The Middle Ordovician Majiagou Formation in the northern Ordos Basin consists of a sequence of subtidal and peritidal carbonates that are extensively dolomitized. The diagenetic evolution of the Ma5 5 Sub-member was investigated to determine the factors controlling reservoir quality in the northern Ordos Basin. Dolomite in the Ma5 5 Sub-member is classified into three main types on the basis of crystal size: dolomicrite (D1), micritic–silty and fine silty dolomite (D2), and silty (and coarser) dolomite (D3). The Dolomite D1 was produced by dolomitization in a low-salinity diagenetic environment and transformed by near-surface freshwater leaching during shallow burial. The Dolomite D2 developed as a result of post-penecontemporaneous seepage–reflux dolomitization and was affected by later burial dolomitization. The Dolomite D3 was produced during burial dolomitization. Although the original porosity of the Dolomite D1 was low, freshwater diagenesis increased the porosity significantly. As a result of the transformation of montmorillonite to illite, there was a deficiency in the Mg 2+ necessary for Dolomite D2 formation, resulting in the nonuniform dolomitization of this type of dolomite. The intercrystalline pores of the Dolomite D2 are mostly filled with montmorillonite and illite, reducing its porosity and permeability and consequently making the Dolomite D2 a poor reservoir. The Dolomite D3 has a higher porosity and contains various types of dissolution and intercrystalline pores, generating a higher permeability. This dolomite is a potential reservoir in the study area and a possible target for oil and gas exploration. Overall, the results of the study suggest that the quality of a potential dolomite reservoir is strongly controlled by the diagenetic history of the host carbonates.

中文翻译：

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鄂尔多斯盆地北部马家沟组中奥陶统马55亚段白云岩化及油气勘探意义

鄂尔多斯盆地北部中奥陶统马家沟组由一系列广泛白云石化的潮下带和潮带碳酸盐岩组成。研究马五 5 亚段成岩演化，确定鄂尔多斯盆地北部储层质量控制因素。马五 5 亚段白云岩按晶体大小主要分为 3 种类型：白云岩（D1）、泥晶-粉质细粉质白云岩（D2）和粉质（粗粒）白云岩（D3）。白云岩 D1 是在低盐度成岩环境中通过白云石化作用产生的，并在浅埋过程中通过近地表淡水浸出转化。白云岩 D2 是准同生期后渗流-回流白云石化作用的结果，并受到后期埋藏白云石化作用的影响。白云岩 D3 是在埋藏白云石化过程中产生的。虽然白云岩 D1 原始孔隙度较低，但淡水成岩作用显着增加了孔隙度。由于蒙脱石向伊利石转变，白云石D2形成所必需的Mg 2+ 缺乏，导致该类白云石的白云石化不均匀。白云岩 D2 的晶间孔隙多被蒙脱石和伊利石充填，降低了其孔隙度和渗透率，使白云岩 D2 成为较差的储层。白云岩 D3 孔隙度较高，含有多种溶蚀孔和晶间孔，渗透率较高。该白云岩是研究区的潜在储层，也是油气勘探的可能目标。全面的，