

3. Reservoir thickness and quality remain poorly

defined for the North Madura Platform due to the paucity of well control. The concept that carbonate reservoirs will occur on the crest of the Platform due to periodic exposure appears to have been validated by high flow rates from wells in this area (cumulative 7500 bopd plus 12 mmscfd from two tests in Bukit Tua-1, 3700 bopd plus 3 mmscfd from one test in Jenggolo), but more study is needed to determine reservoir characteristics. Better seismic imaging with a 3D data volume may help delineate reservoirs and define sweet spots, in combination with more well control and additional rock data.

4. Correction for shallow velocity anomalies and

careful attention to time/depth conversion of seismic data is critical to accurately map prospects and properly position exploration wells.

ACKNOWLEDGEMENTS The authors wish to thank the Gulf Resources Management, partner Petronas Carigali (Ketapang) Ltd., and Pertamina for approval to publish this paper. The contributions by the Gulf exploration staff, particularly Chuck Caughey, Jim Parkin and the drafting team are gratefully acknowledged. We also recognize John N. Wilson and Christopher C. Gibson-Robinson for recognizing the exploration potential of the North Madura Platform and leading early phases of exploration. REFERENCES CITED Ardhana, Wayan, 1993. A Depositional Model for the Early Middle Miocene Ngrayong Formation and Implications for Exploraton in the East Java Basin, Proc. of the Indonesian Petroleum Assoc., 22nd Annual Convention, v. 1, p. 395-443. Bransden, P.J.E., and S.J. Matthews, 1992. Structural and Stratigraphic Evolution of the East Java Sea, Indonesia, Proc. of the Indonesian Petroleum Assoc., 21st Annual Convention, v. 1, p. 418-453. Courteney, S., Cockcroft, P. Miller, R., Phoa, R.S.K., and A.W.R. Wight, ed., 1989. Oil & Gas Fields Atlas, v. IV, Java, p. 2-9. Pertamina BPPKA, 1996. Petroleum Geology of Indonesian Basins Principles, Methods, and Application, v. IV, East Java Basins, 107 p. Phillips, Thomas L., Noble, Ron A., and Frank F. Sinartio, 1991. Origin of Hydrocarbons, Kangean Block Northern Platform, Offshore N.E. Java Sea, Proc. of the Indonesian Petroleum Assoc., 20th Annual Convention, p. 638-661. Soetantri, Bambang, Samuel, Luki, and G.A.S. Nayoan, 1973. The Geology of the Oilfields in North East Java, Proc. of the Indonesian Petroleum Assoc., 2nd Annual Convention, p. 149-175.

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FIGURE 1 - Location of North Madura Platform and Ketapang PSC, offshore East Java, Indonesia. Wells in the vicinity of the

Ketapang PSC are shown.

Fi

gu

re

1

TUBAN-1

JENU-1

JS 13-1

JS 10-1JS 6A-1

JS 31A-1 KE-1

Poleng Field

Camar Field

KE-17KE-2

KE-23KE-8KE-9

KE-7

JS 20-2

JS 19W-1

JS 20-3

AROSBAYA-1

JS 33A-1

KETAPANG-1

JS 14A-1

JS 13A-1

GUGUL

PAMOROH

KONANG-1

KONANG-2

LERPAK TANJUNG

KECODUR & W. KECODURDUKO

SUMBERLANTUNG

DURBUK

XX-1

MS-1-1

CAMPLONG-1

PAKAAN-1

RANCAK-1

GIGIR-1

PRANCAK

KERTENGENEH

MANDALABUJUR

CASSIOPEIA-1

PASIAN-1

MADURA

113°00'E 114°00E

7°

00

'S

6°

30

'S

112°30'E 113°30'E

0

KM

2510

JS 1-1

JS 1-2 / CAMAR-2

BUNKU-1

JS 1-3

KE-5

KE-12KE-6

Ujung Pangkah-1KE-2 Field

KE-5 Field

BD-1

BD-2

JS-18-1

KE-4

Payang-1

JS-2-1 JS-3-1

KETAPANG BLOCK

SERGANG-1

JS-19-1JS-44-1

Bukit Tua-1

Bukit Panjang-1JS-8-1 Perawan-1

Jenggolo-1

LamonganPEGAT-2

KUJUNG-1

EAST JAVA

KUTI

KE 11-AKRUKA

Gresik

NGIMBANG-1 BOGOMIRING

LIDAH

LINGSIR SEPAT-1 (LIS-1)

