Origin and accumulation model of ultra-deep marine natural gas in the Shuntuoguole low Uplift of the Tarim Basin, NW China

The Tarim Basin, with its relatively low level of exploration, is one of China’s largest superimposed petroliferous basins. Recent exploration advancements in the Shunbei-Shunnan area highlight the significant potential for ultra-deep marine natural gas. However, uncertainties surrounding the origin, alteration, and accumulation models of ultra-deep marine natural gas have greatly hindered future hydrocarbon exploration. This study systematically analyzes the geochemical characteristics and genesis of ultra-deep marine natural gas in the Shunbei-Shunnan area. Results indicate that the natural gas in this region comprises both primary and oil-cracking gases. Specifically, the Shunbei area’s natural gas is primarily oil-associated, dominated by primary cracking gas with minor contributions from oil-cracking gas, whereas the Shunnan area’s natural gas is predominantly oil-cracking gas. The maturity of marine natural gas varies, being higher near the Manjiaer Depression and relatively lower farther away. A sequential distribution of highly mature dry gas, condensate oil and gas, volatile oil, and light oil, accompanied by a gradual decrease in gas–oil ratio, is observed with increasing distance from the Manjiaer Depression. The marine natural gas in the Shunbei-Shunnan area has undergone various alterations, including cracking, thermochemical sulfate reduction, and hydrothermal fluid alteration. Hydrocarbon alteration is more pronounced in the Shunnan area than in the Shunbei area. The active period of strike-slip faults in the Shunbei-Shunnan area coincides with the major hydrocarbon generation and expulsion phases of the source rock. These strike-slip faults serve as critical conduits for hydrocarbon migration, enabling oil and gas to migrate vertically into the middle and upper Ordovician reservoirs, where they form the primary reservoir spaces for accumulation.