The Chang 7 member of the Upper Triassic Yanchang Formation in the Ordos Basin is considered to hold the main source rocks for conventional and unconventional oil and gas. The lamination or lithology alteration in vertical and lateral directions, even over a short distance, is a common feature in lacustrine source rocks. The differences in the geochemical characteristics of black shales, dark mudstones and interbedded sandstones have been scarcely reported, and their influences on the petroleum generation potential and shale oil potential are not clear. To this end, 22 core samples were collected from the Lower and Middle Chang 7 (C7-3 and C7-2) members of the Triassic from well CYX in the Qingcheng area. By conducting a series of geochemical analyses including TOC, Rock-Eval pyrolysis yields, bitumen extraction and quantification, and the separation and quantification of saturates, aromatics, resins and asphaltenes, along with biomarker analyses, several results were found. Firstly, the C7-3 and C7-2 source rocks are thermally mature and have entered into the stage of hydrocarbon generation. The C7-3 and C7-2 source rocks have a good to very good hydrocarbon generation potential especially the C7-3 black shales. Secondly, terrigenous source input is more abundant in C7-2, whereas the source input of phytoplankton, algae or microbial lipids is more abundant in C7-3. Moreover, a high TOC content basically corresponds to low wax indexes, terrigenous/aquatic ratios (TARs), and Pr/nC₁₇ and Ph/nC₁₈ ratios and high C₂₇/C₂₉ regular sterane ratios, which suggests that the source input of phytoplankton, algae or microbial lipids is favorable for OM accumulation. Third, analyses of the molecular composition of saturated fractions in shales and interbedded sandstones and the production index demonstrate the migration of petroleum from organic-rich source rocks to their organic-lean counterparts. The C7-2 dark mudstones could be considered as a potential “sweet spot” since their oil saturation index (OSI) was the highest among all the studied samples and they are more enriched in light aliphatic fractions.

The concentric layers of ooids from the modern environment are mostly aragonite and those from the ancient are mostly calcite and Mg-calcite. Dolomitic ooids are rare and are usually formed via the replacement of aragonite or calcite. Here, dolomitic ooids were found in the Pliocene Shizigou Formation in the Qaidam Basin, Northern Tibet Plateau. This paper focuses on whether the dolomitic ooids is a primary precipitate. Optical microscope and scanning electron microscope observation, combined with X-ray diffraction and cathodoluminescence analyses, indicate that the primary mineral of the ooid cortices is poorly ordered dolomite. Extracellular polymeric substances and pyrite were found in the ooids, indicating that the microbe participation was involved in the formation of the ooids. Firstly, the ooids grow on the offshore lake floor. Due to the involvement of sulfate-reducing bacteria and dissolved silica, the nanominerals were precipitated on extracellular polymeric substances. Then, the ooids were transported to strong hydrodynamic surf zones, where the random nanominerals were abraded to form flattened plates as a new polished layer. In addition, a comparison between the carbon and oxygen isotopic compositions and minerals of ooids from different periods indicate that the Pliocene lakes had a lower salinity and were more humid than Pleistocene lakes. Therefore, ooids may be an effective proxy for reflecting the climatic change and uplift history of the Tibet Plateau.

The shale oil reservoirs of Member 7 of the Triassic Yanchang Formation in the Longdong Area of the Ordos Basin have attracted widespread attention due to their unique geological characteristics and enormous development potential. As the core factor controlling reservoir storage capacity and hydrocarbon flow efficiency, the precise characterization and quantitative analysis of pore structure are the prerequisite and key for reservoir evaluation and development plan optimization. All samples selected in this study were collected from the shale of Member 7 of the Triassic Yanchang Formation and were classified into two categories: medium-organic-rich shales (total organic carbon, TOC: 2–6%; TOC refers to the total organic carbon content in rocks, indicating organic matter abundance; unit: %) and high-organic-rich shales (TOC: >6%). The mineral composition and organic geochemical parameters of the shale were determined via X-ray diffraction (XRD) and Rock-Eval pyrolysis experiments, respectively. Meanwhile, pore structure characteristics were analyzed by combining low-temperature nitrogen adsorption–desorption experiments before and after extraction, and multifractal analysis was used to systematically investigate the differences in pore heterogeneity of shale and their influencing factors. The results show that the specific surface area (SSA) and total pore volume (TPV) of shale increased after extraction, while the change in average pore diameter (APD) varied. Multifractal analysis indicates that the micropores of shale both before and after extraction exhibit significant multifractal characteristics; after extraction, pore connectivity is improved, but the changes in pore heterogeneity are inconsistent. The pore connectivity of shale first increases and then decreases with the increase in TOC content and pyrolysis parameter S₂ content. The better the pore connectivity of shale, the lower the content of light-component saturated hydrocarbons and the relatively higher the content of heavy-component resins in the extractable organic matter (EOM). Brittle minerals can provide a rigid framework to inhibit compaction and are prone to forming natural microfractures under tectonic stress, thereby promoting pore connectivity. In contrast, clay minerals, due to their plasticity, are prone to deformation and filling pore throats during compaction, thus reducing pore connectivity. This study provides a theoretical basis for the evaluation and development of shale reservoirs in the Longdong Area.