Impact of tectonic deformation on shale pore structure using adsorption experiments and 3D digital core observation: A case study of the Niutitang Formation in Northern Guizhou

Abstract

Tectonic deformation has an evident impact on the pore structure of organic-rich marine shale. To study the impact of tectonic deformation on the pore structure of shale, cores were taken from different tectonic regions in the Niutitang Formation. The paleotectonic stress, pore structures, mineral composition, and geochemical parameters of the shale were determined by conducting the following tests: paleotectonic stress test, core thin-section observation, focus ion beam-scanning electron microscopy (FIB-SEM) observation, vitrinite reflectivity test, low-temperature nitrogen adsorption test, total organic carbon (TOC) test, X-ray diffraction (XRD) test, and nano-CT. The results showed that the shale in the deformed region mainly developed meso-macropores, and the pores were mainly slit-type and conical. The shale in the transition and stability regions primarily developed mesopores and micropores, which were primarily elliptical, slit-type, and columnar. The pore volumes of the shale in the deformed region were 9.54 cm3/kg and 13.9 cm3/kg higher than those in the transition and stability regions, respectively. The shale in the stability and transition regions had average microporous volumes 8.67% and 7.93% higher than that in the deformed region, respectively. The shale reservoir in the stability region developed more micropores and had a higher specific surface area and microporous volume. Furthermore, the organic matter had a greater impact on the micropores than on the meso-macropores. Thermal maturity had a greater impact on the microporous volume than on the macroporous volume. The shale pore connectivity of Well TX 1 was 86.48% and that of Well FC 1 was 22.33%. The pore connectivity of the shale in the stability region was higher than that of the shale in the deformed region. The shale samples from the deformed region had larger pore volumes and primarily developed macropores. The difference in the meso-macropores is significantly greater than the difference in the microporous volume. Hence, the geological tectonism had a greater impact on the meso-macropores than on the micropores in the shale cores.