Organic petrographic characteristics of Tertiary (Oligocene–Miocene) coals from eastern Malaysia: Rank and evidence for petroleum generation

Abstract

Oligocene to Miocene coals from eastern Malaysia were analysed to evaluate their regional ranks, and petroleum generative potential. The current study performs organic geochemical characteristics of the coals and identifies macerals based on their organic petrographic characteristics as observed under reflected white light and blue light excitations. The coals are characterised by relatively high hydrogen index values between 282 and 516mg HC/g TOC. This indicates that these coals are dominated by Type II to mixed Type II–III kerogens, and are thus considered to be generated mainly oil-prone and limited gas-prone. This is supported by the presence of significant amounts (11–31% by volume) of oil-prone liptinite macerals. Suberinite is among the most common of the oil-prone liptinite macerals in coals. These coals likely are to be potential petroleum sources, thus, where the former is abundant, waxy oils and naphthenic oil might be expected. This has been supported by the distribution of n-alkyl chains within kerogen pyrolysates, predicting the generation of a mainly paraffinic and paraffinic–naphthenic–aromatic (P–N–A) high wax oils. The coals are thermally early-mature and coal rank in the region is subbituminous A to high-volatile bituminous C, possessing vitrinite reflectance in the range of 0.50%–0.67%. This maturity has a considerable influence on the proximate analysis, particularly on a relatively low moisture content and relatively high fixed carbon and low volatile matter contents. Although these onshore coals are thermally early mature for petroleum generation, the stratigraphic equivalent of these sediments offshore is known to have been buried to deeper depth and could therefore act as potential source rock for mainly oil with minor amounts of gas.