Sedimentary Facies And Model Of The Fourth Member Of Ordovician Majiagou Formation In Ordos Basin

Majiagou formation of Lower Paleozoic in Ordos Basin is an important natural gas exploration layer series.In recent years,a series of exploration achievements have been made in the fifth member of Majiagou formation.Although the beach facies dolomite in the fourth member of Majiagou formation（“Ma4”）has good potential,its exploration has not made a breakthrough.The study of sedimentary facies of Ma4 is of great significance to guide the natural gas exploration of this stratum.Based on the division of core facies,logging facies and seismic facies,the sedimentary facies of Ma4 are studied by using core,thin section,logging and seismic data.The lithofacies paleogeography of Ma4 is restored by using the methods of single factor analysis and multi factor lithofacies paleogeography comprehensive mapping.2 types of facies are developed in Ma4:limited platform facies and platform margin facies.The restricted platform facies includes 4 types of subfacies:restricted tidal flat,interbeach sea,grey lagoon and mound beach,which are further divided into 8 types of microfacies:gypsum flat,gypsum dolomite flat,dolomite flat,grey interbeach sea,dolomite interbeach sea,grey lagoon,intra platform granular beach and intra platform mound.The platform margin facies includes 2 types of subfacies:platform margin beach subfacies and interbeach sea subfacies,which are further divided into 2 types of microfacies:platform margin granular beach and dolomite interbeach sea.The platform margin granular beach microfacies,the platform inner granular beach microfacies and intra platform mound microfacies are the favorable microfacies of Ma4.The platform margin granular beach microfacies are distributed in Ma4.The granular beach microfacies in the platform are mainly developed in Ma4₁and Ma4₂,and the intra platform mound microfacies in the platform are mainly developed in Ma4₁.Typical sedimentary cycles are developed in Ma4,which are characterized by upward shallowing water depth and upward coarsening grain size.There are obvious differences in sedimentary microfacies in different parts of Ma4.Platform margin facies are mainly developed in the central paleouplift belt,including platform margin granular beach microfacies and cloud beach inter sea microfacies.Limited platform facies are mainly developed in the middle and eastern part of the basin,gray and cloud inter beach marine microfacies are generally developed in the north and dolomite flat microfacies are generally developed in the south;Wushenqi low uplift belt and Mizhi-Jiaxian low uplift belt mainly develop intra platform granular beach microfacies and mound in the platform,and Taolimiao depression,Mizhi depression and northeast of Shenmu mainly develop gery lagoon microfacies.According to the plane and vertical distribution law of sedimentary microfacies,the sedimentary facies models of carbonate platform in each sub stage of Ma4 are established,which are mainly controlled by the sedimentary paleogeomorphic background and the change of relative sea level.Based on the research results of sedimentary facies of Ma4 formation,Yulin-Jingbian and Shenmu-Mizhi two favorable areas for natural gas exploration are optimized.