Thermal effect of diabase intrusion on hydrocarbon generation and accumulation in the Middle Proterozoic Xiamaling shale from the Xuanhua Basin, North China

The thick organic-rich shale in the Mesoproterozoic Xiamaling Formation in the Yanshan area is a potential target for shale gas exploration. A field profile survey revealed several sets of diabase intrusions in the Xiamaling Formation, and these intrusions impacted oil and gas accumulation. This work focuses on the Mesoproterozoic Xiamaling Formation in the Xuanhua Basin in the Yanshan area. On the basis of actual measurements and experimental analysis of the section, the influence of a diabase intrusion with a thickness of approximately 6 m on the hydrocarbon generation capacity of the surrounding rock was analysed. The diabase in the Mesoproterozoic Xiamaling Formation in the Xuanlong Depression can be classified as island arc alkaline basalt that intruded at 1367.7 ± 50.7 Ma and was emplaced through at least two superimposed magma injections. In this study, the Neoproterozoic prototype basin in the Xuanhua Basin is divided into five evolutionary stages. Volcanic eruption activity was very frequent during the deposition of the Xiamaling Formation, the diabase intruded in a back-arc extensional environment at the end of the deposition of the Xiamaling Formation, and the diabase was formed by rapid thinning of the crust and upwelling of deep mantle-derived magma in the back-arc extensional environment. With the diffusion of heat following the intrusion of the high-temperature magma, the temperature of the surrounding rock increased sharply, greatly promoting the thermal maturation and consumption of organic matter in the surrounding rock, ultimately resulting in a gradual decrease in the organic carbon content with decreasing distance from the diabase. The Rock-Eval pyrolysis, kerogen element composition, and Raman spectral parameter results for the surrounding rock indicate that the hydrocarbon generation capacity of the surrounding rock gradually decrease with decreasing distance from the diabase intrusion. In this study, a model of the influence of diabase on the hydrocarbon generation capacity of the overlying organic-rich shale was established, and the degree of damage to the hydrocarbon generation capacity of the organic-rich surrounding rock caused by the diabase intrusion was divided into strong, moderate, weak thermal metamorphic and unaltered zones. A conceptual model of shale gas accumulation and enrichment in the Mesoproterozoic Xiamaling Formation in the Xuanhua Basin was established on the basis of the intrusion process of this diabase and the occurrence and enrichment of shale gas.