Formation Mechanism and Influencing Factors of Micro-Fractures in Tight Glutenite of Kongdian Formation in Bozhong Sag
Many deep wells in the southwestern part of the Bozhong sag reveal thick glutenite of Kongdian Formation.Due to the large burial depth,the glutenite reservoir is obviously densified,and the production of some wells is less than 10 m3/d.A part of wells have high productivity,and the differential evolution mechanism of reservoirs is a key issue that restricts exploration.Aiming at the differential evolution mechanism of the reservoir,this study carried out experiments such as thin section,scanning electron microscope,compaction simulation.It is found that the degree of development of micro-fractures determines the physical properties of reservoirs.And the differential evolution of the reservoir is significantly affected by the composition of gravel and the content of matrix.The glutenite whose parent rock comes from granite,with low matrix content,develops potash feldspar fractures,strong dissolution,and good reservoir properties.The glutenite with high matrix is characterized by underdeveloped fractures,weak dissolution,and low productivity.The glutenite mixed with a large amount of carbonate gravel has strong cementation in the early stage,and the productivity is still poor even after fracturing.The physical compaction simulation shows that intragranular fractures appear in the gravel below simulation burial depth 2 500 meters,which increase with the increase of simulation burial depth;the types and characteristics of fractures can be compared with the characteristics of micro-fractures in the study area.This confirms that glutenite with low matrix content can generate a large number of compaction fractures.Potassium feldspar and plagioclase are quite different in their ability to form fractures:potash feldspar is more prone to form fractures,and which is further corroded by later fluids,forming dissolution enlarged fractures.Plagioclase is prone to kaolinization,sodium zoisite and other secondary transformations,which changes the mechanical properties of minerals,and is not prone to form fractures.
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