鄂尔多斯盆地伊陕斜坡东北部马家沟组马五5亚段白云岩地球化学特征及其成因

鄂尔多斯盆地伊陕斜坡东北部奥陶系马家沟组马五₅亚段优质白云岩储层发育，多口井在该层系钻获高产工业气流，展现出巨大的天然气勘探潜力，而关于该层系白云岩储层的成因机理至今未达成共识.为此，在岩石学特征分析基础上，通过对白云岩碳、氧、锶同位素及微量元素等测试资料的系统分析，深入探讨了鄂尔多斯盆地伊陕斜坡东北部马五₅亚段白云岩的成因机理.结果表明:研究区马五₅亚段主要发育泥、粉细晶白云岩和豹斑灰云岩3种类型，前者主要发育于马五₅亚段底部，后两者主要发育于马五微晶白云岩₅亚段中上部，是主要的天然气储集体；粉细晶白云岩和豹斑灰云岩白云石有序度均较低，平均值分别为0.86、0.68，δ13C值多数与同期海水一致，δ18O值均显著偏负；多数粉细晶白云岩87Sr/86Sr值较同期海水显著偏正，豹斑灰云岩与同期海水一致；总体具有高Fe、Mn、Na、K和低Sr含量特征，豹斑灰云岩Fe、Mn、Na含量介于泥晶灰岩与粉细晶白云岩之间；稀土元素含量总体较低，且稀土元素配分模式相似，轻稀土元素富集，重稀土元素亏损，具明显Eu负异常和弱Ce正异常.综上认为研究区白云石化流体主要为海源性流体，早期可能有大气淡水的混入，两类优质白云岩均应形成于局限台地潮间-潮下带环境中的浅埋藏期回流渗透作用阶段.基于上述岩石学和地球化学特征，建立了研究区马五₅亚段纵向的沉积与成岩序列模式，为研究区优质白云岩储层的预测提供了理论依据. 

Geochemical Characteristics and Genesis of Dolomite in Majiagou Ma55 Submember of the Northeast Yishan Slope,Ordos Basin

The high-quality dolomite reservoirs of the Ma₅5 formation in the Ordovician Majiagou formation in the northern part of the Ordos Basin Yishan slope are developed. Many wells drill high-yield industrial gas streams in this layer, demonstrating huge potential for natural gas exploration. The genetic mechanism of dolomite reservoirs has not yet reached a consensus. Based on the petrological characteristics, the carbon, oxygen isotopic compositions, strontium isotopes and trace elements were discussed deeply to study the origin of dolomites in Ma₅5 formation of Northern Yishan slope Ordos Basin. There are three types of mud microcrystalline dolomite, ine-grained dolomite and leopard spots limy dolomite in the Ma₅5 submember of the study area. The former is mainly developed at the bottom of the Ma₅5 submember, the latter two are mainly developed in the upper middle part of the Ma₅5 submember and are the main natural gas reservoirs; The fineness of fine-grained dolomite and leopard spots limy dolomite is low, with average values of 0.86 and 0.68, respectively.The δ13C value is mostly consistent with seawater in the same period, and the δ18O value is significantly negative; most fine-grained dolomite 87Sr The/86Sr value is significantly more positive than the seawater value in the same period. The leopard spots limy dolomite is consistent with the seawater in the same period; the dolomites are characteried by high Fe, Mn, Na, K content, and low Sr content, and the leopard spots limy dolomite contains Fe, Mn, and Na contents, which are between mud microcrystalline dolomite and fine-crystalline dolomite; the dolomites have very low REE content and similar REE distribution patterns, which are generally characterized by the enrichment of light REE, losses of heavy REE, obviously negative anomaly of Eu and slightly positive anomaly of Ce. In summary, the dolomitization fluids in the study area are mainly sea-sourced fluids. In the early stage, there may be mixed air freshwater. Both types of high-quality dolomite are formed in the shallow burial stage of the tidal period in the intertidal-tidal zone of the restricted platform. Based on the above petrological and geochemical characteristics, the longitudinal sedimentary and diagenetic sequence models of the Ma₅5 sub-section of the study area are established, which provides a theoretical basis for the prediction of high-quality dolomitereservoirs in the study area.