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Abstract: Discovery of a New Deep-Water Petroleum System Seaward of the Mahakam Delta, in the Makassar Strait, East Kalimantan, Indonesia

Sand reservoirs of the Tertiary Mahakam delta have proven reserves of approximately 3 billion barrels oil and 40 TCF gas. Initial offshore exploration was confined to the shallow shelf, where Unocal discovered the Attaka Field (650 MMBO, 2.85 TCF), and where other significant discoveries were made in Tertiary deltaic reservoirs sourced by land-plant derived kerogen. In 1996, Unocal discovered an entirely new system of Tertiary reservoir and source rocks at Merah Besar field, which confirmed commercial oil and gas pay in Pliocene and Miocene deepwater sands. Seismic data show repeated episodes of lowstand delta progradation to Miocene and Pliocene paleo-shelf edges. Sand from these deltas spilled over the shelfslope break and was transported down-slope in slope-canyons. Canyon reservoirs are stacked, laterally confined channel-fill and channel-levee sands interlayered with shales. Lower on the slope, canyons disgorge into fans with laterally continuous sand packages interlayered with shale. Reservoir quality is excellent in canyon and fan sands.

The same turbidity currents that transported coarse clastics into the basin also carried high concentrations of terrigenous organic material comprised mostly of land plant debris including vitrinite, resinite, bituminite, and, minor curinite and sporinite. Overall, these components are consistent with a volatile oil petroleum system. Source rock analyses including TOC, Rock-Eval, pyrolysis, vitrinite reflectance and kerogen microscopy confirm that there is sufficient source organic richness and thermal maturity to have generated and formed sizable oil and gas accumulations in the deep water. The oils are characterized by (1) a relatively high pristane/phytane ratio, (2) a high concentration of angiosperm-indicative biomarkers including oleanane and bicadinanes, (3) a dominance by C29-steranes over C27- and C28-steranes, and (4) the lack of marine-indicative C30-steranes.These attributes indicate that the oil and gas accumulations present in this deep-water petroleum system were largely derived from land-plant organic matter. However, a gradual reduction of land plant input from the inboard into the deep water is observed, suggesting enhanced marine influence from the shelf, through the slope to the submarine fan system.

The sedimentary strata penetrated by drilling at Merah Besar and Seno Fields are thermally immature; as a result, hydrocarbon fluids must have migrated vertically from deeper kitchen areas before becoming entrapped in the overlying late Miocene and early Pliocene sandstone reservoirs. Hydrocarbon migration along faults was facilitated by deep penetrating normal faults that cut Pliocene and Miocene sedimentary sequences. The magnitude of fault displacement along the shelf margin is graphically demonstrated by lowering carbonate pinnacle reefs that were cut from the shelf edge by listric faults and transported thousands of feet down into the basin.These reefs struggled to keep up with rapid subsidence and grew into pinnacle reefs thousands of feet high; though none of them could keep up with the high rate of normal-fault tectonic subsidence and all are now dead and onlapped by bathyal shale in water depths of over 2000'.These pinnacle reefs are highly porous and form excellent gas reservoirs. Down slope from the shelf edge, the large listric normal faults curve beneath the slope and flatten into a detachment surface at about 20,000' depth subsea. Thrust faults then rise from this detachment surface to produce broad anticlines that grow at the same time as the deposition.and growth of the submarine-fans. Seno field was discovered in the first anticline drilled in this trend.

Two major oil fields have been discovered to date within Plio-Miocene slope sediments seaward of the Mahakam Delta. Merah Besar field is a rollover anticline developed between listric-normal and antithetic faults. Pliocene and Upper Miocene slope-canyon sands form excellent reservoirs. Pliocene reservoirs tend to be gas prone, while Miocene sands tend to contain oil in the 35 to 38 degree API range.The gas is thermogenic rather than biogenic, and migration-fractionation is proposed to explain the observed gas and oil segregation. Down slope from Merah Besar, listric faults sole-out into a basal detachment, and ramp-thrusts rise from the detachment surface to form large fault-propagation folds. The first test of a fold took place at the Seno structure and flowed 10,000 bopd from Upper Miocene sand. In summary, exploration in the deep-water Makassar Strait is still in its infancy, though initial results are positive. 

AAPG Search and Discovery Article #[email protected] International Conference and Exhibition, Birmingham, England