Sedimengtary Microfacies Study of Chang21 Section of Yanchang Formation Triassic System in Youfangzhuang Oil-field,Ordos Basin

Youfangzhuang area lies in the west of Northern Shanxi Slope,Ordos Basin.The sedimentary of Yanchang group in upper Triassic system is terrigenous clastic rock which can be divided into ten formations:Chang10～Chang1 from bottom to top.Chang2 formation can be divided into three sand layers: Chang21,Chang22,Chang23.Chang21 is one of oil and gas reservoir in study area.The sedimentary facies type is simple and deltaic plain subfacies developed merely in this duration.According to the well core and well logging date as well as area sedimentary facies date studying and subfacies,microfacies and sedimentary facies evolution researching,this text marks off six microfacies in the studied area,such as upper-river channel facies,channel lateral facies,natural levee,crevasse-splay and marsh facies.The result shows that distributaries channel sand bodies were developed widely for oil and gas accumulation,and it is the main reservoir in the studied area.