Source rock facies distribution predicted from oil geochemistry in the Central Sumatra Basin, Indonesia

The molecular and compound-specific isotopic composition of the n-alkanes in 30 oil samples from the Central Sumatra Basin were analyzed with the purpose of determining facies variations of the source rock in this basin. The oil samples were generated by the Eocene-Oligocene lacustrine Brown Shale Formation. Biomarkers in the oil samples indicate varying contributions from algal and terrigenous organic matter, primarily under oxic, or suboxic depositional conditions. Five source rock facies were identified based on the relative concentrations of n-alkanes, isoprenoids, tricyclic terpanes, and steranes. The individual facies were also distinguished by the compound-specific isotopic composition of the n-alkanes. Source rock depositional environments vary from deep lacustrine, shallow lacustrine, and lacustrine saline to coal and mixed coal and lacustrine shale. Deep lacustrine facies predominate in the Aman and Bengkalis troughs whereas saline and shallow lacustrine facies predominate in the South Aman and Kiri troughs, respectively. Oils derived from coal and mixed coal and lacustrine shale facies predominate in the northern part of the Central Sumatra Basin, in the Tanjung Medan and north of the Balam grabens.