Biomarker analysis of Air Laya coal (AL71) Muara Enim Formation, South Sumatra Basin as a potential coalbed methane (CBM) in Indonesia

Coal samples collected from the Air Laya Mine, Muara Enim Formation, and South Sumatra Basin were analyzed for their organic geochemical characteristics using extraction, fractionation, and compound identification methodsusing gas chromatography-mass spectrometer (GC-MS) instruments. This analysis aims to determine the implications of organic geochemistry such as the origin of organic matter, level of thermal maturity, and ancient depositional environment. The identification of aliphatic hydrocarbon fraction obtained homologous n-alkanes (n-C13-n-C33), branched alkanes pristane and phytane, and pentacyclic triterpane. The abundance of n-alkane compounds with a bimodal distribution type indicates that coal organic matter comes from various sources such as bacteria, algae, macrophytes, aquatic environments, and higher plants. Some parameters such as CPI >1; ACL >27; OEP >1; and the abundance of olean-18-ene, olean-12-ene, urs-12-ene, and C31aß 22S/ (22S+22R) <1 indicated low maturity and a source of terrestrial higher plant organic matter. The value of the ratio n-C31/n-C19 <0.4; n-C29/n- C17 <1; n-C17/n-C31 >1; LHCPI >1; and the abundance of neohop-13(18)-ene, 17ß(H),21a(H)-hopane, 17a(H)-21ß(H)-homohopane (22R), and 17a(H)-21ß(H)-homohopane (22S) indicates the presence of bacterial input in the coal depositional process. The value of the Pr/Ph ratio >3 indicates that the AL-71 coal sample was deposited in an oxic environment. AL-71 coal is included in type III kerogen which indicates the potential of the source rock as a gas-prone and potential to be used as a source of coalbed methane (CBM).