A New Natural Gas Accumulation Model in the Triassic Xujiahe Formation: A Case Study in the Tongjiang-Malubei Area of the Sichuan Basin

The natural gas in the Triassic Xujiahe Formation (T 3 x) is reported to be mainly derived from the T 3 x source rock itself. Here, we report a new natural gas accumulation model, which demonstrates that the T 3 x gas in the Tongjiang-Malubei (TM) area is derived from both T 3 x and underline marine source rocks. The T 3 x gas in the TM area is characterized by CH 4 with a gas dryness coefficient above 0.99, indicating a high thermal maturity. The δ 13 C values of the methane, ethane, and propane in the T 3 x gas in the TM area are −33.7~−29.2‰, −32.7~−28.3‰, and −32.8~−29.5‰, respectively. Compared with the T 3 x gas in the Yuanba area, which was sourced from the T 3 x source rock, the T 3 x gas in the TM area contains heavier δ 13 C in methane and lighter δ 13 C in ethane, showing a partial reversal carbon isotope distribution (δ 13 C 1 > δ 13 C 2 ). According to their chemical and isotopic compositions, the T 3 x gas in the TM area was a mixture of coal-type and oil-type gases. The coal-type gas was mainly derived from the type III kerogen of the T 3 x source rock, and the oil-type gas was derived from the type-I kerogen of marine source rock in the Permian Wujiaping Formation (P 3 w). The oil-type gas migrated upward along the deep-seated faults that connect the P 3 w source rock and T 3 x sandstone reservoirs, and then mixed with coal-type gas in the T 3 x reservoirs, resulting in large-scale gas accumulation. This new gas accumulation model is controlled by a dual gas source supply and a high efficiency migration via the fault system. The findings of this study can help us to better understand the gas accumulation mechanism with the development of late-stage penetrating faults, which not only have implications for future petroleum exploration and development in the TM area, but also affect other analogous areas in the Sichuan Basin.