Characteristics and Significance of Gas Accumulation during the Uplift Process of Xujiahe Formation Tight Gas System, Central Sichuan Basin

Bian, Congsheng; Wang, Hongjun; Wang, Zecheng; Xu, Anna; Xu, Zhaohui; Li, Yongxin

The Upper Triassic Xujiahe Formation contains a set of land clastic rocks with coal-bearing deposits, including six members from bottom to top, among which member I, III and V are source rock and caps and member II, IV and VI are reservoir rocks, which is low porosity and permeability. Many gas fields have been discovered now and geological reserves is more than one trillion squares in Xujiahe formation, which has become the vital important tight gas formation in mid-west china. The geochemical characteristics of these gas fields show that Xujiahe formation is an independent gas system.

Burial history of central Sichuan area indicates that Xujiahe formation underwent rapid buried form Jurassic to early Cretaceous period to the depth of 4.5-5 km, the source rock became thermal mature and generated mass gases, which migrated from source rock to reservoir and accumulated. After late Cretaceous, the whole formation was elevated and eroded, and the total erosion thickness is beyond 1.5 km.

Amounts of gaseous hydrocarbon inclusions have been found in the micro-fractures of quartz formed during uplift period. The homogenization temperature of coeval saline inclusions of the gaseous hydrocarbon inclusions ranges 110-130 Centigrade, and the freezing point temperature is -20--5 Centigrade. The high homogenization temperature and low freeze point temperature suggest that the hydrocarbon inclusions are formed during uplifting period, and the gas accumulation occurred in a large scale. The physical simulation experiment designed according to the lithological structure of Xujiahe formation, proved the occurrence of desorption and gas expansion in the process of uplift, and the quantity of the gas in this period reaches 20-40% of maximum expulsive gas from the source rock.

The carbon isotope value of both methane and ethane in Xujiahe formation is 3-4‰ lower than that generated by the same maturity coal rocks of other basin, where the erosion thickness is less than 500m. Our study shows that the gas with lower maturity were probably generated and absorbed in the coal-source rocks in early stages, which desorbed from the coal series and charged into the reservoirs as pressure and temperature decreased in the process of uplift. Based on this study, the gas resources potential of Xujiahe formation maybe increase about one trillion squares, and reservoirs within source rocks of member I, III and V could have higher gas-bearing abundance than that of member II, IV and VI.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013