Depositional systems and coal accumulation in the Middle Jurassic Yan'an Formation in the Zhenyuan-Jingchuan region,Ordos Basin

The depositional systems and sedimentary microfacies are dealt with on the basis of new borehole and well-log data on the target strata in the Middle Jurassic Yan'an Formation in the Zhenyuan-Jingchuan region,Ordos Basin.There are gradations from the fluvial depositional system to the delta depositional system from the base upwards in the Yan'an Formation.The sedimentary microfacies consist of the channel bar,point bar,overbank and backswamp microfacies in the fluvial depositional system,and distributary channel,flood plain and swamp microfacies in the delta depositional system.The coal accumulation and controlling factors are also discussed in the light of tectonics,environments,palaeoclimates and landforms in the Yan'an Formation.