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.