白云岩-蒸发岩共生体系白云石化过程研究:以鄂尔多斯盆地马家沟组为例OA北大核心CSTPCD

The dolomite reservoir is developed in the typical dolomite-evaporite paragenetic system(DEPS)of the Ordovician Majiagou Formation in the Ordos Basin.Many researchers have conducted numer-ous studies on the characteristics and formation mechanism of dolomite reservoirs in the Majiagou Formation.However,limited research has been done on the dolomitization process and evolution characteristics from the perspective of DEPS.In this study,two dolomite-evaporite sedimentary sequences(S1 and S2)from the Xiweikou section on the southeast margin of the Ordos Basin were selected for high-precision continuous sam-pling.Through sedimentology and petrology analysis,as well as carbon-oxygen isotopes,rare earth elements(REEs)and trace elements,two types of dolomite-evaporite paragenetic assemblages were identified.A detailed study was conducted on the properties,sources,evolution of dolomitization fluid,and the dolo-mitization process.The results show that:(1)in the S1 sampling interval,a Type I paragenetic assemblage is present,consisting mainly of intraclast grain dolomite-dissolved gypseous breccias dolomite-dolomicrite bearing gypsum pseudocrystals-mudstone.This indicates a supratidal environment with strong evaporation.In the S2 sampling interval,a Type Ⅱ paragenetic assemblage is observed,mainly composed of thin-medium thick layers of finely crystalline dolomite-medium(grain)dolomite-argillaceous dolomite bearing gypsum pseudocrystals-mudstone,representing an intertidal-subtidal environment.(2)The REE distribution pat-terns of dolomite in two types of paragenetic assemblages are similar to those of pore water,both showing en-richment of MREE,with slightly negative anomalies-normal of Ce and Eu.There is no significant difference between δ13C and δ180,and the contents of Fecarb and Mncarb are relatively high.It is speculated that the sources of dolomitized fluid are the same,originating from weakly reducing pore water derived from seawater.(3)The sedimentary sequence in the studied geological section changes from type Ⅰ to type Ⅱ from bottom to top,reflecting the typical dolomitization process of paragenetic systems in evaporative tidal flat environ-ments.As sea level rose from S1 to S2,penecontemporaneous sabkha dolomitization in the supratidal zone e-volved into seepage reflux dolomitization in the intertidal-subtidal zone.The concentrations of MREE and Fecarb in dolomite of the type Ⅰ paragenetic assemblage are higher than those in type Ⅱ,indicating the evolu-tion of dolomitization fluid properties from rich Fe2+-Mn2+and MREE to poor Fe2+-Mn2+and MREE.The re-search has enriched the basic theory of dolomite genesis under paragenetic systems and provides theoretical guidance for oil and gas exploration in the Majiagou Formation in the Ordos Basin.