The Middle Assemblage Of Diagenetic Facies And Reservoir Characteristics Of Majiagou Formation In The Eastern Margin Of The Palaeohigh In Ordos Basin

The Ordos Basin is an important oil and gas exploration base in China. With the discovery of a series of large-scale gas fields, it further indicates that considerable rich oil and gas resources are reserved in the horizons of carbonate rocks of Lower Paleozoic, especially in the carbonate rocks of Lower Ordovician. Previous studies mainly focused on the large strata of Majiagou Formation, and the finer stratigraphic studies mainly focused on the upper assemblage of Ordovician, while the middle and lower assemblages were less noticed. Therefore, this paper describes petrological characteristics and diagenetic types of the middle assemblage of Ordovician within the study area taking the formation of the middle assemblage of Ordovician (the Ma-V5to the Ma-V10sub-section of Majiagou Formation) as the object of study, relying on the project of Studying on Sedimentary Evolution and Geological Conditions of Natural Gas Accumulation in the Carbonate Rocks of Lower Palaeozoic in the Ordos Basin through field geological survey, borehole core recording, casting thin-section analysis, porosity and permeability test analysis as well as other technological means, dividing diagenetic evolution stages, analyzing pore evolutionary history during the diagenetic process of rocks in the target zones, and thereby classifying diagenetic facies and clarifying their distributional characteristics in the study area. Under this basis, it studies the impact of diagenetic facies on reservoirs and predicts favorable zones of hydrocarbon storage. This paper has mainly achieved the following understanding:①Of all the diagenetic processes experienced by the eastern margin of the palaeohigh in the Ordos Basin, constructive diagenetic processes include dolomitization, dissolution process and degypsification; destructive diagenetic processes include compaction, pressure solution, cementation, filling, recrystallization, dedolomitisation, silicification and pyritization.②Diagenetic evolution stages are classified based on the types of diagenetic processes in the study area, including four stages, namely penecontemporaneous-early diagenetic shallow burial stage, hypergene stage, middle diagenetic shallow-middle burial stage and late diagenetic middle-deep burial stage. ③Major pore types in the study area include intergranular pores, intercrystalline pores, moldic pores, dissolved fractures, structural joints and contraction joints. Through negative porosity evolution analysis on the typical thin-sections of the study area, the diagenetic evolution sequence and diagenetic model integrated map are established, thereby summing up the relationship between the pore assemblage evolution and diagenetic evolution.④A total of6favorable hydrocarbon gathering areas,2Class I favorable areas,3Class II favorable areas and1Class Ⅲ favorable area are delineated in the area by describing the impact of major diagenetic processes on the development of pores within the diagenetic facies as well as the distribution of porosity and permeability data of reservoirs in the diagenetic facies.