Methane in soil gas and its transfer to the atmosphere in the Yakela condensed gas field in the Tarim Basin, Northwest China

摘要

In this study, by analyzing CH_(4) concentration and Open image in new window in soil-gas profiles, the potentials of CH_(4) gas transfer from ground to atmosphere were studied at four representative sectors in the Yakela condensed gas field in the Tarim Basin, Xinjiang, China. These are: 1) the oil-gas interface sector, 2) fault sector, 3) oil-water interface sector, 4) an external area. Variation in CH_(4) in soil-gas profiles showed that CH_(4) microseepage resulted from the migration of subsurface hydrocarbon from deep-buried reservoirs to the earth’s surface. It was found that CH_(4) from deep-buried reservoirs could migrate upwards to the surface through faults, fissures and permeable rocks, during which some CH_(4) was oxidized and the unoxidized methane remained in the soil or was emitted into the atmosphere. The lowest level of CH_(4) at the soil-gas profile was found at the CH_(4) gas-phase equilibrium point at which the CH_(4) migration upwards from deep-buried reservoirs and the CH_(4) diffusion downwards from the atmosphere met. The Open image in new window and ethane, propane in soil gas exhibited thermogenic characteristics, suggesting the occurrence of CH_(4) microseepage from deep-buried reservoirs. A linear correlation analysis between CH_(4) concentrations in soil gas and temperature, moisture, pH, Eh, Ec and particle size of soil indicated that both soil Eh and soil temperature could affect CH_(4) concentration in soil gas while soil pH could indirectly influence soil methanotrophic oxidation via impacting soil Eh.