Evolution Mechanism of Pore Structures of Organic-Rich Shale Under Tectonic Deformation: A Comparative Study Between Whole Rock and Kerogen Samples

Under tectonic stresses, shale in different stress-strain environments will undergo structural deformation of varying mechanisms and intensities, forming various types of tectonically deformed shales (TDSs). By changing pore structures, structural deformation significantly influences shale's reservoir properties and then the resource potentials. This paper aims to reveal the evolution rules and mechanisms of micro- and mesopore structures of various TDSs. We first propose a TDS classification scheme according to the differences in deformation features and deformation mechanisms. Then, N2 and CO2 adsorption tests were conducted on the whole rock and kerogen samples of various TDSs to reveal the pore structure evolutions of shale during structural deformations and investigate the contributions of organic pore changes. Finally, we studied the mineral composition differences between various TDSs and their effects on the evolutions of shale's micro- and mesopores. Results showed that compared with undeformed shale, weakly brittle deformed shale (BDS) experienced a significant reduction in micro- and mesopores, mainly resulting from the decrease of organic matter content caused by the development of tectonic fractures and the filling of hydrothermal minerals. In strongly BDSs, there was a noticeable decrease in micro- and mesopores from undeformed shale as well. Apart from the negative effects of increasing carbonate minerals, the increased clay minerals also caused a decrease in kerogen content and organic pores. The limited increases of pores from weakly BDSs to strongly BDSs is mainly due to the emerged interparticle pores during shale fragmentation. Ductile deformed shale showed a significant decrease in micro- and mesopores; the collapse of organic pores is the dominant mechanism, with the mixing of clay minerals being an important reason as well. As for the brittle-ductile deformed shale, the evolution of micro- and mesopores is the result of the increase of carbonate minerals and the compression of kerogen.