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This study focuses on the origin of dolomite within the Middle Permian Qixia Formation in the northwestern Sichuan Basin by the approaches of petrographic examination and stable isotopic as well as clumped isotopic analysis. Two types of dolomite (matrix dolomite and dolomite cement) in the upper part of the Qixia Formation at the Chejiaba (CJB) section were classified. The gray matrix dolomite shows a clear inter-finger contact with limestone, while the milky white dolomite cement lines the edges of dissolved pores. Microscopic examination revealed three types of matrix dolomite including floating fine crystalline, planar-e(s) matrix dolomite (Md1); fine to medium crystalline, planar-e(s) matrix dolomite (Md2); and medium to coarse crystalline, planar-a matrix dolomite (Md3), along with saddle dolomite cement (Sd). The presence of Md1 dolomites along stylolites suggests they were formed due to pressure dissolution during shallow burial. The similar isotopic signatures of Md2 and Md3 with the host limestone indicate that the dolomitizing fluids likely originated from Permian seawater. However, elevated diagenetic temperatures suggest an influence from the Late Permian volcanic activity. The Sd dolomite and subsequent calcite cementation with negative δ18O values and radioactive 87Sr/86Sr ratios point to formation in high-temperature, externally-derived, radio-genic strontium-rich dolomitizing fluids, possibly from deep-seated thermal fluids within siliciclastic strata. The formation of dolomite in the Qixia Formation of the northwestern Sichuan Basin was influenced by both original sedimentary facies and faulting activities. The influx of hydrothermal fluids triggered widespread dolomitization, resulting in the creation of numerous dissolved and inter-crystalline pores, which played a beneficial role in reservoir development.

Abundant natural gas resources are stored in the Lower Permian Qixia Formation of the Sichuan Basin. In recent years, great breakthrough for oil and gas exploration has been achieved in the Qixia Formation in the NW Sichuan Basin, and high-quality reservoirs mainly locate in dolomite strata. However, the origin of dolomites in the Qixia Formation is still unclear. Special zebra-textured dolomites were observed in the Qixia Formation from the cores of well ST 18. In this paper, a comparative study of the light and dark belts in the zebra texture was systematically carried out with petrological and geochemical methods. Results show that the light belt is mainly composed of medium to coarse crystalline saddle dolomites and its cathodoluminescence is bright red, while the dark zone is composed of medium to fine crystalline dolomites and its cathodoluminescence is dark red. The δ13C values in light and dark zones are within the δ13C range of contemporary seawater, while the δ18O values are more negative than those of contemporary seawater. It is then indicated that the dolomitization fluid was derived from seawater and the oxygen isotopes were fractionated by the affection of high temperature. The order degrees of dolomites in the light and dark zones are consistently high. It was speculated that the order degrees of dolomites in the light and dark zones had been matured to the same degree during the later burial stage. In addition, the occurrence of pyrite confirmed the involvement of hydrothermal fluids in the formation of dolomites. Therefore, the formation of zebra dolomites in well ST 18 mainly went through the following three stages: (1) Fluids dissolved and dolomitized primary limestones along tectonic fractures to form medium to fine crystalline dolomites in the dark zone; (2) Hot fluids precipitated to form medium to coarse crystalline saddle dolomites in the light zone; (3) The order degrees of dolomites in the dark and light zones had adjusted to the same degree in the later deep burial stage.

A set of organic-rich black rocks (23.8 m), mainly consisting of siliceous rocks, shale and limestones, outcrops between carbonate rocks of the Maokou and Wujiaping formations in the Xibeixiang section, northwestern Sichuan Basin, at the margin of the Upper Yangtze Platform. Detailed conodont stratigraphy, organic petrology and organic geochemistry studies were performed on samples from the Xibeixiang section. The conodonts found in the study area were Jinogondolella prexuanhanensis, J. xuanhanensis and Clarkina posbitteri hongshuiensis. All of them lived at the end of Guadalupian, confirming that the black rocks in the section were deposited during the late period of the Maokou Formation indicating that the Guangyuan-Wangchang Marine Trough began to develop in the late Guadalupian stage on the Upper Yangtze Platform. The detailed study of organic petrology reveals that benthic algae is the main contributor for hydrocarbon in the black rock series, with a small amount of macroplanktonic algae, and the organic matter is type Ⅱ. The organic geochemical analyses of the black rocks show that the TOC content ranges 1.04% to 32.58% and the chloroform bitumen "A" content ranges 0.03% to 1.05%, indicating favorable source rocks. Thermal parameters, such as the vitrinite reflectance (Ro) value ranges 1.0% to 1.4%, the Tmax value ranges 440 to 460℃, the conodont color index (CAI) ranges 1.5 to 2.5, the Ts/Tm ratio ranges 0.35 to 1.43, the moretane/hopane ratio ranges 0.05 to 0.39, the C2920S/(20S+20R) ranges 0.39 to 0.65, and the C29αββ/(αββ+ααα) ranges 0.26 to 0.58, which indicate that these black rocks are mature to highly mature. The integrated study of lithology, hydrocarbon-forming organisms, conodonts and biomarkers indicate that these black rocks were deposited in a relatively reductive deep-water environment with a high salinity.