Sedimentology Dominated Accumulation Mechanism of Marine Shale Gas

Recent shale gas drilling wells in southern China confirms significant resource potential within the Lower Carboniferous marine shale of the Yaziluo Rift Trough, yet exploration efficiency is hindered by poorly understood sedimentological heterogeneity and gas accumulation mechanisms. This study integrates major and trace elements, high-resolution field emission scanning electron microscopy, and adsorption analyses (low-pressure N₂/CO₂, high-pressure CH₄) to systematically characterize sedimentary facies, reservoir properties, and gas enrichment patterns. The trough exhibits an asymmetric "platform-basin" model with three distinct sedimentary facies. Basin facies comprise siliceous shale formed in deep-water anoxic settings, yielding the highest total organic carbon (TOC) content (average 3.63%) controlled by redox conditions and paleoproductivity. Lower slope facies consist of mixed shale in dysoxic environments, with moderate TOC (average 1.80%), dominated by redox conditions. Upper slope facies are calcareous shale in shallow, weakly reducing settings, showing the lowest TOC (average 0.99%), influenced by clay-mediated organic preservation. Reservoir analysis reveals that basin facies are dominated by organic pore, whereas lower slope facies display reduced organic pores but increased inorganic pores and micro-fractures, and upper slope facies shift predominantly to inorganic pores and microfractures. Moving from basinward to upper slope, increasing carbonate content expands dissolution pore networks, yet declining TOC diminishes organic pore development, promoting organic-clay complexes and weakening pore structure. Additionally, three shale associations classified by shale-to-argillaceous limestone ratios correspond to specific sedimentary facies. The lower slope shale association demonstrates optimal gas preservation due to high TOC, argillaceous limestone interlayers acting as direct caps, and fracture-enhanced porosity facilitating gas migration. The upper slope association shows promise for self-sealing bodies via acid fracturing despite lower TOC. In contrast, the basin facies shale association exhibits constrained gas retention capacity owing to clay-dominated mineralogy and absence of argillaceous limestone interlayers. This study emphasizes the critical role of lithofacies heterogeneity and integrated "source-reservoir-seal" configurations in evaluating of shale gas accumulation under "slope-basin" depositional architectures, providing a theoretical basis for reservoir development in analogous geological settings.