The hydrothermal fluid–carbonate rock reaction is frequently regarded to occur in deep-burial diagenesis, and the hydrothermal dissolution is usually distributed and takes place along the faults. Previous studies have suggested that there was hydrothermal fluid activity locally in the Permian Qixia Formation in Sichuan Basin, likely related to the Emeishan basalt eruption. However, the effect of hydrothermal fluids on the carbonate rocks of the Qixia Formation in the central uplift of Sichuan Basin is still unclear. Based on the characteristics and geochemical parameters of the diagenetic minerals, this study aims to reveal the diagenetic alteration related to the hydrothermal fluid–rock reaction in the Qixia Formation and reestablish the diagenetic evolution by using the timing of diagenetic mineral precipitation. The methods include petrographic observation; trace and rare earth element (REE) analysis; C, O and Sr isotope measurement; fluid inclusion temperature measurement and cathodoluminescence analysis. According to the petrographic characteristics, the dolostones are mainly of crystalline structure, namely fine-medium crystalline dolostone, meso-coarse crystalline dolostone, and coarse crystalline dolostone, with the cathodoluminescence color becoming brighter in that order. The limestones from the Qixia Formation are of the bioclastic limestone type, with no cathodoluminescence color. Compared with dolostones, limestones have higher Sr content, lower Mn content, and heavier oxygen isotopes. With the crystalline size of dolostone becoming coarser, the oxygen isotopes of dolostones tend to become lighter. The meso-coarse crystalline dolostone has the highest Mn content and negative carbon isotope. Both limestones and dolostones have an obvious positive Eu anomaly in the Qixia Formation. However, the REE patterns of fine-medium crystalline dolostones are very different from those of meso-coarse crystalline dolostones. It is credible that there were two periods of hydrothermal fluid charging, with different chemical compositions. The first period of hydrothermal fluids could laterally migrate along the sequence boundary. Fine-medium crystalline dolostones were almost completely distributed below the sequence boundary and were dolomitized during the shallow burial period. As products of the hydrothermal fluid–dolostone reaction, the saddle-shaped dolomites in the meso-coarse crystalline dolostones were the evidence of the second period of hydrothermal fluids. As a result, the dolomitization model was established according to the timing of diagenetic mineral precipitation, which can improve that the geological understanding of the effect of hydrothermal fluid activities on the carbonate rocks in the Qixia Formation.

The role of the single sandstone stacking pattern in controlling the physical properties of the sandstones deposited in the distribution channels of the deltaic plain is unclear. This study aims to reveal the effect of the single sandstone packing patterns on the reservoir qualities of sandstones from the Shanxi Formation in the Daniudi gas field of Ordos Basin. Based on the core observation, 2D-image analysis, and thin section identification, the lithofacies were identified, the stacking patterns of the single sandbody were divided, and the differences in minerals composition and diagenesis of different sandstone stacking patterns were analyzed. According to the sedimentary facies analysis, 10 types of lithofacies have been identified in the Shanxi Formation in the study area. The single sandstone stacking patterns include mixed stacking patterns of coarse to medium-grained sandstone, fining upwards stacking patterns of coarse- to medium-grained sandstone, and coarsening upwards stacking patterns of fine- to coarse-grained sandstone. Among these single sandstone stacking patterns, there is a greater percentage of lithofacies with high reservoir quality in the fining upwards stacking patterns of coarse- to medium-grained sandstone. Through a comparative study of the differences in minerals composition, the degree of compaction, and dissolution of sandstones, it is suggested that the high porosities and permeabilities of the coarse- to medium-grained sandstone lithofacies in the fining upwards stacking patterns are caused by the low content of the matrix in sandstone, relatively weak compaction, relatively high amounts of primary pores, and strong dissolution. The relatively high content of rigid clastic particles with coarser-grained size was favorable for the preservation of primary pores. The relatively high primary porosity could provide favorable passages for the late diagenetic fluid, leading to the development of dissolved pores. The study can provide an important basis for the exploration of high-quality sandstone reservoirs in the distributary channels of the delta plain.