Sedimentary System And Reservoir Characteristics Of Chang 7 And Chang 10 Member Of The Triassic Yanchang Formation In The Central Ordos Basin

Oil-gas exploration in replacement sequence of Yanchang Formation of Ordos Basin has recently achieved the significant breakthrough in Chang 7 and Chang 10 Members. To ensure the continuity of oil-gas exploration and development, it is siginificantly important that sedimentary and reservoir characteristics and favorable area distribution of Chang 7 and Chang 10 Members.This research is based on the regional geological data of the Ordos Basin and previous research achievements. Using a variety of methods such as principles of sedimentary, stratigraphic analysis, petroleum geology and basin analysis, combining field geological sections and core observations and second well log interpretations, the study systematically analyzes provenance characteristics, sedimentary system and sedimentary facies of Chang 7 and Chang 10 Memebrs. Then it focus on the reservoir characteristics and divides the reservoir type. Finally distribution zone of favorable reservoir is advanced.The main sediment provenance of Chang 7 and Chang 10 Member in North Shaanxi area is from the northeast of Ordos Basin. The mother rock of provenance is metamorphic rocks and magmatic rocks of Daqingshan and Yinshan mountains. In Chang 10 period, water of basin was shallow, and the major sedimentary facies are delta plain subfacies. In Chang 7 period, the development of the basin reached its culmination, and the major sedimentary facies are lakes and delta deposits.The sediments of Chang 7 reservoir in study area are generally fine. Reservoir rocks of Chang 7 are mainly arkose and lithic feldspar sandstones and feldspar lithic sandstones. Sandstones grain size of Chang 10 Member are more than Chang 7. Reservoir rocks of Chang 10 Member are mostly medium to coarse grained arkose and lithic feldspar sandstone. Chang 7 and Chang 10 reservoirs mainly have intergranular pores and feldspar dissolved pores. They are low and extra-low permeable reservoirs which are characterized by small pores, fine throats and great heterogeneities. The physical property of Chang 10 reservoirs are better than Chang 7 reservoirs.Chang 7 and Chang 10 reservoirs have reach mid-A diagenetic period after experienced diagenetic process which mainly contains processes of compaction, cementation and dissolution. Chang 7 and Chang 10 reservoirs are divided into eight diagenetic facies on the basis of diagenesis types, structure of sedimentary rocks and characteristics of pore and permeability. Five diagenetic facies such as compaction diagenetic facies destroy reservoir property. And three diagenetic facies such as feldspar dissolution have played important role in the improvement of physical properties.Chang 7 and Chang 101 reservoir are divided into four types and six classes on the basis of reservoir microscopic pore structure characteristics and reservoir physical property and heterogeneity. Combining reservoir physical property, sandbody distribution character, diagenetic facies and exploration and development experience, the study proposes favorable reservoir zone of study area. Reservoir physical properties of Chang 71 are the best in Huachi and Wuqi-Dingbian and Zhidan-Ansai areas. Reservoir physical properties of Chang 72 are the best in Wuqi-Dingbian Area. These favorable reservoir zone is large which mainly contains Ⅰ and Ⅱ reservoir. Reservoir physical properties of Chang 101 are the best in Zhidan-Ansai Area. I reservoir is mainly distributed in this area.