Organic geochemical and petrographic characteristics of Tertiary coals in the northwest Sarawak, Malaysia: Implications for palaeoenvironmental conditions and hydrocarbon generation potential

• The Tertiary Mukah and Balingian coals are ranking from lignite to sub-bituminous C coal.
• They were deposited in a lower deltaic plain wet peat-swamp depositional setting.
• They are rich in huminite, low to moderate in liptinite, and low in inertinite.
• A mean random huminite reflectance in the range 0.26%–0.39% is determined.

Tertiary coals from Mukah and Balingian coalfields in the northwest Sarawak, Malaysia were investigated to evaluate their regional rank variation, petroleum generative potential and to reconstruct the palaeoenvironment conditions during peat accumulation. The Tertiary coals are characterized by high total organic carbon contents (TOC) and yield of bitumen extraction ranging from 31.3 to 55.9 wt. % and 25,724.9-92,143.7 ppm, respectively, meet the standard as a good source rock potential. The Mukah and Balingian coals were generally plotted in an area of Type III kerogen and mixed Type II/III kerogens with HI values between 90 and 289 mg HC/g TOC, whereby the coals were derived from plant materials of terrigenous origin. This shows that the Balingian coals are dominated by Type III terrigenous kerogen while Mukah coals are dominated by Type II/III kerogens, and are thus considered to be generate mainly gas-prone and limited oil-prone. This is also supported by macerals composition and open system pyrolysis gas chromatography (Py-GC). The Mukah and Balingian coals are thermally immature in rang from lignite to sub-bituminous C rank, possessing huminite reflectance in the range of 0.26%–0.39%. This immaturity has a considerable influence on the proximate analysis, particularly on relatively high moisture and volatile matter contents and relatively low fixed carbon content.Petrographically, it was observed that the Mukah and Balingian coals are dominated by huminite, with low to high amounts of liptinite and relatively low amounts of inertinite, pointing to predominantly anaerobic deposition conditions in the paleomires, with limited thermal and oxidative tissue destruction. The palaeoenvironment conditions of the coals are generally interpreted as a lower deltaic plain wet peat-swamp depositional setting, which are generally characterised by low TPI and high GI values, and are plotted on the marsh field of the Diessel's diagram. This is usually consistent with generate relatively high ash yield as is the case of the Mukah and Balingian coals.