Distribution and evolution of sedimentary microfacies of submember 5 Majiagou Formation in the Ordos basin

The difference between lithology and depositional environment is the main cause of reservoir heterogeneity in submember 5 Majiagou Formation of Lower Ordovician in the Ordos basin. According rock types, sedimentary structures, color and its relationship with sedimentary environment, two subfacies and seven microfacies in submember 5 Majiagou Formation are classified. subfacies include basin-marginal tidal flat and intra-platform basin. The former can be furtherly divided into four types microfacies, such as intra-platform margin (gypsum) dolomite depression, intra-platform margin dolomite tidal flat, intra-platform margin calcitic tidal flat and intra-platform margin dolomite calcitic-calcitic dolomite tidal flat. The latter can be furtherly divided into three types microfacies, such as basin (gypsum) dolomite depression, dolomite calcitic-calcitic dolomite basin and calcitic basin. In the stage of submember 5-1 Majiagou Formation, Ordos basin basement uplift, the ancient land expand, reduce sea level, shallow sea, the ancient central uplift belt out of the water are the performance of sedimentary microfacies plane. North area is mainly developing intra-platform margin dolomite tidal flat-dolomite depression, intra-platform margin dolomite calcitic-calcitic dolomite tidal flat and calcitic basin. West area develop intra-platform margin dolomite tidal flat-dolomite depression, intra-platform margin dolomite calcitic-calcitic dolomite tidal flat. Center area develop calcitic basin, and mix dolomite calcitic basin and basin dolomite depression. In the stage of submember 5-2 Majiagou Formation, north area mainly developing intra-platform margin dolomite tidal flat and dolomite depression. Secondary microfacies include dolomite calcitic-calcitic dolomite tidal flat, cal-citic tidal flat. In western area, the main microfacies include intra-platform margin dolomite tidal flat and dolomite depression. In center area, the main microfacies include calcitic basin and basin dolomite depression.