Migration of Natural Gas Controlled by Faults of Majiagou Formation in Central Ordos Basin: Evidence from Fluid Inclusions

Taking the Member-5 of Ordovician Majiagou Formation in the central Ordos basin as an example, the relationship between migration direction of natural gas and faulting activities was investigated based on integrated observations of fluid inclusion petrology, laser-Raman spectroscopy, homogenization and ice melting temperature measurements, thermodynamic PVT simulation. The results indicate that the minerals in the veins of structural fractures are calcite, dolomite and magnesite. There are four types of fluid inclusions found in the veins, including methane-rich gas inclusions, CO₂-bearing and methane-rich gas inclusions, methane-bearing aqueous inclusions and brine aqueous inclusions. The homogenization temperatures of the methane-bearing fluid inclusions range from 130.1 to 179.6 ℃, while the homogenization temperatures of the brine aqueous inclusions at the same time as the methane-bearing fluid inclusions range from 112.3 to 173.3 ℃. Combined with the history of hydrocarbon generation and strata burial, the Early Cretaceous is the important period of natural gas accumulation for the Majiagou Formation in studied area. While, Early Cretaceous tectonic movement produced a large number of structural fractures by the northeast-oriented faults. As the fluid inclusions in veins recorded the fluids of natural gas migration, experimental results prove that structural fractures had been channels for the migration of natural gas. The trapping pressures of fluid inclusions and pressure coefficients show a decreasing trend from SW to NE along the faults, which implies that the methane-bearing fluids migrated along the NE-trending fault.