Main factors influencing the development of nanopores in over-mature, organic-rich shales.

Shale cores of the Lower Silurian Longmaxi Formation from wells in the Fuling and Changning-Weiyuan shale gas demonstration zones and the Wuxi shale exploration block of the Sichuan Basin were investigated to determine the key factors controlling nanopore development in over-mature, organic-rich shales. These samples were studied using organic, geochemical, and petrological analyses, as well as low pressure nitrogen and carbon dioxide adsorption, and broad ion-beam-field emission scanning electron microscopy (BIB-FESEM). The results show that organic-rich shales of the Lower Silurian Longmaxi Formation have complex pore structures, in which the surface areas of micropores and fine mesopores (diameter < 10 nm), along with the total mesopore volume, are the most important parameters controlling nanopore development. The samples have micropore and fine mesopore surface areas of 10.03–46.47 m2/g (mean 22.65 m2/g), accounting for a significant portion (up to 99%) of the total surface areas. Mesopore volumes of 6.28 × 10−3 to38.35 × 10−3 cm3/g (mean 18.36 × 10−3 cm3/g) also account for a significant portion (up to 77%) of the total pore volume. The development of differently sized pores depends on various factors. For example, pores with a diameter < 10 nm exhibit a significant positive linear correlation with organic matter content and thermal maturity. Total organic carbon (TOC), quartz, clays, and pyrite contents are the main controlling factors in the formation of medium mesopores (10–25 nm). The generation and distribution of coarse mesopores(25–50 nm) are dominantly controlled by clay contents, especially those of illite and mixed-layer minerals of illite and smectite (I/S). Macropores are typically mineral-associated pores (e.g., pores associated with the dissolution of calcite and albite). For organic-rich shales, the development of organic pores can be evaluated using the micropore surface area per unit mass of TOC, such that the results show an increase with increasing maturity from the early mature to the over-mature stage for type I and type IIa kerogen shales, but an obvious inversion or decrease when the vitrinite reflectance (R o) > 2.5%. For shale samples with type IIb and type III kerogens, the micropore surface area per unit mass of TOC first rises from early mature to the mature stage, and then declines from the mature to over-mature stages of approximately ~2.0% R o. Therefore, abundant organic matter, type I and type IIa kerogen, relatively high quartz and clay contents (especially illite), and early stage mineral dissolution are favorable factors for the development of nanopores in over-mature shales in the study areas. These findings will help improve our understanding of the development of nanopores in over-mature organic-rich shales. • The porosity characteristics of organic-rich, over-mature shales were explored. • Micropore surface area per unit of TOC first increases with shale maturity. • For type I and IIa kerogen shales, porosity decreases when R o exceeds 2.5%. • For type IIb and III kerogen shales, porosity decreases when R o exceeds 2.0%. • Abundant organic matter and early mineral dissolution favor pore development. [ABSTRACT FROM AUTHOR]