Organic geochemical characteristics and depositional setting of Paleogene oil shale, mudstone and sandstone from onshore Penyu Basin, Chenor, Pahang, Malaysia

Abstract

The eastern Chenor Pahang area of the Penyu Basin contains an untapped potential, in part due to poor knowledge of its organic facies characteristics The sedimentary succession of eastern Chenor Pahang area is dominated by fine to coarse grained sandstone occurring in association with oil shale and mudstone in a predominantly fining upward sequence. Detailed geochemical and petrographic characterization was carried out on oil shale and mudstone samples to evaluate the origin and type of organic matter (OM), as well as the paleoenvironmental conditions and organic richness of the Paleogene sedimentary succession. The facies are only enriched in total organic carbon (TOC), potential hydrocarbon yield (S2) and extractible organic matter (EOM) the types of organic matter are important index reflecting the potential of the source rock(s). Based on elemental ratios, the analyzed samples are shown to contain Types I and II kerogens. This finding is supported by the high hydrogen index (HI) of >300 mg HC/g TOC and the presence of abundant liptinitic materials. Biomarker signatures [n-alkanes >n-C25, high C29/C30 17α (H) hopane ratios (0.8–3.3) and high C29 steranes (49–78%)]. Low total sulfur and V/(V + Ni) ratio] suggest a predominantly terrestrial source input with some contribution from aquatic microorganisms as indicated by the common occurrence of amorphous organic matter. Combining this results with n-alkane patterns, kerogen types and organic petrography, predominance of C29 sterane probably indicates a contribution of aquatic organisms such as green and brown algae (consistent with Botrycoccus braunii race A and Pediastrum contribution) and some influence of higher plants OM. Integration of biomarker studies with bulk geochemical and petrographic analysis suggests that the analyzed samples were deposited under suboxic conditions in a fresh water lacustrine environment. This finding is corroborated by the ratios of pristane/phytane, V/Ni and Sr/Br. High bioproductivity and a stratified fresh water column with suboxic bottom water condition enhanced OM preservation in the analyzed mudstone and shale facies. The high OM content in these samples maybe related to the fine-grained matrix of the mud rocks and the paleoclimate in the area.