Source analysis and depositional systems of Upper Triassic early and mid-term Yanchang Formation in Longdong area

Light and heavy mineral combination characteristic,directions of palaeo-flow,sandstone petrography,space distribution of sand bodies of subaquatic distributary channels and sedimentary structure feature were used to analyze source rock,source direction and depositional system of Chang 8-6 oil-beds of Yanchang Formation of the Upper Triassic in Longdong area of Ordos Basin.The palaeo-flow parameters show that the sediments are carried from department by the basin to the center in the depositional period of Yanchang Formation.Three main source directions can be divided according to the light and heavy mineral combination characteristics,as west,southwest and northeastern directions.In addition,there is the secondary source in the south according to ancient flow direction.The quartzite,neutral-basic and neutral-acid volcanic rock,metamorphic rock of Precambrian and shallow metamorphic rock,carbonate rock,gravel rock of Palaeozoic group from basin outer edge are the source rocks.The depositional system of Chang 86 oil-beds of Yanchang Formation of the Upper Triassic in Longdong area can be divided into four systems.One of them is Wuqi-Huachi delta depositional system from northeast;others are Huanxian fan delta depositional system from west,Zhenyuan-Jingchuan braided delta depositional system from southwest and Heshui-Guchengchuan turbidite fan depositional system from south.On the basis of the depositional systems,petrographic characteristics,sand body pattern,sedimentary structure of different depositional systems and the evolution process of the depositional systems are elaborated in detail.