south sumatra basin

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ABSTRACT The swampy, densely forested Merang Triangle had very limited exploration for many years. Numerous plays have been established by a JOB that has been operating in the area of Jambi-Merang, with numerous assistant operators, since 1988. Production has been established from the Talang Akar in the Gelam field, which is unitised with Gulf Indonesia Resources and partners, A field has been established in the Baturaja carbonates and further stratigraphic potential has been highlighted. The various components of the petroleum systems are discussed along with the challenges encountered in evaluating the carbonate reservoir. A Plio-Pleistocene structural uplift, the Sembilang High, which resulted in the erosion of thousands of feet of sediment, is demonstrated. It is shown that this uplift is associated with a regional tilt to the southeast, causing possible re-migration. Also shown is that recent faulting has broken up a carbonate complex and the resultant offreef platform facies being currently structurally higher than the original reef crest. This resulted in earlier drilling at the structural crest encountering the platform facies and missing the build-up. The block comprises an area of many overlapping plays and an interesting petroleum system. INTRODUCTION AND HISTORY OF EXPLORATION The Merang triangle is situated just south of the city of Jambi, overlapping the boundary between the Jambi Province and the Province of South Sumatra. It is a low-lying, swampy area, which was initially covered by tropical rain forest. The earliest exploration to be carried out in this area was by BPM and NIAM. The latter carried out gravity surveys in__________________________________________________________ * JOB Pertamina-YPF Jambi Merang

the Jambi area in the 1920s and following the drilling of the Air Benakat discovery at Tempino in 1931, drilled Sungai Merang-1 near oil seeps in the Merang River in 1934. Minor shows brought them back to drill Sungai Merang-2 in 1937, which was a dry hole. NIAM however, had better luck the following year when they drilled West Berau-1 and Kali Berau 5, both of which produced oil from the Air Benakat Formation. After the 1939-45 war, BPM drilled Bayung Lincir-1, a surface feature near to the road between Palembang and Jambi, this well tested gas at a very low rate with water. After the formation of PN Pertamina in 1960, the area was explored again in the Sungai Merang and Bayung Lincir areas with Sungai Merang-3 being drilled in 1961 and Bayung Lincir 2 in 1967. The later well recovered water with a trace of oil from the Air Benakat. The shows attracted Pertamina to return in 1973 and drill Bayung Lincir 3, which recovered oil, but not in commercial amounts. Pertamina awarded the Corridor Block to REDCO in 1968 and the Jambi Area to Jambi Oil in 1978, leaving the swampy Merang Triangle open (Figure 1). In 1989, part of the Jambi oil block plus the Merang Triangle was awarded to Elf Aquitaine Indonesie (Elf) as the JOB Jambi Merang (Figure 2). A seismic survey carried out in 1990 highlighted the major features in the block. The first well drilled, was Sukajaya-1 in 1991, on a feature that had been highlighted by an earlier survey carried out by Jambi Oil. This was down dip from the Tempino Oil field, which is producing oil from the Air Benakat Formation. The feature was predominantly a nose dipping down from the Tempino Field with a counter dip creating the closure. This well encountered gas in the Air Benakat Formation, but the feature was small at the productive horizon. It also had shows at the Talang Akar level although high temperatures and

