Effects of Pore Water on the Heterogeneity in Nanopore Structures of Deep Marine Shales: A Multifractal Study on As-Received Shale Samples from the Southern Sichuan Basin, China

Shalestrata generally contain a certain amount of pore water thataccompanies the whole diagenetic and thermal evolution processes ofshales. The distribution and occurrence characteristics of pore watersignificantly influence the nanopore structures of shales, which subsequentlyaffects the accumulation and production of shale oil and gas resources.Based on a set of fresh Wufeng-Longmaxi (WL) shale samplesfrom the deep marine strata in the southern Sichuan Basin, this studyinvestigated the effects of pore water on the pore heterogeneity ofshales by a multifractal method. The results show that (1) the deepWL shales have low pore water content (C (PW)) and water saturation (S (EW)), with averagevalues of 7.47 mg/g and 35.21%, respectively, and the C (PW) and S (EW) of the shales aremainly controlled by clay mineral content. (2) Under dried conditions,the pore heterogeneity of high-probability measure areas has negativerelationships with the clay mineral content and pore volumes withpore widths of 4-10 nm, and the pore connectivity is mainlycontrolled by clay mineral content. Under as-received conditions,however, the pore heterogeneity of low-probability measure areas andpore connectivity have positive relationships with clay mineral contentand pore volumes with pore widths less than 4 nm. (3) Under as-receivedconditions, the pore water mainly occupies the nanopore with porewidths less than 4 nm and reduces the effective pore spaces and connectivity,which reduces the pore heterogeneity of high-probability measure areasand enhances the pore heterogeneity of low-probability measure areas.