Depositional History and Petroleum Potential of Ombilin Basin, West Sumatra - Indonesia, Based on Surface Geological Data

Zaim, Yahdi¹; Habrianta, Litto²; Abdullah, Chalid I.¹; Aswan, Aswan¹; Rizal, Yan¹; Basuki, Nurcahyo I.¹; Sitorus, Franky E.²
¹Department of Geology, Institut Teknologi Bandung (ITB), Bandung, Indonesia.
²Department of E&P, PT Radiant Bukit Barisan, Jakarta, Indonesia.

The Ombilin Basin is situated in the Southwest Bukit Barisan Block, West Sumatera Province. The main objective of the study is to understand historical basin formation and sedimentation, including petroleum potential, based on detailed surface geological survey. The survey reveals that sedimentation in this basin was started soon after the rifting of the basin due to regional Paleogene tectonic event. Graben and half graben basin systems occurred in West Sumatra, including the Ombilin Basin, influenced sedimentation processes in the basin.

The initial stage of rift basin evolution marked by sedimentation of the Brani and Sangkarewang Fms. took place during Eocene to Middle Oligocenen deposited in alluvial fan and in lacustrine environment in the lake basin. During syn-rift development in the Oligocene, the Ombilin Basin became shallower and changed from lake into fluvial basin such as meanders, flood plains and marshes of the river. Simultaneously, there was marine ingression into the basin and formed delta at the river mouth where sedimentation of the Sawahlunto Fm. took place. Further syn-rift development has uplifted the basin; it became shallower and changed into low land basin in form of braided river systems where Sawahtambang Fm. sedimentation took place in Late Oligocene. The Ombilin Fm. was deposited during post-rift development in Early Miocene when the basin changed into fully marine environment due to global sea level rise as well as tectonic activities at the end of Oligocene that caused the basin to be entirely covered by marine ingression to overlie unconformably above the Sawahtambang Fm.

Geochemical analyses on thirty shale samples from Tertiary sediments show that the source rock potentials in the Ombilin Basin are mainly from the Sangkarewang Sawahlunto and Ombilin Fms.

Reservoir potentials in the Ombilin Basin are from the Brani, Sawahlunto and Sawahtambang Fms. as well as the pre-Tertiary basement rocks. The Pre-Tertiary rocks are highly fractured due to Tertiary tectonic deformations, and can be as fracture reservoir. The Brani, Sawahlunto and Sawahtambang Fms. are suggested as the main priority for reservoir rocks. SEM examination on samples from these formations indicates the existence of microporosity, which is very important properties for reservoir.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012