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Petroleum Geology of South Sumatra, Indonesia

Philip R. Woodside

The South Sumatra basin resulted from horst-graben structures formed from the end of Cretaceous to the early Tertiary. During the Pliocene-Pleistocene orogeny, prominent northwest-trending folds and strike-slip faults were formed within the basin.

The depositional history of South Sumatra is similar to the prolific Central Sumatra basin; however, the reservoir sandstone facies in South Sumatra are less well developed. Four major orogenic events resulted in the present framework of the South Sumatra basin. These events occurred during (1) a mid-Mesozoic orogeny which metamorphosed, faulted, and uplifted previously deposited sediments into blocks; (2) the Late Cretaceous-early Oligocene tectonic event with resultant horsts and grabens that controlled Eocene sedimentation; (3) the late Oligocene-Miocene, a period of isostatic subsidence until its tectonic quiescence was broken by middle Miocene diastrophism in the Barisan Mountains and by minor structural movements; and (4) the widespread Pliocene-Pleistocene orogeny reflected in he shallow structures in the Tertiary sediments. The southwestern part of Sumatra corresponds to the uplifted range that was folded during the Miocene.

Only the Tertiary Lahat (Lamat) and Talang Akar formations were deposited in rugged pre-Tertiary topography.

The contact between the Sumatra block and the Sunda-Java block is a north-south-trending right-lateral wrench fault. The anticlines within the South Sumatra basin can be grouped into three groups of anticlinal features. The major anticlinora consist of over 150 individual anticlines. To date, about 50 of the 100 structures tested have proved production.

Shales in the upper Oligocene-lower Miocene Talang Akar and lower-middle Miocene Gumai (Telisa) Formations show excellent potential for oil generation. The geothermal gradient ranges from 27.3° to 45.5°C/km. The depth of burial exceeds 1,700 m (5,576 ft). The source rock potential decreases toward the north margin of the basin.

Talang Akar reservoir sandstones, for the most part, form a zone parallel to the hinge belt along the eastern edge of the basin. The most prolific reservoirs are the Talang Akar and the Baturaja Formations. The Baturaja Formation is usually gas prone but is oil productive when the Talang Akar Formation is present beneath it.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.