THE PETROLEUM POTENTIAL OF TERTIARY COALS FROM WESTERN INDONESIA: RELATIONSHIP TO MIRE TYPE AND SEQUENCE STRATIGRAPHIC SETTING

Abstract

More than 500 deltaic sediments from 14 basins in Western Indonesia were subjected to detailed geochemical, petrographic and palynological analysis. The resultant data represent each of the main peat-forming tectonostratigraphic episodes (Paleogene syn-rift transgressive, Paleogene–Neogene post-rift transgressive and Neogene syn-tectonic regressive) of the Indonesian Tertiary. The coals are hydrogen-rich, with average hydrogen index (HI) of 250–300 and atomic H/C ratios of 1.0–1.3 where immature. Paleogene coals from syn-rift transgressive systems tracts are more hydrogen-rich than Neogene coals from syn-tectonic regressive systems tracts, and upon pyrolysis, yield lower gas/oil ratios, implying they are more oil-prone. Pliocene coals from a strongly regressive sequence in the Sumatran fore-arc are very hydrogen-poor with average HI of 138. Indonesian coals display a systematic increase in HI with increasing rank, peaking at a Tmax of c. 430 °C, suggesting that Rock-Eval pyrolysis underestimates petroleum potential in low rank coals. The increase in HI is accompanied by a decrease in S1/TOC ratio and the disappearance of suberinite and resinite, reflecting the assimilation of these macerals into the coal matrix with increasing diagenesis. Petrographic analyses reveal a very poor correlation between liptinite content and HI. The term Detrovitrinite Index (DI) is coined to describe the ratio of detrovitrinite/total vitrinite. A better relationship between HI and DI suggests that vitrinite type is a more important control on petroleum potential than liptinite content.