Organic geochemical characteristics and depositional environment of the Tertiary Tanjong Formation coals in the Pinangah area, onshore Sabah, Malaysia

Abstract

The Tertiary Tanjong Formation coals exposed in the west middle block of the Pinangah Coalfield, central part of southern Sabah were analyzed, and their depositional environments were interpreted. The Tertiary Tanjong coals are humic and generally dominated by vitrinite, with significant amounts of liptinite and low amounts of inertinite macerals. Total organic carbon contents (TOC) of the coals range from 51.2 to 77.7wt.%, and yield of bitumen values ranging from 57,300 to 140,000ppm, which meet the standard as a source rock with good hydrocarbon-generative potential. In support, good liquid hydrocarbons generation potential can be expected from the Tanjong coals based on significant liptinitic content (>15%). This is supported by their high hydrogen index up to 300mg HC/g TOC, consistent with Type II and mixed Type II–III kerogens and Py–GC (S2) pyrograms with n-alkane/alkene doublets extending beyond C30. The coal samples have vitrinite reflectance values in the range of 0.42%–0.66 Ro%, indicating immature to early mature stage for hydrocarbon generation. These vitrinite reflectance values also indicated that the Tanjong coals are with sub-bituminous B–A and high volatile bituminous C rank. Tmax values ranging from 419 to 451°C are good agreement with the vitrinite reflectance data. This is supported by biomarker maturity parameters as suggested by the C32 homohopane. The saturated fraction of the Tanjong coals are characterized by dominant odd carbon numbered n-alkanes (n-C23 to n-C33), high Pr/Ph ratios (8–18), high Tm/Ts ratios (6–28), and predominant of regular sterane C29. All biomarker parameters clearly indicate that the organic matter was derived from terrestrial inputs and deposited under oxic condition.