The purpose of this work was to study the depositional mechanisms and significance of the Longmaxi shale in the Sichuan Basin in southern China. Seven lithofacies were identified based on the detailed observation of outcrops and cores using petrographic and scanning electron microscope examination of thin sections and other data analyses: (1) laminated calcareous mudstone, (2) laminated carbonaceous mudstone, (3) laminated silty mudstone, (4) laminated claystone, (5) laminated siliceous shale, (6) siltstone, and (7) massive mudstone. The laminated mudstone and laminated claystone originated from suspension deposition, and siliceous shale is associated with ocean upwelling, whereas massive mudstone and siltstone were primarily deposited by turbidity currents. The depositional mechanisms have a great effect on the source rock and reservoir properties. Suspension deposition near oceanic upwelling zones can provide favorable conditions for the production and preservation of organic matter and are thus conducive to the formation of high-quality source rocks (total organic carbon content up to 5.4%). The reservoir storage spaces are primarily interlaminated fractures and organic pores with good physical reservoir properties (high porosity, permeability, and brittle mineral content). Turbidity currents may carry a large quantity of oxygen to the seafloor, resulting in the oxidation of organic matter, which is unfavorable for its preservation. The lithofacies formed by turbidity currents have relatively low total organic carbon contents (average: <1%). Structural fractures and intergranular pores are the primary storage spaces that are present in the reservoir. In summary, organic-rich shale and siliceous shale that was deposited from suspension near upwelling zones are key exploration targets for shale oil and gas. The widely distributed, multilayer, tight sandstone is important in the exploration for tight oil. A better understanding of the deposition mechanism and its effect on oil reservoirs may assist in identification of favorable areas for exploration.

Petroleum is produced from a beach-bar sand reservoir in the upper subsection of the 4th member of the Paleogene Shahejie Formation (Es₄ ^s) in the eastern Dongying Depression, penetrated by many wells in the Guangli-Qingnan area and this subsection still has further exploration potential. Using drilling and logging data, we analyzed the sand body types, emphasizing the sedimentary characteristics of the beach-bar sand bodies. Combining these data with the concepts of lacustrine and oceanic hydrodynamics, we explain the formation and distribution of the beach-bar sands in the eastern Dongying Depression. The connectivity between beach-bar sand bodies within each individual hydrodynamic zone is better than that between sand bodies in any two adjacent zones. The tempestite sand bodies developed in this area are characterized by typical storm deposits and are located at the bottoms of the beach-bar sand bodies. They grade upward to normal shore and shallow-lacustrine beaches and bars. We also propose a new simple method to estimate the paleo-water depth using the thicknesses of the bar sand bodies in parasequences combined with the principle of hydrodynamic zonation. Based on the distribution of the beach-bar sands in parasequence set 3, we infer that the paleo-wind direction was from the north with an average paleo-wind force of 6 when the major beach-bar sand bodies formed.

The sandstone at the base of the Upper Pennsylvanian Taiyuan Formation (Ct2) is a prominent gas reservoir in the Daniudi gas field, northeastern Ordos Basin, China. Logs, cores, thin sections, outcrops, scanning electron microscopy, and three-dimensional seismic data are used to delineate sedimentary facies and to divide the succession into sequence-stratigraphic units. The gas-bearing coarse-grained sandstone is interpreted as a fluvial-dominated incised-valley fill. A sedimentary hiatus documented for the first time in the study area forms the sequence boundary between the Lower Pennsylvanian Taiyuan Formation (Ct1) and Upper Pennsylvanian (Ct2). The hiatus and overlying incised-valley fill can be observed at the Heidaigou outcrop, where the presence of bauxite indicates a period of subaerial weathering. The incised-valley fill and hiatus can also be found through the outcrop correlation of Xipo, Chengjiazhuang, Qiaotou, and Heidaigou at the eastern margin of Ordos Basin. Two types of basinward and two types of landward facies shifts are identified between Ct1 and Ct2 based on core observations and cross-well profiles analysis to demonstrate abrupt facies change and punctuation. Further evidence for a hiatus is provided by an onlap at this surface on the seismic data.