Development Characteristics Of Shale Pyrite In The Lower Segment Of Wufeng Formation-Longmaxi Formation In Northeastern Yunnan And Its Shale Gas Significance

This paper selects a well in the northeastern part of Zhaotong City,Yunnan Province and the Meidong section in Xingwen County,Sichuan Province.The upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation shale is selected as the research object,based on its rock characteristics,sedimentary structure and deposition.Based on environmental analysis,the development characteristics of framboid pyrite in organic-rich shale have been deeply studied,combined with element geochemical indicators to discuss its indications to the sedimentary environment,and its environmental restraint mechanism and its significance to reservoir oil and gas are analyzed.The specific understanding is as follows:(1)The analysis of shale reservoir geochemical data and framboid pyrite particle size statistics shows that the early sedimentary environment of the Wufeng FormationLongmaxi Formation in the study area is characterized by warm and humid,low-energy limited,anaerobic deep-water shelf opposite to high-energy,Semi-enclosed,anoxicoxygen-enriched semi-deep water-shallow water shelf phase transition process.Among them,the oxidation of the sedimentary environment obtained by the analysis of the particle size of the framboid pyrite is different from the results of the geochemical data analysis.It is speculated that the sedimentary water may develop a wedge-shaped sulfide redox structure,making the framboid pyrite reflect that the sedimentary environment is not traditional Sedimentary bodies of water.Therefore,when using the framboid pyrite particle size analysis method to determine the sedimentary environment,it is still necessary to combine with the spherical chemical indicators for comprehensive discrimination.(2)Pyrite in the lower part of Wufeng Formation-Longmaxi Formation in northeastern Yunnan mainly exists in three forms: automorphic crystals,strawberry-like aggregates,and metasomatic paleontological fossils,and there is no significant difference in the particle size of the strawberry-like pyrite between the profile sample and the core sample,indicating that The general weathering conditions after diagenesis are difficult to change the structure.However,when encountering strong metamorphism or weathering,the strawberry-like pyrite crystals will still further crystallize and evolve,which is mainly manifested in the formation of magnetite,hematite,and other pseudo-strawberry-like crystals under alkaline oxidation conditions;Under weak oxidizing conditions and high [Fe][S] saturation,the strawberry-like pyrite particle size will become larger;under acidic hypoxic conditions and low [Fe][S]saturation,Strawberry-shaped pyrite will gradually evolve toward the self-shape.(3)In the lower part of Wufeng Formation-Longmaxi Formation in northeastern Yunnan,there are mainly three lithofacies: clayey siliceous mingling,calcareous siliceous mingling and calcareous shale.The TOC content in the reservoir is generally high,ranging from 0.19% to 4.78%,with an average value of 2.03%.Pyrite is relatively developed,with an average content of 2.1%.TOC and pyrite have a good positive correlation,indicating that the development of organic matter is often accompanied by pyrite.Framboid pyrite has developed internal pores.The pore types are mainly intercrystalline pores and organic pores.The pore diameters range from 19 to 705 nm.Most of them are pores below 10 nm,which have a positive contribution to the reservoir pore system.In addition,as a brittle mineral,pyrite can improve the brittleness index of the reservoir,which is conducive to fracturing mining.The paper has 40 pictures,17 tables,and 104 references.