ABSTRACT: Exploration in the Ombilin intermontane basin, West Sumatra

Koning, Tako¹ and Karsani Aulia²
(1) Texaco Angola Inc, Luanda, Angola
(2) PT Caltex Pacific Indonesia, Minas, Indonesia

The Ombilin Basin is an intermontane Tertiary depocenter located within the Barisan mountain range of West Sumatra. Geological and geophysical surveys conducted by Caltex in 1981 marked the beginning of petroleum exploration activity within this basin and led to the drilling of the Sinamar-1 well in 1984. Sinamar-1 reached a total depth of 3018 m, becoming the first exploratory well to be drilled within an Indonesian intermontane basin. The well proved to be a noncommercial oil, gas, and condensate discovery. A second cycle of exploration in the Ombilin Basin occurred in 1994 with the drilling by Apache of South Sinamar-1, an 1140 m dry hole.

Seismic, outcrop and well data indicate that despite the small areal size of the Ombilin Basin (1500 sq km), up to 4600 m of Tertiary sediments fill the basin. The basin was initially an Early Tertiary intermontane trough with deposition of massive debris flows and sand-rich alluvial fans on the basin margins, while a large Eocene lake occupied the basin center. Uplift and erosion since the Middle Miocene has reduced the Ombilin Basin's original larger extent to its present size. Although this basin is located within Sumatra's present-day magmatic arc and is partially covered by volcanics, subsurface temperature gradients are significantly cooler compared to those measured in the Sumatra back-arc basins. Eocene lacustrine shales are the likely source rocks for hydrocarbons tested in Sinamar-1 and probably sourced the two known oil seeps located along the basin margin.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia