Fine characterization of pore evolution of oil shale during thermal simulation

Abstract

Oil shale is a kind of organic‐rich fine‐grained sedimentary, whose pore evolution is affected not only by the evolution of inorganic minerals, but also by the thermal evolution of organic matter (OM). Meanwhile, the interaction between inorganic minerals and OM makes the pore evolution of oil shale more complex. Therefore, it is more important to fine the evolution characteristic of pores in different maturity stages and explore the pore size range affected by each influencing factor. This study selects immature oil shale (Tmax = 430°C, TOC = 8.2%) of Nenjiang Formation in Songliao Basin as a research object, and samples with different maturity are obtained through a series of thermal simulation experiments. Combined with petromineralogy, organic geochemistry and reservoir physical property analysis, the evolution process of pores is determined. The volume and specific surface area evolution of mesopores is different from that of macropores in the immature to mature stage, the volume and specific surface area evolution of which is same in the mature to over‐mature stage. There is a strong positive correlation between volumes of macropores and porosity (R2 = 0.94), between volumes of mesopores and specific surface area (R2 = 0.96). The influencing factors mainly affect the pore size less than 400 nm. Hydrocarbon generation of OM affects the whole pore sizes. Residual oil is stored in pores with pore size of 5–25 nm. The effect of carbonate dissolution and pyrite pyrolysis on mesopores is stronger than that of macropores. The transformation of illite/smectite to illite mainly affects the pore size greater than 100 nm. In the whole process of thermal simulation, the OM thermal evolution, the inorganic minerals dissolution and pyrolysis and the clay minerals transformation are the main influencing factors of pore evolution.