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.
Highlights
Methane (CH4) tration in the is a principal atmosphere greenhouse gas, has risen from and its concen~0.7 ppmv to~1.9 ppmv in the past 300 years [1]
Negative fluxes are a consequence of methanotrophic activity when the speed of diffusion and infiltration of methane from the oil reservoir is lower than that of its methanotrophic oxidation activity
The gas at Dawanqi showed an extreme 13C enrichment can be explained by the fractionation effects of diffusional migration and methanotrophic oxidation
Summary
Methane (CH4) tration in the is a principal atmosphere greenhouse gas, has risen from and its concen~0.7 ppmv to~1.9 ppmv in the past 300 years [1]. For one-third of total radiative forcing, including the indirect effects of CH4 emissions (3 W/m2) such as changes in ozone and stratospheric water vapour concentrations [2]. Numerous studies have shown that geologic CH4 emission could play an important role in the atmospheric CH4 budget, mainly due to CH4 emissions from petroleum seepage through faults, fissures and permeable rocks, mud volcanism, marine seeps, and geothermal manifestations (Klusman et al 1998b, [7,8,9,10]). Gas seepage includes macroseeps, which is the visible gas manifestations such as oil and gas seeps and mud volcanoes (either onshore or shallow/coastal offshore), and microseepage, which is the Geofluids invisible, diffuse exhalation of gas from the ground, typically occurring in correspondence with petroleum fields. The published research has shown that microseepage in sedimentary basins can overcome the methanotrophic consumption occurring in dry soil, especially in the winter season [11, 12]. few
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