BERITA SEDIMENTOLOGI, INDONESIAN SEDIMENTOLOGISTS FORUM Number 15, I/2001
Herman Darman (Shell EP Companies in Indonesia)
In the last decade significant advances in geologic understanding and exploration technology in Indonesia continue to reduce the risks associated with exploration and development in turbidite plays. New plays are defined base on new acquired seismic and well data. Turbidite become very important plays in Sumatra, Kalimantan, Java, and Irian. This paper will give a summary of the last development of the geological concept of the deep-water basins in Indonesia base on exploration data. NORTH SUMATRA Turbidite exploration activities in Sumatra focus on the north Aceh Basin (Fig. 1). This basin is an extension of the Sumatra back arc system and towards the north it becomes part of the Andaman Basin. Maximum sediment thickness is about 15,000 ft. A mounded, hummocky package of high amplitude seismic reflectors has been interpreted as a sand prone lowstand basin floor deposit (Nuraini et al., 1999). North-south trending seismic lines across the basin floor mound show several coalescing lobes of deposition. The sediment lobes filled the lows between previous fans and generally migrate northward to the lower areas. Trapping mechanism becomes the major risk for this new play, as there are no structural closures (Nuraini et al., 1999) and stratigraphic trap become the main potential in this area. EAST KALIMANTAN 1. Offshore Kutai / Deep Water Mahakam Unocal's exploration targets in deep water Kutai Basin are Pliocene and Miocene slope channels and base-of-slope fans, and Miocene shelf-edge reefs (Fig. 2; Lumadyo, 1998). The Merah Besar-1 well (November 1996) was the first potentially commercial deepwater discovery in East Kalimantan, confirming gas pay in Pliocene deep water sandstones. Significant oil discovery at Seno Prospect (West Seno-2) tested >10,000 BOPD, is further confirmation of the validity and potential of this emerging exploration play (Lumadyo, 1998). Core studies from West Seno field shows that most of the sands appear to have been deposited in a middle to upper slope environment in a series of aggrading channel-levee complexes during lowstands of sea level (Saller et al, 2000). Sands are dominantly fine to very fine-grained and poorly to very-poorly sorted. Bed thickness range from less than 1 mm to approximately 6 feet. Reservoir characteristics are related to depositional facies. Table 1 shows the sand classification and properties. Base on geochemical work on numerous hydrocarbon and kerogen samples Dunham et al. (2000) reported that terrestrial organic matter is the source for oil and gas accumulations discovered in the Deepwater Kutai Basin. Organic matter consists of plat leaves, plant fragments and coal fragments are found in well rock samples. This material is clearly derived from terrestrial sources, and was subsequently transported into the deep-water basin by turbidity currents. Specifically, terrestrial organic matters concentrated in the upper fine-tails of turbidite deposits. The same turbidites that carry sand also carry the terrestrial organic matter into the basin. The organic matter is not disseminated within the turbidite; rather, the kerogen becomes concentrated into laminae that can reach up to 15% TOC (Dunham et al, 2000). Though the resin and plant fragments making up the oil-prone source material are often in the coarse to very-coarse sand-size range (1 to 2+ mm), the low density of this material leads to the particles having the hydraulic equivalent of silt-size quartz (Dunham et al, 2000). As a result, the organic matter specifically concentrates in the silt and mud-rich fraction of the turbidite deposit 2. Tarakan Basin Tarakan Basin is located in the north of the Kutai Basin and share similar provenance, tectonic and sea level changes as Kutai Basin. This basin constitutes a passive continental margin with Late Eocene-Recent sediments on continental to oceanic crust, deposited during the Middle-Late Eocene opening of the Sulawesi Sea. Rifting ceased during the early Oligocene with quiet marine conditions prevailing until the Middle Miocene uplift of the Kalimantan hinterland. The latter uplift triggered a massive influx of turbidites in the deep-water area, deposited as unconfined toe of sloe fans ahead of the outbuilding Tarakan delta. During Plio-Pleistocene delta outbuilding, this sequence was buried by rapidly prograding slope deposits, which triggered gravity-driven toe thrusting (Fig. 3). Small basins were formed between thrust ridges and filled by slope deposits. In the southern part of the delta, westward dipping normal faults limited progradation, resulting in excessive thickening of the Pliocene-Pleistocene deltaic sequences an limiting sediment influx into the deepwater area (Hemmes et al., 2000). Several potential reservoir systems are recognized in the deepwater area. These include 1) unconfined toe of slope fans, 2) confined intra-slope fans, and 3) intra-slope channel (-levee) systems (Hemmes et al., 2000). Understanding about the charging system of this region is limited and there is only one well drilled so far (Bougainville-1). EAST JAVA In the west of East Java Basin, Tuban formation rapidly deposited during late Oligocene to early Miocene (stage II - sag, Ardhana, 1993). This shale dominated interval fill subsiding depressions and become strong source rock candidate for much of oil and gas in the western part of the basin although this is not proven (Netherwood, 2000). From early to middle Miocene, Ngrayong turbidite sandstone were deposited in the south of the basin during compressional fault block rotation, uplift and erosion. Figure 4 shows the depositional model of of lower Ngrayong Formation. Within this interval Ardhana (1993) recognized several depositional lobes of Lower Ngrayong unit II, named Nglobo Fan, Kawengan Fan, Bungoh-Grigis Fan, Gondang-Ngasin Sandy turbidite body and Candi turbidite body. Historically, the onshore Ngrayong sandstones were the main reservoir in the East Java basin and host most of the oil in the westerly Cepu region (Netherwood, 2000). They represent the main reservoir of the Kawengan oil field and are interpreted as relatively deep marine turbidite fan deposits (Ardhana, 1993 & Ardhana et al., 1993). Reservoir quality of Ngrayong sandstone is very good. Table 2 summarizes their reservoir characteristics. IRIAN Paleocene part of the giant Wiriagar deep - Ubadari - Vorwata gas accumulation in Papua (Irian) consists of turbidites (Netherwood, 2000). Information on reservoir characteristics are hardly available from publications. CONCLUSION Turbidite plays occur in Sumatra, Kalimantan, Java and Papua (Irian). These turbidite reservoirs show a good reservoir characteristics with high porosity and permeability. Charging systems are not well understood as there are limited well penetrated the source rocks. Stratigraphic trapping mechanism is common and very important in turbidite plays, however the risk is considered high. REFERENCES Ardhana, W., 1993, A depositional model for the Early Middle Miocene Ngrayong Formation and implications for exploration in the East Java Basin, proceedings of Indonesian Petroleum Association 22nd annual convention. Dunham, J. B., Bronw, T. J., Lin, r., Redhead, R. B., Schwing, H. F., Shirley, S. H., 2000, Transport and concentration of gas- and oil-prone kerogesn into deep water sediments of the Kutei Basin, East Kalimantan, Indonesia, in Proceedings of the 2000 AAPG International Conference & Exhibition, Bali. Hemmes, K., Darman, H., Suffendy, L, Meizarwin, 2000, Depositional systems of the deep water Tarakan Basin, Indonesia, in Proceedings of the 2000 AAPG International Conference & Exhibition, Bali. Lumadyo, E., 1998, Deepwater exploration in the Kutei Basin, East Kalimantan, in Proceedings of the gas habitats of SEAsia and Australasia conference, Indonesian Petroleum Association. Netherwood, R., 2000, A geological overview of Indonesia, in Indonesia 2000 Reservoir Optimization Conference, edited by Blunden, T., Shclumberger. Saller, A., Brown, T., Redhead, R., Schwing, H., Inaray, J., 2000, Deepwater depositional facies and their reservoir characteristics, West Seno Field, Offshore East Kalimantan, Indonesia, in Proceedings of the 2000 AAPG International Conference & Exhibition, Bali.