Quantitative Characterization of Deep Shale Gas Reservoir Pressure-Solution and Its Influence on Pore Development in Cases of Luzhou Area in Sichuan Basin

The phenomenon of pressure-solution (PS) is widespread in deep marine shale reservoirs of the Longmaxi Formation in the Sichuan Basin, affecting pore development. However, systematic reports on the study of PS in shale reservoirs are yet to be seen. This study performed large-field scanning electron microscopy, mineral quantitative identification, low-pressure gas adsorption, and high-pressure mercury injection experiments on shale cores from the Longmaxi Formation. The pore structure characteristics and the PS process of deep shale reservoirs were clarified, a semi quantitative analysis method for PS was constructed, and the influence of PS on pore development was explored. Our results demonstrate that PS is widely present in deep gas shale reservoirs, primarily manifesting in the form of mineral transformation and fusion, particularly involving clay and quartz minerals. This process alters the mineral composition and particle size of the shale reservoirs. A semi-quantitative analysis method for PS and the action strength parameter QP has been established, based on the mineral composition and particle size of shale reservoirs. This parameter exhibits a positive correlation with burial depth, water saturation, and quartz content. The primary effect of PS on pore development is that, as mineral transformation results in an elevation of quartz content and an increase in particle size, pore dimensions undergo compression and subsequently diminish. This underscores why shale reservoirs containing over 70% quartz are unfavorable for pore development. Therefore, when the water saturation in the Longmaxi Formation shale reservoir exceeds 40% and the quartz content surpasses 70%, significant risks are present in the exploration and development of shale gas.