Magmatism and hydrocarbon accumulation in sedimentary basins: A review

The circulation of magma is typically accompanied by the transfer of materials and thermal energy that can significantly alter the compositions and properties of sedimentary rocks, thereby influencing hydrocarbon generation and accumulation in sedimentary basins. In this study, the effects of magmatic activity on hydrocarbon accumulation are summarized and clarified for nine sedimentary basins in China from different perspectives. Magmatism has a multi-role relationship with hydrocarbon accumulation processes in sedimentary basins, affecting hydrocarbon generation, migration, accumulation, preservation and destruction. Volcanic ash provides nutrients that promote biological growth and enhance primary productivity. The extremely anoxic environment caused by magmatism can decelerate the oxidation and decomposition of organic matter, which favors the deposition and preservation of organic material. The upwelling of magma can transfer heat upward, enhancing the maturation of organic matter and promoting hydrocarbon generation. Mantle-derived hydrogen-rich fluids can effectively replenish the hydrogen that is consumed by thermal cracking of hydrocarbons. In addition, abundant mantle-derived inorganic CO2 can cause the dissolution of carbonate minerals and cements in reservoirs, generating secondary pores. Furthermore, magmatic intrusions often form effective hydrocarbon traps, and the strata-penetrating magmatic walls and unconformable surfaces can act as predominant hydrocarbon migration pathways. It is noteworthy that the effects of magmatism differ markedly in different tectonic settings. In China, the basins in back arc rift settings (such as the Bohai Bay basin) are undergoing plate subduction. This geological scenario allows for the upward migration of magma from the mantle, along with the transport of thermal energy, mantle-related inorganic CO2, H-enriched materials, and occasionally volcanic ash. In contrast, basins in post-collisional rift settings (such as the Permian Junggar basin) have undergone lower crust delamination. These basins typically feature volcanic ash derived from the crust, as well as materials originated from the mantle. Lastly, basins in compressional settings (such as the Ordos basin) have experienced crustal block collision. The inner parts of these basins often contain significant amounts of volcanic ash and small-scale magmatic intrusions derived from the crust.Although magmatism can have significant positive impacts on hydrocarbon accumulation, there are also some negative aspects, such as porosity degradation in the oil and gas reservoirs, thermal cracking and degradation of oil, and breaching seals. Whether the movements of deep fluid are advantageous or disadvantageous for hydrocarbon accumulation mainly depends on relationship of hydrocarbon migration and charging, the formation of traps, and the composition, properties, and duration of deep fluid movements. The characteristics and movement of magma are important factors that need to be considered in hydrocarbon exploration and the evaluation of hydrocarbon reservoirs in sedimentary basins.