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

Highlights

• Hydrocarbon generating potential of the Miocene coal-bearing sequences in Southeast Sabah were investigated.

• The Kapilit and Tanjong Formation have an excellent hydrocarbon potential but Kalabakan shale is low.

• The sediments have broad thermal maturity range of organic matter.

• Upper Miocene sediments act as favourable source rock to generate gas/oil.

Abstract

The study reports a new organic geochemical evaluation for a petroleum source rock in the Southeastern (SE) Sabah basin. The analyses focus on Kalabakan, Tanjong, and Kapilit Formation deposited in the deltaic-shallow marine environment stratigraphically from the Early to Late Miocene sequences. The basin sediments are part of the hydrocarbon prolific Tarakan basin, extending from the northernmost of Kalimantan Indonesia to the southeastern Sabah (mainly onshore area). The hydrocarbon generation of Kapilit and Tanjong Formation pinnacles the Total Organic Carbon (TOC) contents that range from 0.62 to 81.97 wt% and 0.97–92.96 wt% respectively. The dominant black shale in Kalabakan Formation has lower TOC content between 0.50 and 1.82 wt%. The results in the Rock-Eval S₂ of Kapilit (0.31–351.61 mg HC/g rock) and Tanjong Formation (3.32–239.78 mg HC/g rock) have excellent source rock potential compared to the Kalabakan Formation (0.12–1.19 mg C/g rock) with fair to good source rock potential. All three formations give a wide maturity range of vitrinite reflectance (VR_o) and Tmax values, from immature to over-mature stage. Stratigraphically from youngest to oldest Miocene sequences, the Tmax increases with increasing VR_o values. The dispersion of organic matter that requires high maturity and overburden of thick upper succession over Kalabakan Formation reflects a higher maturity stage than the Tanjong and Kapilit Formations. Pyrolysis results in Kapilit, Tanjong, and Kalabakan Formation has HI values to varies between 50 and 498 mg HC/g, 157–367 mg HC/g and 14–110 mg HC/g TOC respectively. The binary plot of HI versus Tmax supported by Py-GC pyrolysis marks that the Kapilit source rock is dominated by Type II-III kerogen, potentially generating immature to mature oil and gas prone. Tanjong Formation portrays a mature to late mature stage of Type III kerogen with a minor Type II kerogen capable of producing mixing of oil and gas prone, with mainly gas. The Kalabakan Formation shows a post-mature Type III and IV kerogen that generates gas or barren source rock. In terms of bulk kinetic evaluation, both Kapilit and Tanjong Formation are derived from heterogeneous organic matter. The projected petroleum temperature varies substantially due to the compositional variation. The onset of bulk petroleum generation in the Kapilit and Tanjong Formation occur at 112 °C - 128 °C and 127 °C - 142 °C respectively. The results are consistent with the maturity generated where the Tanjong has higher thermal maturity than Kapilit Formation due to stable aromatic Type III kerogen. Generally, all samples show great variability in thermal maturity and hydrocarbon generation potential, which that be influenced by the tectonism and development of sediments in the basin. The Early Miocene source rock yields post mature gas prone and continues to generate hydrocarbon that consists of mature oil and gas prone from younger source rocks.

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.