Research Of Carbonate Reservoir Characterization And Formation Mechanism In Middle-lower Ordovician,North Slope Of Tazhong Area

The lower ordovician carbonate formation is taken as the research formation in Tazhong North Slope area of Tarim basin, which is mainly located in southern Shun Tuo Guo Le block and Katake 4 blcok. Based on a great number of papers and predecessors research achievements, taken the carbonate reservoir geology as the theoretical direction, integrated use sedimentary petrology, structural geology, and reservoir geochemistry theory, this paper makes full use of existing data to study the reservoir characteristics and formation mechanism on tectonic characteristics,petrology characteristics, lithological combination characteristics, sedimentary facies classification, diagenesis, diagenetic evolution sequence, the reservoir space or combination type and distribution, combined with a series of analysis means such as thin section identification, scanning electron microscope, cathodolumine- scence,electron probe, geochemical analysis and so on. Finally, reservoir develop- ment characteristics, main controlling factors and formation mechanism was definite, and the carbonate reservoir in Tazhong north slope was made comprehensive evaluation.On the plane, the Tazhong north slope region nowadays overall performs the monoclinal structure feature from the northwest to southeast. influenced by tectonic activity in sections of the south orogenic belt and TazhongⅠbelt, it shows the tectonic framework which is zoned from east to west and layered up and down.the Shun Nan Guo Le lower uplift and Guchengxu uplift were developed in the northwest and southeast of research area, which shows obvious characteristics of the partition, and there is a ramp transition between them. On vertical zonation, the research area can be divided into upper, middle and lower structural layer, respectively bounded by the top surface of Ordovician(seismic interface T70) and the bottom surface of the Carboniferous(seismic interface T60).The lower structural layer mainly refers to a series of twisting pressure strike-slip faults between the Cambrian and the middle-lower Ordovician; the middle structural layer, which is located between T70 and T60 seismic interface,mainly developsTranstensional strike-slip faults; the upper layer mainly develops compressional fault in the west of Shun Nan Guo Le lower uplift. Overall faults are not well developed in this region, but mostly slightly-deformed fold.By microscopic identification of well cores, the reservoir rock types in Tazhong North Slope are divided into limestone, dolomite, transition rock, and other rocks.After effective screening, removing rocks which accounts for a smaller proportion of lithological rock type, or do not have the original sedimentary lithological structure(such as silicalite, medium-coarse grained dolomite, etc.) affected by late diagenetic fluid, we conclude 8 kind of principal rock combination type. By fine division of sedimentary facies for every well, combined well-tie logs, well log features of different sedimentary facies, and seismic facies characteristics on the plane, we get that middle-lower Ordovician sedimentary phases mainly include restricted platform,open platform,four platform margin facies and Platform margin slope facies, seven sub-phase, nine microfacies in the Tazhong north slope regionDolomitization of middle-lower Ordovician carbonate reservoir in Tazhong north slope have gradually weakened from the bottom up. There are also differences in the extent of the lateral. In shunnan region Dolomitization is not strong, there are mainly calcite dolomite in the lower part of Yingshan formation. The more close to the Guchengxu uplift, the higher dolomite content, and it has developed pure dolomites near to Long 1 well. Since no large-scale evaporite formations founded in research area,we exclud the "evaporation pumping" mode in the Syngenetic or Penecontemporaneous period and the infiltration and circumfluence mode. Dolomite are mainly products of the shallow burial, the burial(transitional environment) and deep burial dolomitization, the reservoir is located in fine-grain buried formed dolomite strata(euhedral / semi-euhedral),which are product of grain bank metasomatism and sustained dolomitization of mud sized dolostone formed under shallow burial environment; the late hydrothermal fluid mainly reform or readjust the formation, and even bring with a certain filling and damaging.According to the measured temperature and salinity of inclusions, rare-earth elements and trace element and other analysis, hydrothermal fluid has a hybrid nature.Magmatism provides heat, and formation water provides the fluid. After mixing of magmatic hydrothermal and brine water, the middle-low temperature and high salinity of hydrothermal silicification are formed, silica may be derived from the deep magmatic differentiation. The hydrothermal reservoirs developed in Shunnan area may be related with multi-period volcanic activity, and hydrothermal reservoir may bemainly formed in the late Hercynian period.