Study On The Genetic Mechanism Of The Middle-Lower Ordovician Karst Reservoirs In The Yubei Area

The Yubei area is one of the exploration and development areas of the Tarim Basin and has good prospects for exploration and development.The reservoir development types in the Yubei area are mainly karst reservoirs.For the Lower-middle Ordovician reservoirs in the Yubei area,they can be observed by drilling cores and thin slices,combined with cathodoluminescence,order analysis,and carbon-oxygen stable isotope analysis.Geochemical methods such as strontium isotope analysis and rare earth element isotope analysis explore the basic characteristics,genetic mechanism and developmental distribution of karst reservoirs in the Yubei area.The results show that:（1）The middle and lower ordovician rocks in the yubei area are mainly composed of microcrystalline limestone,grain dolomite,granular limestone and algal-bonded limestone,among which microcrystalline limestone and crystalline dolomite.It is the main reservoir rock type in the yubei area;the main reservoir space types in the study area include dissolution pores,dissolution pores along the seam,dissolution pores,intercrystalline pores,and unfilled structural fractures,in which the dissolution pores and the fracture pores along the seam are most developed;The reservoir types are mainly karst-slot-cavity reservoirs,including limestone karst-slot-cavity reservoirs and dolomite karst-slot-cavity reservoirs.The distribution of horizons is characterized by limestone karst fractures.The reservoirs are mainly distributed in the eagle.The upper and lower sections of the shan formation also existed in the penglaiba formation.The dolomite karst-slot-hole reservoirs are mainly distributed in the penglaiba formation and the lower Section of the yingshan formation.（2）Dolomites in the yubei area mainly include fine-medium crystal dolomite formed by shallow burial genesis,coarse-matrix unmodified dolomite formed by hydrothermal genesis,and dolomite transformed by atmospheric water oxidized coarse-matrix.（3）The karstification of the middle-lower ordovician in the yubei area is mainly the atmospheric water karst of the caledonian-haixi period.The main signs are:tectonic weathering crack development,cutting and reforming hydrothermal dolomite and suture,with calcite or carbon.Acid slurry filling;development of dissolution fracture or dissolution modification joint,sedimentation sand and calcite filling;irregular crack development,cutting and reforming suture and hydrothermal dolomite,carbonate mud and calcite filling;core There are purple red dolomites;see the dissolution of holes along irregular cracks and sutures.（4）Calcite-earth-earth karst fracture-cavity calcite in the lower-ordovician in the yubei area,in the carbon-oxygen stable isotope analysis,theδ¹³C value and theδ¹⁸O value are both low;the ⁸⁷Sr/⁸⁶Sr value in the strontium isotope analysis It is higher than normal microcrystalline limestone;the distribution of rare earth elements is characterized by rare earth elements.（5）The reservoir development of the Middle-Lower Ordovician in the Yubei area is mainly controlled by paleogeomorphology and faults.The geomorphic divisions in the Yubei area are mainly divided into fault-breaking areas,fault-breaking areas and platform areas.The central platform area has strong karstification and is a type I karst reservoir development area;the eastern faulted area is strong and is a type II karst reservoir development area;the eastern fault area has a weak karstification and is a type III karst reservoir development area.