Evaluating the hydrocarbon potential of source rocks in Northern Borneo, Southeast Asia: a review of organic geochemical and petrological characteristics

Abstract

This study provides a comprehensive review of hydrocarbon source rocks in Northern Borneo by integrating findings from previous organic geochemical and petrological studies across the Sabah, Sarawak, and Brunei basins. Unlike earlier basin-specific research, this work offers a broader regional perspective, evaluating sixteen (16) formations ranging from Late Cretaceous to Pleistocene. The majority of the samples exhibit good to excellent organic richness (mean total organic carbon (TOC) content exceeding 1 wt%, with promising S₂ and S₂/S₃ ratios). The hydrogen index (HI) values (7.1 and 401.8 mg HC/g TOC) and HI vs. Oxygen Index (OI) plots indicate that the organic matter is primarily composed of Type II/III, Type III and Type IV kerogen. Remarkably, coal-related samples consistently show excellent organic richness, predominantly plotting within the Type II/III kerogen area, indicating a strong potential for mixed oil and gas generation. Other lithologies primarily contain gas-prone Type III kerogen, while Type IV is largely inert. The thermal maturity assessments, based on T_max (324–570 °C) and production index (PI) (0.00–0.37), indicate a wide range from immature to overmature source rocks. Vitrinite reflectance (VR_o) measurements (0.15–3.10 %) and T_max vs VR_o plots further confirm these maturity trends. The organic petrological analysis reveals that the vitrinite macerals dominate the liptinite and inertinite across all the formations, with coal-related source rocks exhibiting the highest maceral concentrations, reinforcing their generative capacity. Specifically, the coal-rich Nyalau, Tanjong, Liang, Batu Arang, Balingian, and Bergih formations stand out as highly promising, presenting substantial economic significance for unconventional plays such as coalbed methane (CBM). This review highlights the significant hydrocarbon potential across Northern Borneo, while also emphasizing the need for further assessment of the Brunei Basin to refine the current understanding of source rock distribution and quality within the regional petroleum system.