GRIGIS BARAT-1

GONDANG-1

NGASIN-1

SURABAYAMETATU

SEPAT

PETIKEN

SEKARKORONG

Tambak Boyo-1 Sidayu-1

22ST1

3

NORTH MADURA PLATFORM

MW-1

MBD-1

CD-1

MS 2-1A

MS 2-1KALIMANTAN

J A V A

SULAWESI

SINGAPORE

PHILIPPINES

BALI

P A C I F I C O C E A N

PHILIPPINES

INDONESIA

Jakarta

1000 KMS.

0°

06°S

12°S

IRIAN JAYA

06°N

106°E 112°E 118°E 130°E 136°E

PA

PU

A

N

EW

GU

IN

IA

124°E

BRUNEI

SARAWAK

SABAH

SOUTH CHINA SEA

J A V A S E A

ARAFURA SEA

SULAWESI SEA

B A N D A S E A

I N D I A N O C E A N

E. JAVA BASIN

LOCATIONAREA

ANYER

WALUKU-1

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FIGURE 2 - Informal Stratigraphy of the East Java Basin.

MY

BP

AGE/SERIES

Stratigraphy of theEast Java Basin

La

te

LA

TE

LA

TE

LA

TE

EA

RL

Y

OL

IG

OC

EN

EE

OC

EN

EM

IO

CE

NE

MI

DD

LE

Ea

rl

yE

AR

LY

MI

DD

LE

0

1

5

10

15

20

30

40

Ledok

Madura Strait/Onshore East Java / Madura Offshore, East Java Sea

Platten Lst

carbonate platform and reefs

Rancak Lst.

"CD"

Karren

clastics and volcaniclastics

Vo

lc

an

is

m

MAINPHASES

KUJUNG CYCLE

TUBAN & NGRAYONGCYCLES.

Renewed clastic deposition ;widespread quartz sandoccurs across East Java

WONOCOLO CYCLES

Widespread deep marinemarl with minor sands.

Initiation of back arc basins. Multiple half graben structures with high rates of subsidence. Rifts initially infilled by lacustrine sediments, thencontinuing regional subsidenceleads to marine transgression.

BDTrend

KerekVolcaniclastics

KA

LI

BE

NG

Gulf Resources (Ketapang) Ltd

Figure 2

Metasedimentary &Igneous Basement

NGIMBANG CYCLE

VVV

V

V

Wonocolo Formation

Ngrayong Formation

Tuban Formation

Prupuh - Kujung Unit I

chalky lst.

Kujung Unit II

Kujung Unit III

Kujung Formation

Ngimbang Formation

Lacustrine shale

Ol

d

An

de

si

te

Vo

lc

an

is

m

S

of

mo

de

rn

ar

c

Mo

de

rn

Vo

lc

an

is

m

Modified from CGR 1997

"TB"

LATE MIOCENE - RECENT

Beginning 7 MYBP,local tectonic events controlfacies development. From latestPliocene, modern volcanic arccontributes huge quantities offeldspathic / lithic sand.

Initial regressive phasedeposits clastics, then a majortrangressive and carbonaterich phase as regionalsubsidence continues.

PLEISTOCENE

PL

IO

CE

NE

Vol. 1 - 717

FIGURE 3 - Location of Seismic Data and Well Control for the Ketapang PSC.

KETAPANG BLOCK

113 °12' 00" E 113 °24' 00" E 113 °36' 00" E 113 °48' 00" E

6

°3

0'

00

"

S

6

°4

0'

00

"

S

6

°5

0'

00

"

S

JS-2-1

KE- 4

JS-44A-1

JS-8-1PERAWAN-1

SERGANG-1

JS-3-1

JENGGOLO-1

JS-19-1

PAYANG-1

BUKIT TUA-1

BUKIT PANJANG-1

740000 760000 780000 800000

92

40

00

09

26

00

00

92

80

00

0

MADURA ISLAND

113 °00' 00" E

JS-14A-1

PASIAN-1

CASSIOPEIA-1JS-33A-1

KETAPANG-1Camar-3

Camar-4Camar-1

JS-1-1

KE-2A-6JS-19W-1KE-2A-5

KE-2B-1

JS-20-3

KE-23

93

00

00

0

6

°0

0'

00

"

S

Fi

gu

re

3

1999 VINTAGE

PRE 1999 VINTAGES

0 10

KM.

