The 5th sub-member in 5 Member Ordovician Majiagou Formation in Daniudi Gas Field, Ordos Basin, is deposited in an environment consisting of an ancient epicontinental sea, where very fine crystalline dolostone reservoir has developed. In this study, based on the petrological and geochemical characteristics, the genesis of the dolomite developed in M₅⁵ were studied by analyzing the properties and sources of the dolomitization fluids, and the influence of the paleogeomorphology differences on the distribution of dolostone was also discussed in order to clarify the distribution of the dolostone developed in the lime flat of the epicontinental sea. The dolostone of the M₅⁵ had a crystal structure, mainly including microcrystalline and very fine crystalline. The content of MgO and CaO in dolomite was negatively correlated, indicating that it was the result of replacement. The dolomite was dark red under cathode luminescence, and the distribution mode of rare earth elements showed the negative anomaly of Ce and Eu, indicating that the dolomitization fluid was sea-sourced fluid. The δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr isotope range of limestone was similar to that of Ordovician seawater in the study area, whereas the δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr of dolostone were obviously more positive than that of limestone. The substitute index of the salinity (Z) of the dolomitization fluid was higher than 122, which is higher than limestone (Z = 120.5), indicating that the dolomitization fluid was slightly evaporated seawater. The wormholes observed on the core and the gypsum in the penecontemporaneous period observed in the thin sections indicated that the dolostone was formed in a period when the sea level was relatively low, and it was the result of seepage–reflux dolomitization. By analyzing the correlation between the thickness of dolostone and the paleogeomorphology of the M₅⁵ of the sedimentary period, it was found that the thickness of dolostone at relatively high altitude was significantly larger than that of other areas. The development of dolostone was controlled by sea level, and the local paleogeomorphology controls the distribution of dolostone during the period of low sea level. There were many more limestone–dolostone cycles and larger cumulative thicknesses of dolostone at relatively higher topography. This study provides a theoretical basis for the prediction of the distribution of dolostone reservoirs in the carbonate tidal flat environment dominated by lime flats under the background of the ancient epicontinental sea.

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.

Combined with thin sections and electronic probe microanalyzer (EPMA) analysis, we studied the effect of the chemical composition of tuffaceous materials on the quality of sandstone reservoir in the Dingbei area. Analysis of 100 thin sections from five drilled wells indicates that the tuffaceous materials in sandstones from the Taiyuan and Shihezi Formation are common due to the volcanic eruptions. There are three types of tuffaceous materials in the sandstone: intermediate-basic type, intermediate-acid type, and mixed type. Seven thin sections were selected to analyze the chemical elements of tuffaceous materials by EPMA, with 3–8 test points on each thin section. The results indicate a clear decline in most major elements except for SiO2 and Al2O3. The elemental differentiation of tuffaceous sandstone is largely controlled by the activity of the diagenetic system. Under the slow-moving fluid, the active elements such as K, Na, Ca, Mg, and Fe are lost, and Si and Al are enriched. The SiO2 contents of EPMA results are up to 42%. Under the strong-flow fluid, Si and Al can be transferred to precipitate the kaolinites. In the closed diagenetic system, the contents of active elements such as Na and K are high. Dissolution or alteration of tuffaceous materials with different types under the acid fluid is one of the reasons for the generation of heterogeneous reservoirs. The intermediate-acid tuffaceous materials with high SiO2 content experience transformation without dissolution. The mixed tuffaceous materials are mainly transformed into kaolinite to form intercrystalline pores. Due to a large number of soluble components in the intermediate-basic tuffaceous materials, a large number of secondary pores are developed.