The geochemical composition of thirty two (32) oil samples in the Central Sumatra Basin (CSB) were determined by gas chromatography (GC), gas chromatography coupled with mass spectrometry (GC-MS) and gas chromatography isotope ratio mass spectrometry (GC IR-MS). The carefully selected sample set is representatively distributed among the oils producing grabens in the CSB: Aman, Balam, Tanjung Medan, Kiri and Bengkalis Troughs. The oils are derived from the lacustrine Brown Shale Formation of Eocene - Oligocene age.

The main objective of this study was to perform a detailed geochemical characterization of the sampled oils in order to understand the observed chemical variability within the CSB. Moreover, it aimed at interpreting changes in depositional environments within the source rock that generated these oils.

Combination of molecular and carbon isotopic composition revealed that five source rock facies developed in the studied grabens: (1) deep lacustrine, (2) shallow lacustrine, (3) saline lacustrine, (4) coal, and (5) mixed coal/lacustrine shale. The geographic distribution of the samples indicates that deep stratified lakes developed in the Aman and Bengkalis Troughs. Oils in the Kiri graben are predominantly derived from a shallow lacustrine facies while the oils south of the Aman and Kiri grabens display a more saline lacustrine facies. Coal derived oils and oils derived from a mixed coal/lacustrine shale facies seem to be restricted to the northern part of the Central Sumatra Basin.

A detailed look at the geochemical composition of the oils in the Aman Trough allowed identification of paleoproductivity and paleoclimatic changes that took place during source rock deposition. Isotopic data indicate CO2 limiting conditions prevailed during source rock deposition and that changes occured in paleoclimatic conditions, possibly associated with the Eocene-Oligocene paleoclimatic transition. These changes in depositional conditions had an effect on the geochemical composition of the source rocks in the Aman Trough.

A new sesquiterpane biomarker compound was identified in the oils from the Aman Trough and was tentatively identified as a C15 sesquiterpane.