To elaborate genesis of the Ordovician Ma5⁵ Sub-member dolomites in the Ordos Basin, this study implemented a series of analyses, including petrological investigation and geochemical (constant and trace elements, and isotopes) characterization. Three main types of dolomites were identified, namely micritic dolomite, micritic silty dolomite and silty dolomite. Specifically, the micritic dolomite is featured by dull red luminescence, low degree of order, high Sr isotope value, positive linear MgO–CaO correlation, high contents of Na and Sr elements, low contents of Fe and Mn elements, obvious negative δ¹³C excursion and slightly positive δ¹⁸O value, all of which indicate a syngenetic–penesyngenetic closed, high salinity environment. In comparison, the micritic silty dolomite is characterized by no or extremely dull red luminescence, moderate-to-high degree of order, higher δ¹³C value, slightly lower ⁸⁷Sr/⁸⁶Sr ratio, and sharp decrease in contents of K, Al, Fe and Mn elements, all of which indicate this type of dolomite to be the product of seepage–reflux dolomitization. Moreover, difference in geochemical characteristics between micritic silty dolomites in the depression and those in the platform flat suggests the Mg-rich brine from the tidal flat to be the dolomitization fluid for the platform flat while the Mg-rich brine from the overlying Ma5⁴ Sub-member to be the dolomitization fluid for the depression. Meanwhile, the silty dolomite is observed to have no or extremely dull red luminescence, high degree of order, heavy δ¹⁸O value, high Sr isotope, lower Na content, and higher Fe and Mn contents, which are attributed to burial dolomitization in the fresh water environment. The distribution of dolomite was demonstrated to be controlled by sedimentary microfacies, and the dolomite in the West Jingbian dolomitic flat area turns out to have the largest distribution area and thickness as well as the best reservoir capacity. Massive fractures and vugs were formed in the dolomitization process, which were the basis of high-quality reservoir formation, involving generation of a relatively small amount of intercrystalline micropores during the syngenetic–penesyngenetic dolomitization, intercrystalline pores after the penecontemporaneous dolomitization, and intercrystalline and dissolution pores during the burial dolomitization. Moreover, dissolution during hypergene and burial periods contributed to most of the reservoir space, while filling effect during the burial period significantly reduced porosity.

为了奥陶纪MA5的精心创⁵鄂尔多斯盆地小组成员白云岩，本研究实施了一系列的分析，包括岩石调查和地球化学（常量和微量元素，以及同位素）表征。确定了三种主要类型的白云岩，即泥晶白云岩、泥晶粉砂质白云岩和粉砂质白云岩。具体而言，泥晶白云岩发光暗红色，有序度低，Sr同位素值高，MgO-CaO呈正线性相关，Na、Sr元素含量高，Fe、Mn元素含量低，δ ¹³ C明显为负偏移和略正 δ ¹⁸O 值，所有这些都表明同基因-同基因同基因的封闭、高盐度环境。相比较而言，泥晶粉质白云岩的特点是无或极暗红色发光，中到高度有序，δ ¹³ C 值较高，⁸⁷ Sr/ ⁸⁶ Sr 比值略低，K、Al 含量急剧下降, Fe 和 Mn 元素，所有这些都表明这种类型的白云石是渗流-回流白云石化的产物。此外，坳陷泥晶粉质白云岩与台坪泥晶白云岩地球化学特征的差异表明，潮坪富镁卤水为台坪白云石化流体，而上覆马五⁴富镁卤水为台坪白云石化流体。子构件为洼地白云石化液。同时，观察到粉质白云岩没有或非常暗淡的红色发光，高度有序，重δ ¹⁸O 值、高 Sr 同位素、较低的 Na 含量以及较高的 Fe 和 Mn 含量，这归因于淡水环境中的埋藏白云石化。白云岩分布受沉积微相控制，西靖边白云质平地白云岩分布面积最大，厚度最大，储集能力最好。白云石化过程中形成大量裂缝和孔洞，是形成优质储层的基础，包括同生-准同生白云石化过程中产生少量晶间微孔、准同生白云石化后的晶间孔以及晶间和溶蚀作用埋藏白云石化过程中的孔隙。而且，