Pore characteristics and evolution mechanism of shale in a complex tectonic area: Case study of the Lower Cambrian Niutitang Formation in Northern Guizhou, Southwest China

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The Lower Cambrian Niutitang Formation is one of the most important shale reservoirs in the Northern Guizhou. To study the characteristics of pore structures and the relationships between shale's physical properties and pore structures in the complex tectonic area, from the perspective of hydrocarbon generation and evolution, the mineral composition, pore structure characteristics of shale samples in northern Guizhou were analyzed using X-ray diffraction (XRD), optical microscopic composition analysis, focused ion beam-scanning electron microscopy (FIB-SEM), vitrinite reflectivity and nitrogen adsorption experiments. Results indicate that the pore structures can be categorized into conical, ink bottle, spherical, elliptical, beaded, irregular, and plate-like intersecting microfractures. Shale samples are organic-rich with an average total organic carbon content of 4.39%. Organic matter is consumed within the shale matrix during the hydrocarbon generation and evolution, which increases the organic pore size and the proportion of organic pores. The high TOC content is conducive to developing organic pores. In addition, the average content of brittle minerals is greater than 70%. High content of brittle minerals during diagenesis is conducive to developing the residual pores at the edges of mineral particles and dissolved pores within mineral particles. Thus, the high content of brittle minerals and TOC content is favorable for the development of meso-macropores. The content of clay minerals within the shale samples ranges from 6.3% to 42.2%, with an average of 20.02%. Micropores and mesopores mainly occur in the clay minerals. Pores in clay minerals are easily compacted under tectonic stress. Meanwhile, high thermal evolution degree and the vitrinite reflectance Ro greater than 3.5 is not conducive to developing pores. Hence, high TOC content, high brittle minerals content, and thermal evolution degree ranging from 0.6% to 3% make the Lower Cambrian Niutitang Formation shale reservoirs favorable for shale gas exploration.