Micro-petrological characteristics and its sedimentological significance of thin coal seam and mudstone in Pinghu Formation in the northern part of Pinghu Slope, Xihu Sag

The Pinghu Formation in the northern part of the Pinghu Slope is rive-tidal deposits. The hydrodynamic conditions were complex, and there is a controversy on the origin of sandbodies for distributary channels and tidal channels. Through the analysis of micro-petrological characteristics of coal seams and mudstones, the rock structure and mineral composition, the arrangement of clastic particles in mudstones, hydrodynamic conditions, and water medium environment were discussed, and the types of lithologic facies were divided, after which the sandstone microfacies were divided based on sandstone-mudstone integration, and hydrodynamics was analyzed. In particular, the thin coal seam can be categorized into the strawberry-shaped pyrite coal seam indicative of a closed-basin environment, the pure coal seam reflecting an interdistributary bay, and the silty laminated coal seam formed in natural levee. Mudstone can be categorized into horizontal lamellar siderite formed in a closed sag, siderite concretion formed in an interdistributary bay, and crisscrossed lamellar siderite in sandstone. The widely developed siderite in mudstone indicates that an iron-rich and restricted lagoon was developed during the deposition of Pinghu Formation in the north of Pinghu Slope. Minerals in mudstone are mainly clays mixed with kaolinite and illite, indicating a fresh-marine mixed water medium environment. The micropaleontological assemblage in mudstone and macerals of coal indicate that the interbedded box-shaped and bell-shaped sandbodies are channel deposits under delta background, and the sandbodies are the complex of multi-phase channel superposition. The microscopic petrological analysis of thin coal seams, mudstones, and related sandstones shows that the Pinghu Formation was formed in a typical gentle-slope water-restricted shallow background, in which terrigenous organic matter was rich, mudstones were carbon-abundant, and sandbodies were dominated by one-way flow sedimentation. This study provided new data and a reference for reconstructing the sedimentary framework, understanding the genesis, and predicting sand bodies and thin coal seams in the Pinghu Formation in the Xihu Sag.