Quantifying the relative contribution and evolution of pore types to shale reservoir space: Constraints from over-mature marine shale in the Sichuan Basin, SW China

Pore types significantly affect the pore properties of shale reservoirs, but their contribution to shale pore space in over-mature marine shale remains controversial because organic and inorganic pores fail to be quantitatively characterized. In this study, mercury intrusion porosimetry, low-temperature N2 and CO2 adsorption, and image analysis were conducted on over-mature marine shale in the Lower Silurian Longmaxi Formation to quantify the contribution of pore types to pore volume (PV) and surface area (SA). The full-scale pore size distribution of over -mature shale indicates that mesopores account for the largest PV, while macropores are developed in organic matter (OM)-poor shale. By analyzing the influence of multiple factors and heterogeneity of different pore types, OM pores, pyrite framboid pores, clay mineral pores, and dissolution pores are identified as dominant types in OM-poor/moderate/rich shale. The PV and SA of these four pore types are quantitatively characterized by extracting parameters of shale components and pores using image analysis. In OM-poor shale, clay mineral pores contribute the most to the PV and SA, followed by OM pores and dissolution pores, and finally pyrite framboid pores. In OM-rich/moderate shale, the contribution of OM pores to the PV and SA is dominant and increases with the increasing total organic carbon content. The evolution of pore types throughout the maturation sequence is revealed based on previous studies. Especially at the over-mature stage, clay mineral pores contribute the most to pore space in OM-poor shale, while OM pores make a major contribution to pore space in OM-rich/moderate shale.