Pore structures of shale cores in different tectonic locations in the complex tectonic region: A case study of the Niutitang Formation in Northern Guizhou, Southwest China

Abstract

The Cambrian Niutitang Formation shale reservoirs are widely distributed in the complex tectonic area in northern Guizhou. Fifteen organic-rich Niutitang Formation cores were selected from the tectonic deformation zones and tectonic stability zone in northern Guizhou, including the FC1 well related to fold, TM1 well related to faults, and TX1 well in the tectonic stability zone. Pore structures and reservoir physical properties of shale were tested via microphysical experiments and low-temperature nitrogen adsorption (LTNA) test. Results indicate that the thermal evolution maturity of shale is in the over mature stage, with RO distributed between 2.30 and 3.74% and the total organic carbon content (TOC) between 3.8 and 7.5%. Shale cores in the tectonic stability zone have higher TOC content (6.25%) than that of shale (5.16%) in the tectonic deformation zones. Shale cores in the tectonic deformation zones have higher thermal evolution maturity Ro (2.98%) than that of shale (2.76%) in the tectonic stability zone. The pore size of shale cores ranges from 4.15 to 15.62 nm. The total pore volume of shale cores ranges from 9.71 to 33.20 cm3/kg, and the meso-macropore volume ranges from 7.4 to 28.21 cm3/kg. Shale cores in the tectonic deformation zones have higher average pore size, total pore volume, and meso-macropore volume percentage than that of shale cores in the tectonic stability zone. Meanwhile, the specific surface area of shale cores (SBET) ranges from 7.34 to 30.37 m2/g, and the micropore volume of shale cores ranges from 2.29 to 5.82 m3/kg. Shale cores in the tectonic stability zone have higher micropore volume, specific surface area, and percentage of micropore volume than that of shale cores in the tectonic deformation zones. Shale cores in the tectonic stability zone mainly develop micropores and mesopores. Mesopores and macropores are more susceptible to geological tectonism than micropores. The pore structures of shale in the tectonic stability zone have been effectively preserved during tectonic movements. The research results have certain guiding significance for the exploration of shale gas in the complex tectonic regions.