Geological Characteristics of Lower Paleozoic Shale Gas Accumulation in the Yuxi Region, Southern Sichuan Basin: In View of High-Density Methane Inclusions.

Abstract

Twelve shale fracture veins and forty-nine fluid inclusion assemblages within the veins of 3800–4200 m in three wells located in different tectonic zones in the Yuxi Region, southern Sichuan Basin were selected in this study. The burial and thermal histories of single wells were reconstructed, and time–temperature–pressure of oil and gas filling were clarified using microscopy observation, Raman microprobe analyses, geochemical tests, and fluid inclusion microtemperature measurement. The shale fracture veins of the Wufeng-Longmaxi Formation in the Yuxi Region are mainly formed vertically and horizontally, where the vein-forming fluids are derived from endogenous fluids. A large number of methane inclusions, bituminous inclusions, and methane-bearing bituminous inclusions within the veins confirm the process of oil cracking gas and kerogen cracking gas. The homogeneous temperature (Th) of the aqueous inclusions contemporaneous with the bituminous inclusions ranges from 109.3 to 174.1 °C, which were trapped during 220 to 250 Ma. The homogeneous temperature of the aqueous inclusions contemporaneous with the methane inclusions ranges from 137.3 to 226.8 °C, which were trapped during 160 to 195 Ma and 51 to 56 Ma. The trapped pressure calculated by high-density methane inclusions (0.246–0.293 g/cm3) is between 82.9 and 140.1 MPa, with a pressure coefficient between 1.64 and 2.07. The formation pressure coefficient is nearly two, indicating that the current overpressure is inherited from the overpressure at the maximum burial depth. The earlier the fracture vein opening, the less the damage to the shale gas accumulation, and the more opening-closing phases, the lower the homogeneous temperature of the gas–liquid two-phase aqueous inclusions coeval with the high-density methane inclusions and the greater the degree of damage to the shale gas accumulation. The results provide a basis for further study on the genesis of overpressure and the migration of shale gas.