Geochemical analysis of Ordovician natural gas in Shunbei area of Tarim Basin and its exploration significance of oil and gas

After years of exploration, oil and gas breakthroughs have been made in the strike-slip fault zones in the northeast of Shunbei area, forming hydrocarbon types and phase distribution patterns characterized by oil reservoirs in the north, condensate gas reservoirs in the south, and dry gas reservoirs in the east. Therefore, it is of great significance to discuss the genesis mechanism, source and thermal maturation of the gas from the perspective of the overall oil and gas distribution for the further continuous promotion of ultra-deep oil and gas exploration. By systematically collecting gas samples from different fault zones, this study analyzes the geochemical characteristics of natural gas in Shunbei area in detail. The results show that the natural gas in Shunbei area is less affected by TSR, except that there is strong modification by thermal-chemical sulfate reduction (TSR) in the part locations of the fault zones. The natural gases in the No. 1 fault zone and in the northern and middle sections of the No. 5 fault zone are mainly the crude oil associated gas from the primary kerogen cracking, while the natural gases in the southern section of No. 5 fault zone and No. 4 fault zone are mainly the mixture of early kerogen cracking gas (oil-associated gas) and late crude oil cracking gas. The natural gas in Shunbei No. 12 fault zone originated from deeper high-temperature crude oil cracking gas, and the crude oil cracking grade has reached wet gas cracking. The natural gases in the study area mainly comes from the source rocks of the Lower Cambrian Yurtus Formation, and the parent material of gases has the characteristics of benthic algae or mixed source of benthic algae and planktonic algae. Finally, the regression equation of thermal maturity calculation based on carbon isotope of methane is established during the process of hydrocarbon generation in the source rocks of Yurtus Formation. The research results can provide important reference for the next ultra-deep gas origin, source and thermal maturity analysis.