ABSTRACT: Oil Production from Pre-Tertiary Basement Rocks in Indonesia: Examples from Sumatra and Kalimantan

KONING, TAKO, Texaco Canada Petroleum Inc., Calgary, Alberta, Canada

Oil production from pre-Tertiary basement rocks is rare within the Tertiary back arc (foreland) basins of Western Indonesia. The Beruk Northeast field is the only field in the prolific Central Sumatra basin that produces from pre-Tertiary basement. The field was discovered in 1976 by the drilling of Beruk Northeast No. 1, which tested 1680 BOPD from fractured basement metaquartzites. Approximately 2 million barrels of oil have been produced from metaquartzites, weathered argillites and weathered granite. The basement rocks have K/Ar radiometric age dates varying from Early Permian to Early Cretaceous, indicating a complex pre-Tertiary geologic history. The Beruk Northeast field presents unusual production problems due to the great variability in reservoir rocks, the presence of at lea t four

separate oil-water contacts, and possible unrecognized water-bearing fracture systems.

The Tanjung field in the Barito basin, South Kalimantan, was discovered in 1938 and has produced 21 million barrels of oil from the Pre-Tertiary basement rocks. Oil occurs in volcanics, pyroclastics, and metamorphosed sandstones and claystones that are locally deeply weathered and fractured.

Both the Beruk Northeast and Tanjung fields occur within faulted anticlines. The overlying thickness of Tertiary section in both fields is less than 2000 m. The likely oil source rocks for both fields are adjacent Tertiary shales. The Beruk Northeast and Tanjung fields indicate that pre-Tertiary basement is a valid exploration objective in the Tertiary basins of Western Indonesia and that, whenever feasible, all exploration wells in these basins should be drilled into basement.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)