Effect of Sodium Carbonate on Kerogen Pyrolysis Behavior and Products: Insight from Thermal Simulation Experiment

Abstract

The organic–inorganic interactions during the thermal pyrolysis of organic matter significantly influence hydrocarbon generation in shales, but research on the controlling mechanisms of alkaline minerals on kerogen pyrolysis is relatively scarce, which limits the understanding of hydrocarbon generation mechanisms in alkaline lacustrine basins. This study focuses on natural alkaline mineral (sodium carbonate), kerogen, and whole-rock samples from the Fengcheng Formation shale in the Mahu Sag of the Junggar Basin, China, investigating the effects of alkaline minerals on the kerogen pyrolysis behavior and products. The results indicate that alkaline minerals enhance the total oil and gas yields from kerogen, significantly increasing heavy hydrocarbon and wet gas yields, whereas whole-rock minerals reduce total oil and heavy hydrocarbon yields while increasing total gas and wet gas yields. Specially, our observation can be shown as followings: (1) Alkaline minerals inhibit kerogen condensation and promote the neutralization of organic acids, reducing hydrogen (H₂) yield and increasing carbon dioxide (CO₂) yield; (2) Throughout the thermal evolution stages, alkali-containing kerogen lowers the activation energy for wet gas generation and enhances wet gas yield, while alkaline minerals catalyze wet gas decomposition into methane, thereby increasing methane yield under high temperatures; (3) Alkaline minerals increase the polarity of C–C bonds, lowering the activation energy for light hydrocarbon generation and significantly boosting intermediate hydrocarbon production above 360 °C; (4) A carbocation mechanism and electron transfer mechanism exist between alkaline minerals and kerogen, promoting kerogen depolymerization into heavy hydrocarbons and substantially increasing heavy hydrocarbon yield; (5) Alkaline minerals catalyze kerogen pyrolysis to generate abundant alkyl and hydrogen radicals, which facilitates the cleavage of aliphatic and aromatic groups in asphaltenes and resins within crude oil, leading to increased saturate and aromatic hydrocarbon content.