Sedimentary Facies Of Chang 8 Oil Formation Of The Triassic Yanchang Formation In Ordos Basin

The Yanchang formation belonging to Triassic was a most important oil-bearing stratum of Ordos basin. The reservoir types mainly composed of lithology reservoir and structure-lithology reservoir, and these two types reservoir had important research value and economical value. It had very important theory meaning and actual meaning to reveal space-time distribution of sedimentary system, to discover high production, high content reservoir on the base of study of sedimentary systems characters, subfacies characters and mircrofacies association characters, vertical evolution patterns of facies and distribution laws of favorable reservoir sand.Based on data collecting and digesting, and analysis of sedimentary evolution setting, integrated with observation of outcrops and cores, the lithology facies of study area could be identified. Logging facies could be recognized by analyzing logging data, so the facies system and sedimentary facies of study area could be confirmed, microfacies could be divided. Based on controlling analysis of sedimentary facies on petroleum, forecast of favorable zone could be done.Through study of facies and sedimentary system, the main facies composed of fluvial system, lacustrine system and delta system. 10 subfacies and 19 microfacies could be subdivided by further study.There were three directions source in study area, the first source was northern Yinshan and Wulangeer uplift that presented paleo-granite, gneiss, biotite-amphiboleschist of pre-Sinian, and the second source was western Zuozishan, Helanshan, Daxiaoluoshan that presented Sinian quartzite, Cambrian-Ordovician metamorphic rock, beside paleo-granite, gneiss outcrop, and the third source was southwestern Longxian-Baoji ancient land that presented paleo-granite, gneiss, quartzite of Sinian, metamorphic rock and carbonate of Cambrian-Ordovician.When Chang 8 deposited, the base of Ordos basin was subsiding. Meandering delta presented on northeastern of study area, braided river and braided delta presented on southwestern area, and Meandering delta presented on southern part. Chang 8₁ inherited sedimentary character of Chang 8₂ on the whole, except the sedimentary scope changing. Lacustrine basin scope of Chang 81 was larger than that of Chang 82, and the scope of deep lake is larger than Chang 82. The distributary channel and its sand developed in delta of Chang 8 oil member were favorable reservoir for petroleum collecting.