The formation mechanisms and processes of geochemical anomalies used as proxies in surface geochemistry exploration(SGE) have not been well understood. Previous studies cannot realize 3D measurement of microseeping hydrocarbons from reservoirs to the surface,which made it difficult to understand the features and pathways of deep hydrocarbon microseepages. Understanding the processes of hydrocarbon microseepages will contribute to the acceptance and effectiveness of surface geochemistry. Based on a simplified geological model of hydrocarbon microseepages,including hydrocarbon reservoir,direct caprock,overlying strata and Quaternary sediments,this work established a 3D experimental system to simulate the mechanisms and processes of deep hydrocarbon microseepes extending to the surface. The dispersive halos of microseeping hydrocarbons in the subsurface were adequately described by using this 3D experimental system. Results indicate that different migration patterns of hydrocarbons above the point gas source within the simulated caprock and overlying strata can be reflected by the ratio of i-butane to n-butane(i-C_4/n-C_4),which follow diffusion and infiltration(buoyancy) mechanisms. This is not the case for vertical measurement lines far from the point gas source. A vertical gas flow in the form of a plume was found during hydrocarbon microseepage. For sampling methods,the high-density grid sampling is favorable for delineating prospecting targets. Hydrocarbon infiltration or buoyancy flow occurs in the zones of infiltration clusters,coupling with a diffusion mechanism at the top of the water table and forming surface geochemical anomalies. These results are significant in understanding hydrocarbon microseepage and interpreting SGE data.
WANG GuojianTANG YupingCHENG TongjinTANG JunhongFAN MingLU Li
In this study,by analyzing CH4 concentration and δ13CCH4 in soil-gas profiles,the potentials of CH4 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 CH4 in soil-gas profiles showed that CH4 microseepage resulted from the migration of subsurface hydrocarbon from deep-buried reservoirs to the earth's surface.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.The lowest level of CH4 at the soil-gas profile was found at the CH4 gas-phase equilibrium point at which the CH4 migration upwards from deep-buried reservoirs and the CH4 diffusion downwards from the atmosphere met.The δ13CCH4 and ethane,propane in soil gas exhibited thermogenic characteristics,suggesting the occurrence of CH4 microseepage from deep-buried reservoirs.A linear correlation analysis between CH4 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 CH4 concentration in soil gas while soil pH could indirectly influence soil methanotrophic oxidation via impacting soil Eh.