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pressures along with some H2 S prevented a full evaluation of that interval. Sukajaya-1 also demonstrated drilling difficulties as the shales of the Gumai Formation were over pressured. The second well, Muara Sabak-1, was drilled along strike from the East Ketaling (Air Benakat) Oil Field with a resultant dry hole. A third well, Kukulambar-1, was drilled to test a structural closure at Talang Akar level and a carbonate (Baturaja) play. Here a platform limestone was encountered, which after acidizing, recovered gas and condensate, but the Talang Akar Formation lacked reservoir. Meanwhile, Asamera, which is now Gulf Indonesia Resources Ltd (Gulf), discovered the Gelam gas/condensate field in the Talang Akar Formation. This field crosses over into Jambi Merang, Elf demonstrated this with the drilling of Gelam 4 in 1992. The field is now on production and forms part of the Corridor Gas Project supplying gas to Duri for the steamflood operations there. A final well Merang-1 was dry. Elf had evaluated, without success, the extension of two oil trends into the block and a large structural high near the West Berau discovery of NIAM. They had encountered tight limestone at Baturaja level in Kukulambar-1, Muara Sabak-1 and Merang-1, while the Baturaja at Gelam, although having porosity, was water wet. Elf sold its remaining interest to Saga Petroleum Indonesia (Saga). Saga pursued the Baturaja carbonate by drilling the Pulau Gading feature, which had both structural and stratigraphic closure at Baturaja level. Pulau Gading-1 discovered gas and condensate in the Baturaja Formation, uneconomic gas flows from the Talang Akar Formation and gas shows in fractured basement. The porosity in the Baturaja Formation was predominantly at the top of the buildup, but wherever there was porosity there was gas and a gas/water contact was not confirmed. Saga sold out initially to Cue Energy (Cue), but finally to YPF-Indonesia (a subsidiary of YPFRepsol). JOB Pertamina-YPF-Jambi Merang has since drilled two delineation wells on Pulau Gading. These wells demonstrated the areal extent of the hydrocarbon bearing porosity. Further evaluation of the remaining prospects has been carried out. This has been achieved in several ways, for instance by studying the seismic amplitudes to determine porosity distribution, and by flattening on Intra-Gumai events

to reveal the paleo-configuration of the carbonate buildups, and this has highlighted the potential of other buildups such as Sungai Kenawang. The petroleum system of this carbonate play is discussed, with the other plays in the block, later. TECTONIC HISTORY The main structural elements of the Jambi sub-basin are horsts and grabens, formed in the Palaeogene, which align northeast to southwest as opposed to the usual north-south trend of the South Sumatra Basin. In the Jambi Merang area, the Merang High and the Ketaling High represent horsts (Figure 3). The rift between the Merang High and the Ketaling High forms the East Ketaling depocentre. To the southeast of the Merang High is the Merang depocentre. These horsts were sources of sediment into the basins until they were eventually sub-merged and carbonates developed on these old highs. The area was subjected to Mid-Miocene wrenching and then further subsidence. Finally, it was subjected to PlioPleistocene compression, which formed northwestsoutheast trending folds, however, this compressional phase is also associated with a regional tilt towards the southwest and the uplift of the Sembilang High, followed by a considerable amount of erosion over the last few million years. Interpretations of three seismic lines that are aligned parallel to the horst and graben are shown in Figures 4, 5 and 6, Line A (Figure 4) runs down the East Ketaling Deep, line B (Figure 5) runs along the Merang High and line C (Figure 6) runs down the Merang Deep from the Sembilang High through the Gelam Field. Line A clearly demonstrates the regional tilt, the blue horizon is near base Muara Enim and thus represents a time of less than 10 m.y.a. This indicates that the tilting phase is younger than the near base Muara Enim event and probably took place at the same time as the Plio-Pleistocene compressional phase. This was followed by a period of erosion. The yellow horizon represents an Intra-Gumai event that can be mapped regionally; below this are a series of downlapping events demonstrating the sediments were derived from the Sunda Craton to the northeast. The green horizon is the top of the Talang Akar Formation and is approximately the top of the syn-rift sequence. Line B, being predominantly on the Merang High, only has a thin section of Talang AkarVol. 1 - 804

Formation. Parts of the Merang High were submerged prior to the end of the Talang Akar. The Muara Enim section clearly comprises parallel beds dipping to the southwest, as seen in the previous section, and the downlapping sequences are clear below the Intra-Gumai event. Line C is closer to the Sembilang High and the amount of recent uplift and erosion can be seen to be many thousands of feet and thus this area was buried much deeper. This section also goes through the Gelam Field, which is situated on the basement high at the western end of the seismic line. This recent tilting will have had an effect on any hydrocarbons that migrated prior to this tilting event, resulting in remigration. A similar remigration has been observed in the Kaji-Semoga area as described by Hutapea (1998). Line D (Figure 7) is aligned perpendicular to the previous three lines and shows the East Ketaling Deep, the Merang High and the Merang Deep.

Eventually, these carbonate buildups were drowned and the Gumai shales act as a regional seal over the area. Within the Gumai Formation, there are numerous downlapping events. These contain glauconitic sandstones, which are potential reservoirs, although within the Jambi-Merang block they are tight. These sands, however, are productive to the north in Sungai Gelam and in G rissik to the south. The downlapping events on the seismic profi


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