Methane in Soil Gas and Its Migration to the Atmosphere in the Dawanqi Oilfield, Tarim Basin, China

Abstract

Methane microseepage is the result of natural gas migration from subsurface hydrocarbon accumulations to the Earth’s surface, and it is quite common in hydrocarbon-prone basins. In this study, by analyzing gas concentrations and isotope composition of soil gas, the potentials of CH4 gas transferred to the surface were studied at three measurement transects in Dawanqi oilfield, Tarim Basin, China. It was found that CH4 from deep-buried reservoirs could migrate upwards to the surface through faults, fissures, and permeable rocks, during which some CH4 was oxidized and the unoxidized methane remained in the soil or was emitted into the atmosphere. Soil gas samples had mean concentrations of 907.1, 62.3, 21.7, 11.0, and 5.8 ppmv for CH4, C2H6, C3H8, C4H10, and C5H12, respectively. The C1/C2+ (13.3 for soil gas and 3.75 for absorbed gas) and gas wetness ratio (12% for soil gas and 26% for absorbed gas) suggested that the hydrocarbons were derived from a thermogenic process. According to isotope composition analysis, the δ13CCO2, δ13CCH4, and δDCH4 values for the soil gas from Dawanqi oilfield varied from -15.5 to -17.2‰, -11‰ to -17‰, and -150 to -189‰, respectively. The extreme 13C enrichment in CH4 is possibly because of the fractionation effects of diffusional migration and methanotrophic oxidation. Soil gas and absorbed gas showed high CH4 concentrations at the edge of the fault block, which indicated that fault was conductive to gas migration. Also, gas migrated from the surface to the atmosphere in the center region of the fault block because of the high permeability and shallow depth of the reservoir in Dawanqi oilfield.