Liquid hydrocarbon generation potential from Tertiary Nyalau Formation coals in the onshore Sarawak, Eastern Malaysia

Tertiary coals exposed in the north-central part of onshore Sarawak are evaluated, and their depositional environments are interpreted. Total organic carbon contents (TOC) of the coals range from 58.1 to 80.9 wt. % and yield hydrogen index values ranging from 282 to 510 mg HC/g TOC with low oxygen index values, consistent with Type II and mixed Type II–III kerogens. The coal samples have vitrinite reflectance values in the range of 0.47–0.67 Ro %, indicating immature to early mature (initial oil window). Tmax values range from 428 to 436 C, which are good in agreement with vitrinite reflectance data. The Tertiary coals are humic and generally dominated by vitrinite, with significant amounts of liptinite and low amounts of inertinite macerals. Good liquid hydrocarbons generation potential can be expected from the coals with rich liptinitic content ([35 %). This is supported by their high hydrogen index of up to 300 mg HC/g TOC and Py-GC (S2) pyrograms with nalkane/alkene doublets extending beyond C30. The Tertiary coals are characterised by dominant odd carbon numbered n-alkanes (n-C23 to n-C33), high Pr/Ph ratio (6–8), high Tm/ Ts ratio (8–16), and predominant regular sterane C29. All biomarkers parameters clearly indicate that the organic matter was derived from terrestrial inputs and the deposited under oxic condition.