N

Vol. 1 - 718

FIGURE 4 - Regional Tectonic Setting for the East Java Basin.

BD SOUTH H-GRABEN

0 50

Kilometers

100

111°00 E109°00 E

7°

00

S

5°

00

S

3°

00

S

9°

00

S

JAVA

KARIMUNJAWA ISL.

: HIGH AREA

: INTERMEDIATE AREA

: LOW AREA

: RECENT VOLCANIC ARC

LEGEND :

113°00 E 115°00 E 117°00 E 119°00 E

PASIRBASIN

BARI

TO S

HELF

PATE

RNO

STER

HIG

H

SE

BU

KU

SH

EL

F

MERATUS

BARITO

BASIN

RIDGE

ASEM-ASEM BASIN

LAUTISL.

KALIMANTANSULAWESI

SOUTHMAKASSAR

BASIN

MAKASSARHALF GRABEN

MURIAH TROUGH

FLO

REN

CE

TRO

UGH

WE

ST

BAWEAN ISL.

EAST CEPU HIGH

MADURA BASIN

CD H-GRABEN

BASINWEST CEPU HIGH

DO

ANG

TR

OU

GH

SALAYARBASIN

HIGH

2000

m

FLORESBASIN

SIBARU H

IGH

PULAU LAUT RIDGE

PAGERUNGAN HALF GRABEN

LOMBOK BASIN

JS-5

TROUGH

MAS

ALIM

A TR

OUGH

MASALIMA

MDA H-GRABEN

BD RIDGE

PORONG HALF GRABEN

NORTHERN PLATFORM

NORTH MADURA HIGH

KETAPANGBLOCK

CARBONATEPLATFORM

CENTRAL BASIN

NGIMBANG

JS-1 RIDGE

JS-1 TROUGH

BAWEAN ARCH

TUBAN TROUGH

TRO

UGH

FLO

RENC

E

EA

ST

KARIMUNJAWA ARCH

SUNDA - LAND

SOUTHERN BASINCENTRAL HIGH

SOUTH HIGH

Fi

gu

re

4

ADANG FAULT ZONE

CENTRAL DEEP

MADURA

Vol. 1 - 719

FIGURE 5 - Distribution of Major Lithologies in pre-Tertiary basement, East Java Basin.

MBD-1

CD-1

MS 2-1AMS 2-1

Tuban

Bojonegoro

Lamongan

Mojokerto

Ngawi

BloraAROSBAYA-1NGRAYONG

BANYUBANG

BANYUASINLEDOK

DANDANGILO

NGIMBANG-1

DERMAWU-1

MUDI-1

KLUWEH(gas)

WONOCOLO,KAWENGAN,NGUDAL, &KINDANGAN

METES

LUSIPETAK

KEDINDING

BALUN - TOBO

SEMANGGI NGLOBO

DANDER-1

BODJONEGORO-1

PEGAT-2

KUJUNG-1KEMBANG BARU-1

KEMBANG BARU-2

WUNUT-1

KE 11-G

PETIKEN

METATU

SEKARKORONG

BOGOMIRING

LIDAH

GUGUL

PAMOROH

KRUKA

KUTI

LINGSIR SEPAT-1 (LIS-1)

KONANG-1KONANG-2

PORONG-1

GRIGIS BARAT-1

GONDANG-1

NGASIN-1

SEPAT

LERPAK TANJUNG

KECODUR & W. KECODUR

DUKO

SUMBERLANTUNG

DURBUK

KE 11-C

KE 11-E

BD-1

XX-1

BD-2

MS-1-1

CAMPLONG-1

PAKAAN-1

RANCAK-1

GIGIR-1

KE 11-A

PRANCAK

KERTENGENEH

MANDALABUJUR

0 5025

KM

JAVA

MADURA

KETAPANG BLOCK

SURABAYA

7°

00

S

114°00E

Tuffaceous Ss.

