Pore-micro fracture structure, porosity and gas- bearing property of deep shale under lithofacies-formation pressure coupling

Formation pressure-lithofacies type are the most critical factors influencing micro pore structure and porosity in shale reservoir. However, how these two factors jointly affect shale gas accumulation remains unclear. Scanning electron microscopy (SEM), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), on-site desorption and low-temperature N₂ adsorption (LTNA) are integrated to analyze coupling effects of pressure variation and lithofacies on reservoir quality and gas-bearing characteristics of deep-buried shale. Three lithofacies are identified in Longmaxi deep-buried shale: siliceous lithofacies (S), argillaceous lithofacies (CM) and mixed lithofacies (M). Pores with pore size < 4 nm are the main contributors to the specific surface area (SSA), and pores between 4 nm and 30 nm are the main contributors to the pore volume (PV). Pressure variations directly affect the size and number of organic matter pores but have no impact on intraparticle pores. The variability in interparticle pores indicates that the S lithofacies has a stronger resistance to compaction compared to the M lithofacies. Porosity and micro structure of deep-buried shale reservoir are influenced by lithofacies type, burial depth and pressure variation. Organic-rich S shale and organic-poor S shale demonstrate good reservoir properties under over-pressure and well-preserved conditions, with organic-rich S shale having the strongest resistance to compaction. Organic and inorganic pores are largely lost during compaction of organic-rich M shale. CM lithofacies also has a poor material foundation and the weakest resistance to compaction, making it difficult to preserve original porosity and pore structure during compaction. The decrease in formation pressure results in macropores making almost no contribution to pore volume, while the contribution of mesopores is further enhanced. As the formation pressure decreases, the contribution of micropores to pore volume is gradually increased. As shale porosity decreases, porosity associated with macro pores declines first, followed by that associated with mesopores. Free-gas content in the CM and M lithofacies declines rapidly as porosity decreases. Both adsorbed and free gas decrease sharply as porosity is lost.