Origin of deep strata gas of Tazhong in Tarim Basin, China

Tazhong is located in the interior of the Taklamakan desert in the Tarim Basin. Deep carbonates are targeted exploration areas with abundant gas, and the largest Ordovician gas-condensate field has been discovered. The hydrocarbon is mainly distributed in the reef-shoal complex of the upper Ordovician Lianglitage Formation and in the interbedded weathered reservoirs of the lower-middle Ordovician Yingshan Formation. Despite the deep burial depth of the reservoirs and strong heterogeneity, the reservoir bodies are large, multiple-layered, and enrichment in hydrocarbons indicates a great exploration potential. The Ordovician gas-condensate reservoir in the Tazhong consists mainly of methane with a gas dryness coefficient of 0.91–0.98. The Ordovician gas in the Tazhong belongs to secondary (oil cracking) gas. The comprehensive analysis of natural gas composition, stable carbon isotopes and adamantane content showed that the natural gas was from paleo-reservoir cracking at depth. Basin geothermal history, burial history, fluid inclusion data and geological analysis show that the gas accumulation occurred 5 Ma ago. The sulfur isotope of sulfides, the presence of 2-thia-adamantane and existence of light CO2 carbon isotopes indicate that the H2S was formed via TSR. The natural gas accumulated through migration along faults.