Geochemical characteristics of lacustrine source rock: An insight of Banuwati Formation from Sunda and Asri Basins, Indonesia

Abstract The Sunda and Asri Basins are petroliferous in Indonesia. Despite producing billions of barrels of oil equivalent since their initial production, research on the source rock organic geochemistry of these basins remains limited, particularly concerning the sapropelic kerogen of the syn-rift Banuwati Formation. A comprehensive study of the Banuwati Formation in the Sunda and Asri Basins was conducted using kerogen and biomarker data. Various techniques, such as carbon analysis, Rock-Eval pyrolisis, vitrinite reflectance (Ro), gas chromatography (GC), and gas chromatography-mass spectrometry (GC-MS), were employed on samples from six wells. The Banuwati Formation exhibits source rock with good to very good quantities. Biomarker analysis offers a novel interpretation, suggesting that the Banuwati shale source rock was deposited in an oxic-suboxic environment. The formation is primarily composed of Type I organic material, characterized by the presence of biomarkers such as C26 tricyclic terpane, C30 4-methylsterane, and a possible botryococcane peak, indicative of a lacustrine setting. Maturity assessments based on Ro and Tmax values show conflicting results. Tmax values imply maturity, while Ro values suggest immaturity. This discrepancy is likely due to Ro suppression, a phenomenon often observed in Type I kerogen samples.