Shale Gas Significance Of Pyrite In Wufeng-longmaxi Formations,Case Study In Western Hunan And Hubei

Deposit in shallow-deep shelf environment of the Wufeng-Longmaxi shale has undergone complex evolution of diagenesis and tectonic,the main organic matter in the area is type II₁ kerogen with high TOC content and high maturity.Moreover,general development of pyrite is the biggest characteristic in the shale of this area.At present,the domestic research on pyrite in the shale is very weak,therefore,carrying out research on western Hunan and Hubei Wufeng-Longmaxi pyrite of the shale is very important,which contributes to the rapid research and evaluation of the sedimentary water reducing strength in the study area and the space provided to migrate organic matter during diagenetic process,which effects organic matter enrichment condition in the shale and the organic matter supply capacity during the thermal cracking to gas process.The core samples,optical microscope and scanning electron microscope showed that Longmaxi shale developed euhedral-subhedral pyrite,SEM observations of pyrite generally occurs framboid pyrite mainly distributed in the organic or in the clay mineral’s pores;the core of shale pyrite of Wufeng formation was no obvious crystal structure,SEM observations of pyrite also shows framboidal shape of pyrite.Unlike the Longmaxi formation,the organic rich shale in Wufeng formation develop small particles euhedral pyrite,however,larger grain size of framboid pyrite develop in shale of Longmaxi formation.According to the observation and analysis of different pyrite morphological,organic rich shale of Wufeng-Longmaxi formation mainly developed into three types pyrite:type I,syngenetic framboid pyrite（diameter<6μm）;type II,diagenetic framboid pyrite（diameter>6μm）;type III,diagenetic euhedral pyrite.Due to the different framboid pyrite represents the different water environment,we found that water environment in the study area is mainly located in the euxinic environment,anoxic environment,oxygen-dysoxic environment and oxygen environment.The relationship between pyrite characteristics and three parameters of TOC,clay minerals and gas content was analyzed by SPSS software.It was found that pyrite had a positive correlation with these three parameters.Therefore,it can be inferred that the characteristics of pyrite have a certain indication to shale oil and gas.By analyzing the core data,it is found that the desorption gas content of the euxinic environment is the highest,the desorption gas content of anoxic environment is second,and the desorption gas content of the oxygen-dysoxic environment is third,while the desorption gas content of the oxygen environment is not good.The study area is mainly located in the euxinic environment.Through observing the development of type II and III type of pyrite,desorption gas content of high value corresponding to the development of type III pyrite,the desorption gas content will decrease where the type II pyrite develop.III type pyrite of Wufeng formation is more developed than the Longmaxi formation.Therefore,the desorption gas content of Wufeng Formation is higher than that of Longmaxi Formation.And the Longmaxi formation appeared with type III pyrite is featured with higher desorption gas content than that of other position of the Longmaxi formation.In summary,the particle size,morphological characteristics and distribution of type I,II and III pyrite directly indicate the organic content in the shale,menwile,it has important significance for gas content of the shale formation,can be widely used in the rapid evaluation of shale gas.