The Lower Paleozoic marine shale in southern China has undergone several strong tectonic transformations in an extensive region outside the Sichuan Basin. Although some shale strata underwent strong deformation, they still contain a significant amount of shale gas. The gas preservation mechanism in the strongly deformed shale has become the focus of attention. In this paper, the Lower Cambrian gas-bearing shale samples with a strong deformation taken from an exploration well in northern Chongqing, China, were investigated on their pore types and structure, with the aim to reveal the reason for the gas preservation. The pore types of the Lower Cambrian shale are dominated by microfractures and interparticle (interP) pores occurring mainly between clay minerals and between organic matter (OM) and clay minerals, while pores within OM that can be observed by FE-SEM (field emission-scanning electron microscopy) are rare. The shale has a low porosity, with an average of 1.56%, which is mainly controlled by the clay mineral content. The adsorption experiments of low pressure N₂ (LPNA) and CO₂ (LPCA) indicate that the shale is rich in micropores and small mesopores (<2–3 nm) provided mainly by OM, but mesopores with a size range of 3–50 nm are underdeveloped. The shale, as revealed by LPNA data, has dominant slit-like or plate-like pores and an obvious low-pressure hysteresis (LPH), indicating a low gas diffusion. The deformed shale samples with a removal of OM by oxidation and their isolated kerogen further indicate that the LPH is completely related to OM, without any relationship with minerals, while an undeformed shale sample, taken from another well for a comparison, has no obvious LPH for both of its OM-removed sample and kerogen. Based on a comprehensive analysis of the relative data, it is suggested that the nanopores related to OM and clay minerals in the shale were significantly altered owing to the deformation, with a result of the pores being squeezed into the slit-like shape and converted into micropores. This extraordinary pore structure of the shale formed during the deformation process should be the main preservation mechanism of shale gas.

Current models of sedimentary environments and organic matter (OM) enrichment for the Lower Cambrian black shales in the Yangtze Platform have not yet incorporated its northern carbonate platform margin where the related research is lacked. This study focuses on the SNZ1 well in the northern carbonate platform margin, utilizing total organic carbon (TOC) content and major and trace element data to reveal the main controlling factors of OM enrichment during the Early Cambrian. The results show that the shale stratum is tentatively ascribed to the Lower Cambrian Stage 3 and that, during its deposition, the redox transitioned from anoxic to suboxic–oxic conditions, the hydrodynamic conditions weakened initially and then strengthened, the primary productivity first increased and then decreased, the paleoclimate shifted from arid–cold to warm–humid conditions, and the terrigenous clastic input gradually diminished. Overall, the OM enrichment is primarily controlled by preservation conditions. By systematically analyzing the data from the intraplatform basin to the deep-sea basin across the Yangtze Block, a model of the sedimentary environments and OM enrichment during the Early Cambrian was suggested. Additionally, this study highlights the intrinsic link between the expansion of oxygenated surface water and the Cambrian explosion. These results provide critical insights for shale gas exploration in this region.

The Lower Cambrian shale has been considered an important target for shale gas exploration in South China. However, systematic research on the differences in pore structure and their controlling factors in shales from different sedimentary facies is still lacking, which hinders the accurate prediction of favorable areas. In this study, pore types, structural parameters, heterogeneity and connectivity of the Lower Cambrian shale samples from the shallow shelf, deep shelf and slope were qualitatively and quantitatively characterized by using field emission scanning electron microscope (FE-SEM), low-pressure CO₂ adsorption (LPCA), low-pressure N₂ adsorption (LPNA) and multifractal theory, in order to reveal the reservoir characteristics and differences among these sedimentary facies. The results show that organic pores dominate the pore space in shales from different sedimentary facies, but their morphology and abundance are controlled by the depositional environment. The shallow shelf shales are characterized by scattered organic pores and abundant intragranular pores. Deep shelf shales feature uniformly distributed bubble-like organic pores and have the highest non-micropore volume. Slope shales contain the most developed micropores, with organic pores mainly occurring within organic matter (OM). Total organic carbon (TOC) is the primary factor controlling pore development. It affects pore connectivity and heterogeneity by regulating the development of pore space. Micropore connectivity increases with TOC content. In contrast, non-micropore connectivity first increases and then decreases as TOC increases. The influence of mineral components on shale pore structure varies with sedimentary facies. Clay minerals promote micropore development in shallow and deep shelf shales by adsorbing abundant OM. Carbonate minerals inhibit micropore development through diluting TOC content and cementing pore. However, dissolution of carbonate minerals can contribute to non-micropore space in shallow shelf shales. Deep shelf and slope shales with TOC between 4% and 6% have optimal pore volume, high connectivity, and low heterogeneity, and are the main intervals for forming high-quality shale reservoirs. This study clarifies the pore structure characteristics and controlling factors of shale reservoirs in different sedimentary facies, providing a theoretical basis for predicting and exploring shale gas sweet spots in the Lower Cambrian of South China.