Organic matter evolution and pore structure response of overmature marine shales:Evidence from pyrolysis experiments
This study focused on the overmature shale of the Longmaxi Formation in the Sichuan Basin,using a closed-system quartz tube pyrolysis experiments to systematically investigate the pore structure evolution of marine organic-rich shale with Type I kerogen.The research employed quantitative analysis of gas products from experiments and examined pore structure of solid samples after pyrolysis using X-ray photoelectron spec-troscopy and argon ion polishing coupled with scanning electron microscopy.This work thoroughly explored the evolution of organic matter in shale and its corresponding pore structure changes during the overmature stage.The findings indicated that:(1)The organic matter within the overmature shale retained hydrocarbon genera-tion potential,with pyrobitumen likely being the most significant contributor to methane formation during the pyrolysis process.(2)When EqVRO<3.56%,the degree of organic matter graphitization is less than 15%,and the gas generation capacity and reservoir properties of shale reservoirs continue to improve with the increase of simulation temperature.When EqVRO=3.56%,the gas generation capacity of shale reaches its maximum value,and the degree of shale porosity and organic matter pore development also reaches its maximum value.When EqVRO>3.56%,the degree of organic matter graphitization exceeds 15%,and the methane yield and or-ganic matter porosity begin to decrease,indicating that the condensation effect of tar asphalt is enhanced,lead-ing to a decrease in pore volume and the destruction and transformation of nano pores.(3)Based on the actual drilling data in southern Sichuan,the shale source and reservoir were damaged during the over mature graphiti-zation stage after RO value exceeds 3.6%.It is believed that RO value of 3.6%is the favorable exploration limit for shale gas in the southern Sichuan.
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