Depositional Controls on Source Rock Character in the Aman Trough, North Central Sumatra, Indonesia

Rodriguez, Norelis D.¹; Philp, Richard P.²
¹Energy Technology Company, Chevron, Houston, TX.
²Conoco Phillips School of Geology and Geophysics, The University of Oklahoma, Norman, OK.

The Central Sumatra Basin (CSB) is a prolific oil producing basin in Indonesia where a single source rock has been identified, The Brown Shale Formation of the Paleogene Pematang Group. Despite the presence of a single source rock, the oils in the CSB and particularly in the Aman Trough, exhibit variable molecular and isotopic compositions, previously described as reflective of lateral facies variations within the source rock. Understanding these facies changes and how they are expressed in the geochemical composition of the oils is key during the hydrocarbon evaluation of a prospective area. Potential source rock intervals can be predicted through inversion of the geochemical composition of oil samples, which becomes more significant when rock samples are not available for direct geochemical analyses.

Molecular and isotopic data from 15 oil samples from the Aman Trough allowed identification of both vertical and lateral facies changes within the Brown Shale Formation as well as paleoproductivity and paleoclimatic changes that took place during source rock deposition. Biomarker data indicate the Brown Shale Formation was deposited in a fresh to brackish water stratified lake with mixed contributions from algal and terrigenous organic matter. Isotopic data indicate CO2 limiting conditions prevailed during source rock deposition and changes in paleoclimatic conditions, possibly associated with the Eocene-Oligocene paleoclimatic transition, had an effect on the geochemical composition of the source rock in the Aman Trough.

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