Hydrocarbon Generation Potential of Oligocene Oil Shale Deposit at Onshore Penyu Basin, Chenor, Pahang, Malaysia

This is the first study that focuses on the evaluation of a newly discovered oil shale deposit in the eastern Chenor area in the state of Pahang, Malaysia. Previously, this deposit was reported as carbonaceous/coaly mudstone. However, in this study, organic-rich immature carbonaceous mudstone containing above 15 wt % total organic carbon (TOC) is evaluated as oil shale, and that below 3.5 wt % TOC is termed mudstone. Oil shale and mudstone, which are significant sedimentary facies for oil and gas exploration, were investigated using organic geochemical and petrological methods, as well as computed tomography (Micro-CT), pyrolysis, and bulk kinetic techniques, to evaluate their hydrocarbon generation potential. The vitrinite reflectance values are less than 0.5% R-o in all of the analyzed samples, indicating low maturity stage. This is corroborated by T-max values ranging from 383 to 429 degrees C. However, based on the kinetic simulation model, the average predicted geological temperature for the onset of hydrocarbon generation is 109 degrees C, while the peak of hydrocarbon generation is 153 degrees C. The extractable organic matter and hydrocarbon contents results show that the oil shale samples possess excellent petroleum potential compared to very good values for the studied mudstone. This is consistent with the plots of TOC content versus extractable organic matter, and hydrocarbon yields versus TOC content, commonly used in estimating the hydrocarbon generative potential of source rocks. The TOC of these oil shales is consistent with that of the Tertiary oil shale deposits in China. The analyzed oil shale samples are characterized by a high hydrogen index (HI) of up to 700 mg HC/g TOC (average 517.4 mg HC/g TOC), suggesting oil prone type I and type II kerogens. However, the open pyrolysis-gas chromatography (Py-GC) result of these samples displays a predominance of n-alkene/n-alkane doublets extending to the long-range homologous series (C7-C33) with considerably high aromatic compounds, which indicates type II/III kerogens' (mixed oil and gas) generative potential. Similarly, bulk kinetic analysis of the analyzed samples suggests typical petroleum source rock facies derived from heterogeneous (type II/III kerogens) organic matter assemblages. Thus, based on the pyrolysis and bulk kinetic result, the studied oil shale can be classified as 'cannel coal' derived from terrestrial plants materials. This is supported by organic petrological and Micro-CT studies which revealed that the samples are composed mainly of liptinite (sporinite, cutinite, amorphous organic matter, and resinite) and vitrinite, with varied trace amounts of inertinite.