Typical dolomite reservoirs exist in the upper assemblage of the Ordovician Majiagou Formation in the southeastern Sulige gas field, Ordos Basin, however, the current understanding of dolomite genesis needs to be clarified. This study confirms the dolomitization characteristics, diagenetic environment, and genesis of dolomite through the use of core observations, thin section identification, and geochemical data (carbon and oxygen stable isotopes, strontium isotopes, and major and trace element contents). The results showed that dolomite samples from the study area includes micritic dolomite, powder crystal dolomite, and micrite to powder crystal dolomite, in which the main reservoir space consisted of intergranular pores, intercrystalline pores and various dissolved pores. The δ13C value of the dolomite samples ranged from −7.26‰ to1.28‰ with an average of −0.45‰, which is equivalent to that of seawater during the Ordovician. The δ18O value ranged from −9.94‰ to −6.32‰ with an average of −7.86‰, which is clearly more negative than that of the Ordovician seawater. The 87Sr/86Sr ratio varied from 0.70867 to 0.71033 with an average of 0.70946, which is significantly more positive than that of Ordovician seawater. The Mg/Ca ratio was lower than that of the seawater and close to 1:1. Overall, all of the samples were characterized by high Fe and Mn contents, while low Sr and Na contents. These results clearly indicate that the dolomitized fluid was closely related to seawater or a sea-source fluid. Furthermore, the restored paleosalinity, paleotemperature, and paleodepth also indicate that the dolomite was formed in a marine reducing environment. Combined with the petrological characteristics and geochemical indicators, two dolomitization models (mixed water dolomitization and burial dolomitization) were established for the upper assemblage of the Ordovician Majiagou Formation in southeastern Sulige gas field. We infer that mixed water dolomitization was dominant before the Carboniferous, whereas burial dolomitization was dominant after the Early Permian.

中文翻译：

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鄂尔多斯盆地苏里格气田东南部奥陶系马家沟组上组合白云岩成因：来自C、O、Sr同位素及主微量元素的证据

鄂尔多斯盆地苏里格气田东南部奥陶系马家沟组上组合存在典型的白云岩储集层，但目前对白云岩成因的认识有待进一步阐明。本研究通过岩心观测、薄片鉴定、地球化学数据（碳氧稳定同位素、锶同位素、主微量元素含量等）证实了白云岩化特征、成岩环境和白云岩成因。结果表明，研究区白云岩样品包括泥晶白云岩、粉晶白云岩和泥晶至粉晶白云岩，其中主要储集空间为粒间孔、晶间孔和各种溶蚀孔。白云岩样品的δ13C值为-7.26‰至1。28‰，平均为-0.45‰，与奥陶纪海水相当。δ18O 值为-9.94‰～-6.32‰，平均值为-7.86‰，明显低于奥陶系海水。87Sr/86Sr 比值从 0.70867 到 0.71033 不等，平均值为 0.70946，明显高于奥陶系海水。Mg/Ca 比低于海水，接近 1:1。总的来说，所有样品的特点是铁和锰含量高，而锶和钠含量低。这些结果清楚地表明，白云石化流体与海水或海源流体密切相关。此外，恢复的古盐度、古温度和古深度也表明白云岩形成于海相还原环境。结合岩石学特征和地球化学指标，建立了苏里格气田东南部奥陶系马家沟组上组合两种白云石化模式（混合水白云石化和埋藏白云石化）。我们推断，在石炭纪之前，混合水白云石化占主导地位，而早二叠世之后，埋藏白云石化占主导地位。