Study On The Mechanism Of Natural Gas Accumulation In The Southern Songliao Basin Based On Three-dimensional Reconstruction Of Core Nano-CT Scan

It has been found that the high CO₂ gas-bearing horizons are mainly the Lower Cretaceous Yingcheng Formation and Quantou Formation in the Songliao Basin.Most people believe that the origin of inorganic CO₂ is dominated by mantle sources,supplemented by crust-mantle mixing in the southern Songliao Basin;Only a few people believe that it is the result of the late degassing of inorganic CO₂ gas adsorbed by volcanic rocks in the deep.In view of the divergence of opinions on this understanding,the volcanic rock samples were ground to different particle sizes,heat each sample separately,record the amount of CO₂ gas at different temperature points,and confirmed whether the CO₂ from volcanic rock degassing can become one of the sources of inorganic CO₂gas in the Songliao basin.The inorganic CO₂ gas reservoirs,hydrocarbon gas reservoirs,and mixed gas reservoirs were developed in the deep lower Cretaceous Yingcheng Formation in the southern Songliao Basin,while hydrocarbon secondary gas reservoirs were found in the overlying Denglouku Formation sandstone reservoir.The reasons for hydrocarbon accumulation in the Denglouku Formation were studied through diffusion experiments and displacement experiments.Due to volcanic rock degassing experiments and gas diffusion experiments,the relationship between natural gas migration and rock pore throat structure characteristics cannot be characterized microscopically.Therefore,with the use of CT scanning technology,three-dimensional modeling of core pores through VGStudio MAX3.0 software,and use Avizo2019.1software to extract and analyze rock fractures and three-dimensional network model display of throat to explore the microscopic pore throat structure characteristics and natural gas accumulation relationship.Through this study,it was confirmed the CO₂ from volcanic rocks degassing could be one of the sources of inorganic CO₂ gas in the Songliao Basin.The reasons for the formation of the hydrocarbon reservoirs of the Denglouku Formation and the controlling effect of the micro-pore-throat structure of the rocks on natural gas migration were clarified.After detailed research in the thesis,the following results and cognitions have been obtained:?1?Grind volcanic rock samples into three different particle sizes,heated each sample separately,and recorded the amount of CO₂ gas at 6 temperature points from 150°C to 600°C.The volcanic rocks of the Yingcheng Formation increase geothermal temperature with the increase of the burial depth,and can degas inorganic CO₂ gas.The CO₂ released from volcanic rocks can be one of the sources of inorganic CO₂ gas in the Songliao Basin,but the amount of inorganic CO₂released at temperatures<200°C is low,and it contributes little to the existing high CO₂ gas reservoirs.?2?The CH₄ and CO₂ diffusion coefficients of the Yingcheng Formation volcanic rock samples from 5 wells in the study area were measured,and the results showed that the CH₄diffusion coefficient was greater than CO₂.After the formation of the Yingcheng Formation hydrocarbon gas,the deep-source faults are activated due to the influence of regional tectonic movements.The mantle-derived inorganic CO₂ migrates to the Yingcheng Formation volcanic hydrocarbon gas reservoir,displacing and replacing the hydrocarbon gas.The hydrocarbon gas replaced by CO₂ migrates upward to the Denglouku Formation sandstone layer with good reservoir properties and enriches to form a secondary gas reservoir.?3?In the conventional test method,the pores of the tested rock are mainly large and medium pores,and the obtained porosity value is relatively low.The measured rock sample porosity,permeability and gas diffusion coefficient have an abnormal negative correlation.Therefore,the volcanic rock reservoir samples of the Yingcheng Formation in wells CS1 and W21 in the southern Songliao Basin were selected for micro-and nano-CT scanning experiments to study the microstructure characterization of the volcanic reservoir samples,and accurately identified the control effect of volcanic rock microstructure characteristics on gas diffusion and migration.VGStudio MAX3.0 and Avizo2019.1 software were used to carry out three-dimensional modeling of the rock pores,extract the three-dimensional spatial crack distribution in the rock and establish a three-dimensional throat network model.The analysis results explain the positive correlation between the diffusion coefficient of natural gas and the development of rock pore throats and fissures.The analysis results well explain the positive correlation between the diffusion coefficient of natural gas and the development of rock pore throats and fissures.The development of micro-and nano-scale pores,fractures and throats greatly improves the diffusion and migration ability of reservoir gas.The reasonable combination of pore throats and fractures is the basis for the efficient development of volcanic gas reservoirs.