The Depositional Characteristics and Evolution Rules of the Middle and Lower Jurassic in the South of Ordos Basin

Through synthesizing predecessor's researches,field outcrops,core and well log data,the characteristics of the deposition and evolution of the lower and middle Jurassic in Maling Oilfield were studied,the characteristics of the sedimentary facies were discussed,it was considered that the lower and middle Jurassic Fuxian Formation and Yanan Formation were fluvial facies and fluvial-lake-delta facies.The fluvial facies mainly included low curvature arenaceous fluvial channels and braided arenaceous fluvial channel;fluvial-lake-delta facies mainly included upward distributary channels,crevasse-splay,flood plain,inter-channel depressions,swamp and natural levee.It is pointed out that the characteristics and evolution of paleogeographic,and the sedimentary basin of the Jurassic Fuxian Formation and Yan'an Formation in southern Ordos Basin were through the full evolutionary process of incising,infilling and rising.It indicates that the fan fluvial sedimentary system is predominant at the Yan 10 substage of Fuxian Stage,and the fluvial lake delta sedimentary system is developed from the Yan 9 to Yan 6 substage,and paludal coal-bearing sediments are predominant at the substage Y4+5 for river rejuvenation and basin rising.