SEDIMENTARY FACIES OF MEMBER 2 OF SHANXI FORMATION OF LOWER PERMIAN IN THE MIDDLE-EASTERN ORDOS BASIN

Sedimentary sequence and tectonic features of outcrops and cores in Member 2 of Shanxi Formation in the east edge of Ordos Basin are studied with modern sedimentary mechanisms.Rhythmic stratification,block stratification and lateral accretion cross stratification are widely found in outcrops and cores of Member 2 of Shanxi Formation.Outcrop sands are lenticular,cutting each other and piling to large sand belts,due to frequent migrations of braided-river sands.Based on studies of rock sedimentary facies,well logging sedimentary facies,sandbody thickness,palaeotopography and palaeogeologic background,it is pointed out that,when Member 2 of Shanxi Formation deposits,sandy braided-river and shallow water delta system is formed,which can be divided into alluvial plain braided-river,braided-river delta plain,braided-river delta front,frontal braided-river delta or seashore shallow lake facies from north to south.Braided-river main channel,alluvial marsh,delta front debouch bar are distinguished.Main sand belt with large width and thickness proportion in main channel of braided-river is favorable for petroleum exploration.