Source rock pyrolysis and bulk kinetic modelling of Miocene sedimentary sequences in southeastern Sabah, Malaysia: The variability of thermal maturity to oil-gas producing kerogen
Introduction
The distribution of significant oil and gas resources in Tertiary basin are known to be effective source rocks of Southeast Asia region (e.g. Beddoes, 1981; Madon and Abolins, 1999; Abdullah, 2003; Alias et al., 2012; Abdullah et al., 2017b; Doust, 2017; Adepehin et al., 2018; Burhannudinnur and Kurnianto, 2021). Over the years, sedimentary basins in Malaysia have proven to be petroliferous and commercially produced, especially in the Malay, Sarawak, and Sabah basins. Nevertheless, the SE Sabah basin is incompletely explored, and none of the hydrocarbon production is active in the eastern area. Previous exploration conducted by several companies in 1960 shows that several wells were drilled offshore of SE Sabah to increase the national oil production (Petronas, 1999). However, most of these wells are reported as dry wells. The enigmatic and complex structure gives a challenge on basin and stratigraphic interpretation due to the influence of active tectonics since the Mesozoic era, where Sabah is lying adjacent to actively moving plates in the Southeast (SE) Asian region (Van Hattum, 2006).
The source rock played a major role in controlling petroleum accumulation, and a comprehensive study is needed to minimize the risks of hydrocarbon exploration (Wang et al., 2019, 2020). Some petroleum evidence will indicate a presence of a petroleum system in the target area like existing oil and gas seep to the surface or in a well. The reinforcement of this research is based on the hydrocarbon-bearing in the Tarakan basin that northern part, known as the Tidung sub-basin, is extended to the SE Sabah basin. The Tarakan basin has proven as one of the main hydrocarbon productions in the Kalimantan state. Yet, insufficient hydrocarbon exploration to deduce the commercial accumulation of hydrocarbon in the SE Sabah basin.
This research paper focused on evaluating the source rock using a geochemical and kinetic approach to distinguish the hydrocarbon generation potential and thermal maturity of three different types of Miocene sedimentary sequences. The unexplored source rocks comprised Kalabakan, Tanjong and Kapilit Formation (Fig. 1) covered along Kalabakan-Silimpopon road, Kalabakan Village, and near Maliau Basin Conservation Area (MBCA). The recent discovery of the onshore Cenozoic sediments in the southern part of Sabah mainly focused on Tanjong Formation that covered in Pinangah and Imbak Canyon area (Zulkifli et al., 2008; Mustapha et al., 2011; Alias et al., 2012). Therefore, this research provided a new data finding of undiscovered hydrocarbon prospects in the SE Sabah basin and, at the same time to amend outdated framework from the previous exploration.
Section snippets
General geology
In northern Borneo, Sabah state is flanked by three marginal basins: the Sulu Sea to the north-east, the Celebes Sea to the south-east, and the South China Sea to the west. The general geology of Sabah is still debated related to the different proposed tectonic models that implied a different interpretations mechanism (Hutchison, 1996; Balaguru et al., 2003; Hall, 2013; Lunt and Madon, 2017). The Proto-South China Sea block drifting to the NW Borneo-SW Philippines in the pre-Neogene period
Stratigraphy and petroleum system
Based on the general stratigraphy modified after Hall (2013), the three Miocene sedimentary rocks rest un-conformably upon Labang and Kuamut mélange Formation that occurring in circular sub-basin of SE Sabah (Fig. 2). Further deposition overlying them were Simengaris, Umas-Umas and Balung Formations in Early Pliocene age.
The Kalabakan Formation predominantly consists of thick-bedded grey shale-mudstone with thin siltstone layer and heterolithic sandstone that established to be distal equivalent
Samples and methods
All thirty rock samples were collected from non-weathered exposed formations around the study area in the SE Sabah region. Further organic geochemical analyses were conducted to investigate the characteristic of the Miocene source rock and its hydrocarbon generation potential.
Results and discussion
Parameters from pyrolysis SRA used for the interpretation of hydrocarbon in this study are shown in Table 1. The perspective for hydrocarbon generating potential is identified depending on several aspects: the richness of hydrocarbon, the type of organic matter and the thermal maturity of the kerogen.
Conclusion
The organic geochemical and bulk kinetic studies in the Miocene Formations of SE Sabah basin show significant variability in thermal maturity and hydrocarbon generation potential. The deltaic-shallow marine environment of Kapilit and Tanjong Formation, especially coals have an excellent hydrocarbon generation potential of organic rich coal-bearing sediments as contributed by high TOC content (43.14–92.96 wt%) and high concentration of S2 (84.24–351.61 mg HC/g rock). The shaly facies including
Credit author statement
Nur Amalina Ahmad Thana’Ani; Conception and design of study, acquisition of data, analysis and/or interpretation of data, Drafting the manuscript. Khairul Azlan Mustapha; Conception and design of study, acquisition of data, analysis and/or interpretation of data, revising the manuscript critically for important intellectual content. Mohammed Idris; acquisition of data.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This research is part of the first author Master's degree project. University of Malaya is acknowledged for providing fund to support this research under special university internal grant (grant number BK085-2016). The author gratefully acknowledges all reviewers for their constructive scientific reviews and comments.
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