Organic geochemical aspects of aliphatic hydrocarbon fractions for determining coal bed methane exploration strategies of Sawahlunto and Sangatta coals

Sawahlunto (SL-02) and Sangatta (KD-04) coals were analyzed for the presence of aliphatic hydrocarbon fraction biomarkers to assess their organic geochemistry and its implications for selecting exploration strategies for coal bed methane (CBM). Structural identification using the gas chromatography–mass spectrometry (GC–MS) method revealed the distribution of n-alkanes, isoprenoids, sesquiterpenoids, and triterpenoids. The organic matter in both coal samples originated from higher terrigenous plants, as indicated by the dominance of long-chain n-alkanes, carbon preference index (CPI) values greater than 1, low vs. high carbon preference index (LHCPI) values less than 1, terrigenous/aquatic ratio (TAR) values over 5, and the dominance of 8β(H)-homodrimane over 8β(H)-drimane. An oxic depositional environment is suggested by a Pr/Ph ratio greater than 1, the presence of hopanes (C31 and C32), and the low abundance of 8β(H)-drimane in both samples. CPI values greater than 1, OEP values greater than 1, LHCPI values less than 1, the abundance of 17α(H),21β(H)-homohopane (22S) compared with that of 17α(H),21β(H)-homohopane (22R), and the presence of several unsaturated biomarkers in the KD-04 sample, such as olean-12-ene, olean-18-ene, and neohop-13(18)-ene, indicate that the coal is relatively immature. A cross plot of the Pr/n-C17 ratio against the Ph/n-C18 ratio in the Hunt diagram classifies the sample as humic coal derived from higher terrigenous plants, deposited in an oxic environment, and as a type III kerogen with gas-generating potential in both the SL-02 and KD-04 coals. These biomarker data and parameters are highly relevant for methane gas miners in SL-02 coal from the Sawahlunto coal mine, West Sumatra, and KD-04 coal from the Sangatta coal mine, East Kalimantan. The CBM is estimated to have a biogenic origin.