Research Of Carbonate Microfacies In The Second Member Of Ordovician Majiagou Formation, Eastern Ordos Basin

Ordos basin is located in the west of North China plate, the Ordovician sedimentary thickness is about600-800m, the type of sediments is epeiric sea carbonate platform. ordos basin contains a large number of oil and gas resources, the finding of soft asphalt and asphalt mud of the second Member of Ordovician Majiagou Formation of Zhenjia1well in the East of Ordos Basin is the cause of this study. Otherwise, previous research on microfacies of carbonate rock classification of the second Member of Majiagou Formation are not detailed enough, and analysis of sedimentary environment still need to be further studied.The papar comes from the National973project " Potash-forming mechanism^conditions and Late evolution of ancient epicontinental sea basin of ordos basin in the Ordovician ". The study discuss carbonate microfacies and reservoir characteristics in the second Member of Ordovician Majiagou Formation, Eastern Ordos Basin, summarize different microfacies combination and the variation of different facies belt and discuss the characteristics of carbonate reservoir in the study layers.The rocks developed in the Member2of Majiagou Formation in the eastern of Ordos basin represented the typical types in the carbonate platform, and the full coring Zhenjia1Well in the eastern of Ordos Basin is regarded as backbone well, Yu9and Longtan1Well are auxiliary wells, Shanxi Liu Lin Sanchuan River section is a reference section. According to the mineral composition, microscopic structure and fossils of Member2of Majiagou Formation of Zhenjia1Well, we can get the analysis of petrology characteristics. And then carbonate rocks of Member2of Majiagou Formation can be divided into limestone and dolomite, limestone can be subdivided into grain limestone, particulate quality of plaster limestone, particulate plaster limestone, Plaster limestone, dolomite can be subdivided into micritic dolostone, crystal powder dolomite and aplite-dolomite. Combining with mineral composition and the type and content of particles and matrix of carbonate rocks, the type of carbonate rocks microfacies can be divided into10types:MF1/Sparry bioclastic grainstone, MF2/Brachiopod grainstones, MF3/Bioclastic packstone, MF4/microcrystalline limestone with little debris, MF5/Plaster limestone, MF6/Plaster peloidal grainstone, MF7/Laminated mud powder crystal dolomite, MF8/Micritic dolostone, MF9/Crystal powder dolomite, MF10/Gypsiferous Dolostone. According to the types of microfacies and relations of microfacies combination, their sedimentary environment can be classified as:Ⅰ. open marine platform; Ⅱ. restricted marine platform; Ⅲ. Platform evaporites.By dividing microfacies combination, sedimentary facies characteristics, sea level changes, sedimentary facies evolution in the study area is open marine platform-evaporate platform-open marine platform-restricted marine platform-evaporate platform-open marine platform-restricted marine platform.There are multi period regression transgression cycles in the Member2of Majiagou Formation depositional period. And the evolution of the sedimentary facies’ characteristics show the study area experienced3times transgression. Lithology is mainly bioclastic particles limestone in the lower member2of Majiagou formation, with a large thickness, containing brachiopods, gastropods and other fossils, and sedimentary environment is open marine platform, transgressive reached the maximum value in the early deposition. With the lowering of sea level, deposits changed to dolomicrite and crystal powder dolomite, with a decrease of bioclastic and layered distribution, sedimentary environment is restricted marine platform near the wave base. Sedimentary environment of lower member2of Majiagou formation changed from open marine platform into restricted marine platform. In the middle of the second member of Majiagou formation, lithology changed from crystal powder dolomite into dolomicrite. After that, deposits changed into grain-refining gypsum and dolomicrite, with a large changes of lithology and thickness, and sedimentary environment changed from open marine platform into restricted marine platform and then changed into evaporate platform environment. In the upper of the second member of Majiagou formation, lithology changed from dolomicrite into crystal powder dolomite, and sedimentary environment changed from open marine platform into restricted marine platform, with a complete transgressive regressive cycles.Based on the analysis of the petrology, pore type, the characteristics of diagenesis, physical property parameters, we can get that reservoir pore types is mainly dolomitization of intergranular pore, intergranular micropores and intergranular dissolved pores. Hole, cave and primary pore is relatively rare. The reservoir space are mainly intergranular pore, intergranular dissolved pores, and part of it is paste, salt crystal moldic pore, crack hole. Porosity is mainly ultra-low porosity, permeability is mainly low permeability. Diagenesis includes dissolution, recrystallization, dolomitization, pressure solution, the compaction and cementation, and properties are largely affected by the influence of diagenesis. The carbonate rock reservoir is mainly class Ⅲ and class Ⅳ reservoir. Class Ⅲ reservoir in the study area is more widely distributed. Most belong to low porosity and low permeability reservoir.