RIFT PLAY IN OMBILIN BASIN OUTCROP, WEST SUMATRA

摘要

1. Structurally and tectonically, the Ombilin basin is a rift basin in half graben form. 2. Its sedimentations followed the basin evolution phases of: synrift and postrift. 3. The petroleum system of the basin is evidently effective. The early synrift sediments of Sangkarewang lacustrine shale serve as the best source rock. It is organically rich and slightly mature to mature. The postrift Ombilin shallow marine mudstone serves as an effective cap-rock. 4. The potential reservoirs are: weathered / washed granite of Lassi and Tigotumpuk area (prerift); turbiditic quartz feldspathic sandstone, conglomeratic delta fan quartz sandstone, alluvial fan conglomerate and conglomeratic sandstone, and amalgamated braid plain conglomeratic sandstones, all of Sangkarewang formation. (early synrift); amalgamated meandering quartz sandstone of Sawahlunto formation and amalgamated braid plain coarse to conglomeratic quartz sandstone of Kulampi member of Sawahlunto formation (late synrift); and massive sequence of cross bedded, fining upward quartzose to feldspathic sandstone of Sawahtambang formation (postrift). Their position and location controls the quality of the reservoir i.e. porosity preservation. 5. The Ombilin basin, with all surface / outcrop data might serve as good model for a rift related proven petroleum system.