Slate

Rhyodaci tesSlate

Fi

gu

re

5

TUBAN-1

JENU-1

JS 28-1

KUTILANG-1

JS 13-1

JS 10-1JS 6A-1

KE-5JS-20/Poleng Field

KE-17UJUNG PANGKAH-1

KE-2

KE-23KE-8

KE-9

KE-7JS 20-2

JS 19W-1

JS 13A-1

PLATUNGAN

TAWUN GEGUNUNG

JATIROGO-1

BLIMBING-1

112°00E

6°

00

S

Quartzite

Phyll i te + Ss.

Px-Basalts

Metashale + Siltstone

Phyll i te

Phyll i teJS 2-1

JS 33A-1

KETAPANG-1

MONTOR-1

JS 14A-1

CASSIOPEIA-1

PASIAN-1

MetaSiltstone

Gabbro

Diabase

Meta- tuf f+ lava

Meta-Sediments

JS 31A-1

KE-6

KE-1

KE-12 JS 20-3

KE-4

JS 1-1

JS 1-2 / CAMAR-2

113°00E

Basalt

Andesite

Basic Igneous

PAYANG-1

JS 8-1JS 44A-1

PERAWAN-1

SERGANG-1MonzoniteVitr ic Tuff

Diorite

BUKIT PANJANG-1

JS 19A-1

BUKIT TUA-1JENGGOLO-1

JS 3-1

Meta-greywacke

: Acidic Igneous Rocks

: Intermediate Igneous Rocks

: Basic Igneous Rocks

LEGEND :

: Low Grade Metamorphic Rocks

Vol. 1 - 720

FIGURE 7 - Interpreted Paleogeography during deposition of the upper Kujung Unit 1 (Early Miocene) in the East Java Basin.

FIGURE 6 - Interpreted Paleogeography during deposition of the upper Ngimbang Formation (Late Oligocene) in the East Java Basin.

Vol. 1 - 721

FIGURE 8 - Maturation Model for Hydrocarbon Source Rocks in the Central Deep of the East Java Basin. The geothermal gradient is assumed to be 1.80 degrees F per 100 feet.

FIGURE 9 - Maturation Model for Hydrocarbon Source Rocks in the Madura Straits Portion of the East Java Basin. The geothermal gradient is assumed to be 1.51 degrees F per 100 feet.

CENTRAL DEEP MATURATION MODEL

Age

Depth (f

0 5000

10000 15000 20000 25000

55 5

5 40 3

0 20 1

0 0t = 0

Basement L. Ngimbang Type I/II U. Ngimbang Type II/III Kujung Kujung II Kujung

Tuban Ngrayong Wonocolo Pliocene Pleistocene

Mature (oil) 0.7 to (%Ro) Peak Generation (oil) 1 to 1.3 (%Ro) Main Gas Generation 1.3 to 2.6 (%Ro)

GG. 1.8 F/100'

MADURA STRAITS MATURATION MODEL

Age

Depth (f

0

5000

10000 15000 20000

30000

55 5

5 40 3

0 20 1

0 0t = 0 Basemen

t L. Ngimbang Type I/II U. Ngimbang Type II/III Kujung I Kujung II Kujung III Tuban Ngrayong

Wonocolo

Pliocene

Pleistocene

Mature (oil) 0.7 to (%Ro) Peak Generation (oil) 1 to 1.3 (%Ro) Main Gas Generation 1.3 to 2.6 (%Ro)

GG. 1.51 F/100'

25000

Vol. 1 - 722

FIGURE 10 - Interpreted Distribution of Hydrocarbon Kitchen Areas and Migration Pathways for the North Madura Platform Area of the East Java Basin.

FIGURE 11 - Cross Section A-A’ Reconstructed from Seismic Lines to show Structural Configuration and Distribution of Mature Source Rocks A). At the Present Day. B). During Sedimentation of the Tuban Formation, 15.0 million years B.P.

Vol. 1 - 723

FIGURE 12 - Top Wonocolo Time Structure Map. Note prominent north-south trending channels. Seismic line segment located in west center area is shown in Figure 13.

0 10KM.

N

MADURA ISLAND

48

0

560

720

640

560

400

80

160

480

56

0

56

0

64

0

1040

1120

72

0

400320

640

72

0

560720

800

800

480

400

320

540

113° 12' E 113° 24' E 113° 36' E 113° 48' E

6°

30

'

S

6°

40

'

S

6°

50

'

S

C.I. 20 ms

JENGGOLO-1 BUKIT TUA-1

PAYANG-1

BUKIT PANJANG-1

JS - 44A - 1

JS - 8A - 1 PERAWAN-1

JS - 19 - 1

JS - 2 - 1 JS - 3 - 1

SERGANG-1

KE - 4

KETAPANG BLOCK

92

80

00

0

92

60

00

0

92

40

00

0

740 000 760 000 780 000 800 000

SEISMIC LINE

Fi

gu

re

12

CHANNELS

CHANNELS

Vol. 1 - 724

FIGURE 13 - Seismic Line Segment from the west central part of the Ketapang PSC. Note prominent Wonocolo channel in the center of the line at about 500 ms., causing sag in deeper horizons. Line segment is located on Figures 12, 14, and 15.

99GKT- 023

Time

i n

Mi l l i seconds

2.5 KMSW NE

Fi

gu

re

1

3

Top Wonocolo

Top Ngrayong

Intra- Tuban

Top BasementTop Kujung III

Top Kujung IITop Kujung I

Top Rancak

CHANNEL

Vol. 1 - 725

FIGURE 14 - Uncorrected Time Structure Map for Intra-Tuban Horizon. Note presence of sag caused by shallower

Wonocolo topography. Seismic line segment located in west center area is shown in Figure 13.

Fi

gu

re

14

11

00

1100

11

00

11001200

1100

1000

800

700

1000

1100

1100

1000

11

00

1100

11

00

1000

900

800800

700

80

0

900

1000

1100

1100

1100

1100

1000

900

900

1200

1200

1100

1000

800

600

700500600700

700

600

600

500

70

0

1100

1300

1200

1400

1600

1100

1800

1800

1600

1400

1200

1200

1300

1200

1040

10

40

10

40

JENGGOLO-1 BUKIT TUA-1

PAYANG-1

BUKIT PANJANG-1

JS - 44A - 1

JS - 8A - 1 PERAWAN-1

SERGANG-1

KE - 4

JS - 19 - 1

JS - 2 - 1

JS - 3 - 1

92

80

00

0

92

60

00

0

92

40

00

0

740 000 760 000 780 000 800 000

0 10

KM.

N

C.I. 20 ms

113° 12' E 113° 24' E 113° 36' E 113° 48' E

6°

30

'

S

6°

40

'

S

6°

50

'

S

KETAPANG BLOCK

SEISMIC LINE

70

0

AREA OF SAG

Vol. 1 - 726

FIGURE 15 - Corrected Time Structure Map for Intra-Tuban Horizon. Wonocolo topography is no longer

recognizable. Seismic line segment located in west center area is shown in Figure 13.

1000

1000

900

900

1100

1200

1100

12

00

1300

1200

800

600

1000

700 7

00

11

00

1100

1400

1200

1600

1400

1600

1400

700

600

800 800

700

400700

600500

9001000

1000

900

70

0

600

500

400

1000

500

500

600

800

1000

500

50

0

500

400

600

700

800

1000

11

00

1100

12001100

110011

00

900

1000

1000

900

11001100

1000

900

1100

1100

JENGGOLO-1 BUKIT TUA-1

PAYANG-1

BUKIT PANJANG-1

JS - 44A - 1

JS - 8A - 1 PERAWAN-1

SERGANG-1

KE - 4

JS - 19 - 1

JS - 2 - 1

JS - 3 - 1

92

80

00

0

92

60

00

0

92

40

00

0

740 000 760 000 780 000 800 000

0 10

KM.

N

C.I. 20 ms

113° 12' E 113° 24' E 113° 36' E 113° 48' E

6°

30

'

S

6°

40

'

S

6°

50

'

S

KETAPANG BLOCK

MADURA ISLAND

SEISMIC LINE

Fi

gu

re

15

1800