Geochemical characteristics of a tropical lowland peat dome in the Kota Samarahan-Asajaya area, West Sarawak, Malaysia

Augered tropical lowland peat samples (KS.TP.02 to 10) were characterized, classified (von post) and subjected to source rock analyses and biomarker gas chromatography mass spectrometry analyses. Lateral variations in the organic matter types within the top 0–0.5 m layer were found to occur from the margin to the mid-section to the center/near-center (i.e., thicker areas) of the studied peat dome. The organic matter types of these three regions correspond to kerogen types II, III, and II, respectively. The lateral vegetation succession, phasic community zonation and organic matter types (II and III kerogen) are most likely associated with tropical lowland peat domes. The Pr/n-C₁₇ and Ph/n-C₁₈ ratios for the peat samples from the same locations are low and indicate anoxic to suboxic and reducing depositional environmental conditions for the wet tropical lowland peats. The S/(S+R) ratios are relatively low, indicating the immaturity of the peats in terms of hydrocarbon production. The dominant biomarker hopane compounds have 13–39 carbon atoms, with dominant peaks exhibiting an odd over even carbon number predominance, indicating the terrestrial depositional environment of the peats. The Pr/Ph ratios most likely indicate anoxic (the marginal, relatively wet “moat area”) to suboxic (the mid-section and the basin center) depositional environmental conditions. The ββ hopanes biomarkers that indicate immaturity include ββ C30 hopane (17β, 21β (H)-Hopane) and ββ C31 hopane (17β, 21β (H)-Homohopane). Other common hopanes present are C29 hopane (Norhopane), 22Sαβ C31 hopane (17α, 21β (H)-Homohopane) and 22Rαβ C31 hopane (17α, 21β (H)-Homohopane).