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 lithofacies and pore structural characteristics of shale reservoirs directly affect the exploration and development of shale gas. To clarify the exploration and development potential of the Lower Cambrian Shuijingtuo Formation (SJT) shale in the Middle Yangtze region, China, this study employs integrated experimental approaches, including optical and scanning electron microscopy (SEM) observations, X-ray diffraction (XRD) mineralogical analysis, and low-pressure gas (N₂/CO₂) adsorption, to classify mudstone lithofacies within the SJT and elucidate pore structural characteristics and dominant geological control across different lithofacies. The research results show that (1) Six main types of shale lithofacies are found in the STJ, including low-TOC massive calcareous mudstone (LMCM), low-TOC laminated mixed mudstone (LLMM), medium-TOC massive mixed mudstone (MMMM), high-TOC massive mixed mudstone (HMMM), high-TOC laminated siliceous mudstone (HLSM), and laminated argillaceous mudstone (LAM). (2) The pore types of SJT mudstone primarily include organic pores, intragranular clay mineral pores, and microfractures. The pore structure of mudstone is mainly controlled by clay mineral content and TOC content. However, the controlling factors of pore structure vary among different mudstone lithofacies. LMCM and LLMM are dominated by intragranular clay mineral pores, with their pore structures mainly controlled by clay mineral content. The pore types of HLMM and HLSM are organic pores, with pore structures predominantly controlled by TOC content. (3) The SJT mudstone gas reservoir exhibits diverse types, including HLSM, LAM, and LLMM. HLSM is characterized by the highest brittleness index and elevated pore volume (PV) and it can be considered the optimum lithofacies in the study area. Additionally, LLMM has the highest PV and relatively high brittleness index, positioning it as another significant reservoir target in the study area. Therefore, the Lower Cambrian shale gas reservoirs in the Middle Yangtze region exhibit diverse reservoir types. These research findings provide a scientific basis for the next phase of shale gas exploration planning in the Lower Cambrian.

Nanopores hosted in secondary organic matter (SOM) serve as critical spaces for hydrocarbon storage and migration within marine shales. Scanning electron microscopy (SEM) could enable direct visualization of nanopore structures within shale reservoirs, but analyzing large volumes of data solely through manual methods is time-consuming, labor-intensive, and highly subjective. Using SEM image data from Wufeng-Longmaxi Formation shale samples with varying maturity in the Sichuan Basin, this study proposes an improved U-Net model integrated with a multiscale input module and a hybrid attention mechanism for high-precision automated segmentation and quantitative characterization of SOM-hosted nanopores of shale reservoirs. Compared with the standard U-Net, the improved model shows significantly higher segmentation accuracy (porosity error of 0.10 vs 0.16), especially for small pores, and improves boundary delineation while reducing misclassifications from mineral grains. This enables rapid, reproducible, and large-scale quantitative analysis of nanopore networks. In the studied shale samples, SOM primarily occurs in the interparticle pores of rigid grains, intraparticle pores of clay platelets and pyrite framboids, and the associated nanopores display predominantly bubble, irregular, or spongy shapes. Moreover, the SOM-hosted nanopores are relatively sparse in the studied shale samples with the relatively low maturity [Equivalent vitrinite reflectance (EqVRo) < 1.3%]. At the EqVRo ranging from 1.3 to 3.0%, a dense pore network is formed within the SOM due to the intense thermal cracking, and the number of nanopores reaches the maximum. At the higher maturity (EqVRo > 3.0%), the graphitization of SOM and the coalescence of aromatic structures into tightly stacked graphite-like layers lead to the degradation and collapse of larger organic pores. This study not only proposed an efficient tool for nanopore characterization of shale reservoirs, but also established a pore evolution trend of SOM-hosted nanopores, serving as a critical microscopic indicator for predicting favorable zones in shale gas exploration.

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.