Basin modelling and bulk kinetics of heterogeneous organic-rich Nyalau Formation sediments of the Sarawak Basin, Malaysia

Abstract The investigation of the Nyalau Formation and its offshore equivalents (Cycles I and I) using 1D-basin modeling was carried to evaluate the timing and maturity aspects of their petroleum system. The Nyalau Formation and its offshore equivalents show a very remarkable source rock sequence for oil and gas in the Sarawak Basin. About twenty typical samples of the Nyalau Formation, with its maturity range beginning at immature to early-mature were evaluated, adopting bulk and quantitative pyrolysis procedures to reveal the kerogen characteristics and define the inherent optical properties of the organic matter and the effect it has on petroleum generation. Pyrolysis data identified two organic facies that indicate petroleum generation tendencies; Kerogen Type II/III having fairly high values of hydrogen index (HI) above 200 mg HC/g TOC whilst the kerogen Type III is comparatively with low HI values below 200 mg HC/g TOC. The aforementioned is reinforced with respect to distribution appertaining to energy activation a derivative of the bulk kinetic analysis. This study in a wide distribution of energy activation of kcal/mole to 69 kcal/mol which is unswervingly correlated heterogeneously to the organic matter. Also, the bulk kinetic analysis shows substantial variation in petroleum formation tract with regards to onset as well as the Nyalau's source rocks samples peak generation temperatures. At the temperatures between 121 and 131 °C petroleum generation for kerogen Type II/III commenced while at the temperatures of 134–145 °C for kerogen Type III. There is a variation of the peak generation temperatures (geological temperature maximum) in the range of 135–142 °C for Type II/III kerogens, whilst 153–167 °C for kerogen Type III. A notable transformation ratio of about 33% is reached as a result of these peak generation temperatures. The results were then integrated into the basin modelling, thereby predicting the timing, generation, and expulsion of hydrocarbons from the Nyalau equivalent source rocks in the offshore basin. This models suggests the onset of petroleum generation from Nyalau source rocks began in the Early Miocene (23-16 Ma), and the generation peaked at the Middle Miocene (18-12 Ma). The evolution and the expulsion of hydrocarbon as modeled indicate the timing of the expelled petroleum commenced in Pliocene and continued to present-day (5–0.0Ma). One dimensional basin models and bulk kinetics provide a quantitative way of predicting the timing, generation, and expulsion of hydrocarbon from Nyalau Formation within the Sarawak Basin.