Dolomitization Controlled by Paleogeomorphology in the Epicontinental Sea Environment: A Case Study of the 5th Sub-Member in 5 Member of the Ordovician Majiagou Formation in Daniudi Gas Field, Ordos Basin

The 5th sub-member in 5 Member Ordovician Majiagou Formation Daniudi Gas Field, Ordos Basin, is deposited an environment consisting of ancient epicontinental sea, where very fine crystalline dolostone reservoir has developed. In this study, based on the petrological and geochemical characteristics, genesis dolomite developed M55 were studied by analyzing properties sources dolomitization fluids, influence paleogeomorphology differences distribution was also discussed order to clarify lime flat sea. had a crystal structure, mainly including microcrystalline crystalline. content MgO CaO negatively correlated, indicating that it result replacement. dark red under cathode luminescence, mode rare earth elements showed negative anomaly Ce Eu, fluid sea-sourced fluid. δ13C, δ18O, 87Sr/86Sr isotope range limestone similar seawater study area, whereas obviously more positive than limestone. substitute index salinity (Z) higher 122, which (Z = 120.5), slightly evaporated seawater. wormholes observed core gypsum penecontemporaneous period thin sections indicated formed when sea level relatively low, seepage–reflux dolomitization. By correlation between thickness sedimentary period, found at high altitude significantly larger other areas. development controlled level, local controls during low level. There many limestone–dolostone cycles cumulative thicknesses topography. This provides theoretical basis for prediction reservoirs carbonate tidal dominated flats background