鄂尔多斯盆地大牛地地区马五5亚段发育优质白云岩储层,白云岩成因是天然气勘探中储层预测的关键.结合岩石学特征,通过对碳氧同位素、微量元素及流体包裹体等测试资料的分析,深入探讨了大牛地地区马五5亚段白云岩的地球化学特征及成因机制.研究表明:①马五5亚段主要发育泥晶白云岩和粉细晶白云岩,泥晶白云岩主要发育于马五5亚段底部,粉细晶白云岩主要发育于马五5亚段中上部.②泥晶白云岩中发育水平藻纹层、鸟眼及针状膏岩假晶等原始沉积构造,孔隙不发育,白云石有序度较低,微量元素Sr、Na含量高,Fe、Mn含量低,岩石形成时期的盐度与泥晶灰岩基本一致.推断泥晶白云岩为准同生阶段的产物.③粉细晶白云岩中常见残余砂屑和雾心亮边结构,孔隙发育且以晶间孔为主,白云石有序度相对偏高,与泥晶白云岩相比,Sr、Na含量低,而Fe、Mn含量高,岩石形成时期盐度与泥晶灰岩相似.推断粉细晶白云岩为回流渗透作用下的产物.④马五5亚段白云石化流体主要为海源性流体,无大气淡水及热液流体的参与.基于上述岩石学和地球化学特征,建立了大牛地地区马家沟组马五5亚段准同生和回流渗透白云石化模式,为研究区优质白云岩储层的预测及地质勘探方向选择提供了理论指导.

Since the discovery of high quality dolomite reservoirs in Submember Ma55 of the Ordovician Majiagou Formation (O1m55) in Daniudi area of the Ordos Basin, the dolomite genesis has become the key to natural gas exploration in this interval. Based on petrological characteristics, the geochemical characteristics and genesis of O1m55 dolomite in Daniudi area are systematically studied with geochemical test data such as isotopic compositions, trace element contents, and fluid inclusion data. The results show that: (1) There are two types of dolomite, i.e., micrite dolomite and very fine-fine crystalline dolomite in O1m55, and the former mainly developed at the bottom, the latter mainly developed in the upper-middle part. (2) In the micrite dolomite, the original sedimentary structures such as horizontal algae lamina, bird-eye and acicular gypsum pseudocrystal are developed, the pores were not developed. The order degree of dolomite was low, with an average value of 0.63. The average value of δ13C and δ18O are -0.65‰ and -6.65‰ respectively. The content of Sr and Na (151 μg/ g and 488 μg/ g respectively）is high, and the content of Fe and Mn (1 202 μg/ g and 49 μg/ g respectively) is low. The salinity (24.2，) during the formation period is basically consistent with that of micrite limestone. It is inferred that micrite dolomite should be the product of the quasi syngenetic stage in tidal flat environment of limited platform. (3) In the very fine-fine crystalline dolomite, the sedimentary structures of residual sand debris and "fog core and bright edge" are common, and the intercrystalline pores are developed. The order degree of dolomite is relatively high, with an average value of 0.89. The average values of δ13C and δ18O are -1.33‰ and -8.91‰ respectively. Compared with the micrite dolomite, the content of Sr and Na（78 μg/g and 426 μg/g respectively）is low, while the content of Fe and Mn ( 1 974 μg/g and 77 μg/g respectively) is high. The salinity (22.6，) during the formation period is similar to that of micrite limestone. It is inferred that the very fine-fine crystallinefine dolomite should be the product of reflux infiltration in shallow burial stage in tidal flat shoal environment. (4) The dolomitization fluid of O1m55 is mainly sea-sourced, without the participation of atmospheric fresh water and hydrothermal fluid. Based on the above petrological and geochemical characteristics, the quasi syngenetic and reflux infiltration dolomitization models of the O1m55 in Daniudi area are established, which provides a theoretical guidance for the prediction of high quality dolomite reservoirs and the selection of geological exploration directions in the study area.