Accumulation characteristics and large-medium gas reservoir-forming mechanism of tight sandstone gas reservoir in Sichuan Basin, China: Case study of the Upper Triassic Xujiahe Formation gas reservoir

Tight sandstone gas reservoirs of Xujiahe Formation in central Sichuan Basin have generally high water saturation. The characteristics of natural gas migration and accumulation, as well as gas reservoirs with high water saturation formation mechanism, were investigated using test data from natural gas geochemistry analysis and nuclear magnetic resonance (NMR) high pressure filling simulation experiment. The findings reveal that: (1)The Xujiahe Formation established the foundation for oil and gas near-source accumulation by interacting superimposed large areas of source rock and reservoir. As the maturity of source rocks increases, the wetness becomes smaller and the δ13C1 value becomes heavier of natural gas, indicating a near-source accumulation. (2)The gas saturation of Xujiahe Formation gas reservoirs ranges from 50% to 65%, with a pore radius of the reservoir space 0.1–10 μm. The larger the proportion of relatively large pore size, the higher the gas saturation will be. (3)As charging pressure is increased, natural gas accumulates in tight sandstone by progressively aggregating from large pore diameter to small pore diameter space, culminating “three-stage” charging characteristics of rapid increase, moderate increase and basically stable charge. The coupling between low-pressure drive and relatively large pore reservoir is the fundamental reason why the Xujiahe Formation of natural gas can form large-medium gas reservoirs but high water saturation in central Sichuan Basin. The findings will provide theoretical and technical support for the exploration of tight sandstone.