The origin, migration and accumulation of the Ordovician gas in the Tazhong III region, Tarim Basin, NW China

Abstract The Ordovician large-scale gas reservoir in the Tazhong III region (burial depth > 6000 m) is a typical deep strata gas reservoir in the Tarim Basin of NW China. The origin, migration and accumulation of the Ordovician natural gas in the Tazhong III region were investigated by integrating geological, geophysical, and geochemical parameters. The parameters that were successfully analyzed included the gas compositions and carbon isotope ratios, production data, burial and geothermal histories, oil compositions and properties, together with the diamantane index. The results indicated that the Ordovician natural gas in the Tazhong III region originated from Cambrian-Ordovician paleo-oil cracking, accompanied by a large amount of H 2 S generated from thermochemical sulfate reduction (TSR). When the oil cracking gas migrated vertically into the Ordovician reservoir through the Tazhong I fault and strike-slip faults, it was preferentially charged in the pinnate fault zones of strike-slip faults with high porosity and permeability, leading to the enrichment of natural gas and an abnormal distribution of the compositions and properties of hydrocarbon. Then, oil cracking gas intruded into and altered the previous oil reservoir from northeast to southwest through the strike-slip faults, unconformities and pore-fracture reservoir belt, leading to the differential accumulation of hydrocarbon in districts of the Tazhong III region. The Ordovician natural gas migration-accumulation model in the Tazhong III region indicates a great exploration potential in the deep or super-deep strata of the Tarim Basin. Further exploration activities in the Ordovician reservoir of the Tarim Basin should be conducted in the regions where Cambrian-Ordovician source rocks are well developed, and the stress fracture zones-related deep fault should be emphasized.