Depositional, mineralogical, and maturity controls on pore types, porosity, and pore-size distribution in mudrocks
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Mudrocks act as seals and aquiclude for conventional petroleum systems and sources, seals, and reservoirs for unconventional petroleum resources. They are also candidates for nuclear-waste disposal and carbon capture and storage sites. This investigation improves our understanding of the interplay of depositional, diagenetic, and hydrocarbon generation processes in the origin and evolution of OM-rich mudrock pores by providing (1) a process-based understanding of the origin and control on the formation of different pore types, (2) quantitative methods to characterize pore systems in mudrock reservoirs, and by establishing (3) a link between sediments and depositional processes to pore system variations. Pore evolution models of mature mudrocks and a classification scheme of OM pores were developed during this research. Four main categories of OM pores comprise the OM-pore network: primary OM pore, convoluted OM pores, OM bubble pore, and OM spongy pore, with the latter two related to thermal maturation processes. The original interparticle and intraparticle mineral pores, which are a function of depositional and early diagenetic processes, determine the mineral pore network before petroleum generation, expulsion, and migration begin. The mineral pore network constrained petroleum migration and OM redistribution, controlling the modified-mineral-pore and OM-pore network during thermal maturation of OM. After OM maturation, the pore evolution is closely related to OM conversion. The morphology of mineral and OM pores varies with stages of OM maturation. Predominant pore types changed from primary mineral pores, to modified mineral pores containing relic OM, to coexisting modified mineral pores, OM bubble pores, and OM spongy pores, and finally to OM spongy pores. Pore sizes in OM-rich mudrocks range across at least five orders of magnitude (from 1 nm to approximately 100 μm). In general, OM pores are smaller than mineral pores. OM pores are related to kerogen and maceral types and subsequent thermal maturation processes. Mudrocks with well-sorted and coarse grains contain more abundant mineral pores than those with poorly-sorted and finer grains. Primary mineralogy, texture, and fabric determine diagenesis such as compaction and cementation and could indirectly affect sizes of OM pores up to two orders of magnitude differences.