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.

中文翻译：

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表陆海环境下古地貌控制的白云石化作用——以鄂尔多斯盆地大牛地气田奥陶系马家沟组五段五亚组为例

鄂尔多斯盆地大牛地气田奥陶系马家沟组5亚段5亚段沉积于古陆表海环境，发育极细晶白云岩储层。本研究基于岩石地球化学特征，通过分析白云石化流体的性质和来源，研究了M ₅ ⁵发育白云岩的成因，并探讨了古地貌差异对白云岩分布的影响。为明确陆缘海石灰滩发育的白云岩的分布。M ₅ ⁵的白云岩具有晶体结构，主要包括微晶和极细晶。白云石中MgO和CaO的含量呈负相关，说明是置换的结果。白云岩在阴极发光下呈暗红色，稀土元素分布方式呈Ce、Eu负异常，表明白云石化流体为海源流体。灰岩的δ ¹³ C、δ ¹⁸ O、⁸⁷ Sr/ ⁸⁶ Sr同位素范围与研究区奥陶系海水相似，而δ ¹³ C、δ ¹⁸ O、⁸⁷ Sr/ ⁸⁶白云岩的 Sr 明显高于石灰岩。白云石化液的盐度（Z）替代指数高于122，高于石灰石（Z=120.5），说明白云石化液为微蒸发的海水。岩心上观察到的虫洞和薄片观察到的准同生期石膏表明白云岩形成于海平面较低的时期，是渗流-回流白云石化作用的结果。通过分析白云岩厚度与M ₅ ⁵古地貌的相关性沉积期发现，海拔较高的白云岩厚度明显大于其他地区。白云岩的发育受海平面控制，低海平面时期局部古地貌控制着白云岩的分布。在相对较高的地形上，有更多的石灰岩-白云岩旋回和更大的白云岩累积厚度。该研究为古陆表海背景下以石灰滩为主的碳酸盐岩滩涂环境中白云岩储层分布预测提供了理论依据。