The Reservoir Characteristics And Controlling Factors Of Jurassic Shale In Qaidam Basin

Shale gas is an unconventional natural gas,which is free occurrence in the natural factures or inter-particle pores,or adsorbed in the surface of organic matter and the clay particle.It was thought that the shale in the 7th Member of the Middle Jurassic Dameigou Formation in Qaidam Basin was regarded as the main part of the Northwest Jurassic source rocks,with a high abundance of organic matter,high degree of evolution,hydrocarbon volume characteristics.Recently,the shale gas reservoir is becoming an important exploration target within the Qaidam Basin,especially in the northern margin,where Oil&Gas Survey has discovered shale intervals cumulative thickness of 141 m,the thickest the section thickness of 58.12 meters,in CHAIYE 1.But the research of Jurassic source rocks in northern Qaidam Basin is mainly for conventional oil and gas resources,shale gas has not been carried out,particularly for shale thickness,composition characteristicsof shale minerals,gas content,evaluation of shale gas reservoir,etc.Therefore,according to rock-mineral identification,scanning electron microscopy,X-ray diffraction,mercury testing,testing,gas adsorption methods and so on in this article,the study analyzed difference of brittle shale mineral content,abundance and maturity of organic matter evolution degree in order to discuss about shale gas reservoir characteristics and controlling factors,providing a scientific basis for the shale gas resource evaluation and preferred exploration blocks.The shale of the 7th Member of the Middle Jurassic Dameigou Formation in the study area is divided into laminated shale,laminated sub-carbonate shale,laminated sub-silty shale,laminated carbonaceous shale,laminated sandy dolomitic shale,laminated carbonate silty shale,laminated silty shale,based on 3 unit analysis method.The mineral characteristic of black Organic-rich shales showed that it contained high siliceous mineral content,high clay mineral content,low carbonate mineral content,in where of the clay minerals kaolinite and illite mixed layer is dominant,illite followed.It was inferred that diagenetic stage of dark shales is A of late diagenetic stage.By the comprehensive analysis of pyrolysis,kerogen elements and kerogen maceral,the main type of organic matter is type ? and ?,? type followed,TOC content ranges from 2%to 6%,and maturity of organic matter(R0)ranges from 0.5%to 0.8,in the low mature—mature stage,a good source rock quality.Througn the observation of well core and scanning electron microscopy,the reservoir space is divided into rock pores and cracks,while the pores include intergranular pores,interparticle pores and organic pores,and cracks include diagenetic cracks and tectonic fractures.Shale reservoir characteristics is controlled by sedimentary environment,mineral assem-blages and pyrolysis of organic matter,where sedimentary environment has an impact on horizontal and vertical distribution of shale reservoir and favorable mineral assemblage and pyrolysis of organic matter has a positive connection with formation of reservoir pore and fractal dimension.