The coal-sourced petroleum system in the Barito Basin (S Borneo): Factors controlling quantity and quality of oil from Eocene and Miocene coals

Abstract

The Barito Basin in southern Borneo contains large Eocene and Miocene coal deposits as well as a minor petroleum system. Previous authors have distinguished two oil families, termed Tanjung and Warukin. Coal has been discussed as the source for both, but there is no consensus on the lithology of the source rock of the Tanjung family.

Various geochemical parameters including biomarkers, the composition of pyrolysates and carbon isotope ratios were used for oil-source correlation. Biomarker fingerprints (pristane/phytane ratio, abundance of non-hopanoid triterpanes and bicadinanes, predominance of C₂₉ homologues in sterane distribution) and the clear dominance of dibenzofurans over fluorenes and dibenzothiophenes in oils clearly indicated their origin from coal. The excellent agreement between the oil and coal parameters confirms a relation between Tanjung oil and Eocene coals and between Warukin oil and Miocene coals.

The factors controlling the oil potential of marginal mature Eocene coals (0.52–0.59 %Rr) and immature Miocene coals (≤0.40 %Rr) in the Barito Basin were investigated using Rock-Eval pyrolysis of solvent-extracted coals and petrographic and biomarker data that have recently become available in papers investigating the depositional environment of these coals. The study revealed that all coals contain significant amounts of heavy, oxygen-rich bitumen components, which form part of the Rock-Eval S2 peak (S2_bitumen) and showed that the type of oil generated from Eocene (high wax paraffinic oil) and Miocene coals (paraffinic-naphthenic-aromatic mixed oil with moderate wax content) differs significantly. It is concluded that the S2_bitumen is at least partly derived from resins and that the generated oil type is controlled by the prevailing resin producing vegetation, which is palms in Eocene coals and Dipterocarpaceae in Miocene coals. Other liptinite macerals and the mire type do not appear to play a significant role.