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Depositional, Mineralogical, and Maturity Controls on Pore Types, Size, and Distribution in Mudstones


Qualitative and quantitative analyses of mudstones in this study have revealed controls on pore type, size, and distribution in organic matter (OM)-rich mudstones because they affect permeability, petrophysical properties, fluid transport mechanisms, and production in unconventional plays. The relationships between facies, depositional cycles, depositional processes, texture, fabric, thermal maturation, bulk mineralogy, total organic content (TOC), kerogen and maceral types were investigated in a series of subsurface lacustrine (Yanchang Fm), and marine (Pearsall Fm and Eagle Ford Gp) samples across a range of maturities. In addition, immature and/or low-maturity Boquillas, Barnett, and Woodford mudstone samples were artificially pyrolyzed in the laboratory to compare the pore evolution in carbonate- (e.g. Eagle Ford) vs. siliciclastic- (e.g., Barnett) dominated mudstones. We have identified important differences and origins of mineral pores and OM pores, especially during petroleum generation and migration. The primary mineral pores are fully or partially filled with generated and migrated petroleum during bitumen and oil-window stage maturation, and residual OM is retained within the mineral pores. Some of these pores were previously misidentified as OM pores but they are in fact modified mineral pores with residual OM. In the Eagle Ford Gp, the abundance of mineral pores related with coccolith fragments implies good inter-connectivity of pore spaces (permeability). Mudstones with well-sorted grains and a higher percentage of coarser grains have more abundant mineral pores. The size distribution of mineral pores is found to be closely related to sorting and grain size. Pore network evolution models of marine and lacustrine mudrocks are different because of different kerogen and maceral types, rock texture, and diagenesis. The sizes of OM pores in lacustrine mudstones are 1 to 2 orders of magnitude smaller than those in marine mudstones which display abundant early cementation. Pore evolution and pore type heterogeneity in mudstones is a function of the initial mineral pore network, maturity, kerogen types and their maturation transformation ratios. The primary mineral pore network is a function of grain size, texture and fabric, depositional and diagenetic processes. Subsequent changes in mineral pores and development of OM pores are caused by petroleum generation and the interactions between fluids and mineral surfaces.