Comparative Study On The Origin Of Quartz And Brittleness Index In Marine And Marine Continental Shale

Taking the marine shales of Longmaxi and Niutitang Formation in South China and the marine continental shale of Taiyuan Formation and Shanxi Formation in YuShe Wu-Xiang block of Qinshui Basin as the research objects,the differences of organic geochemical characteristics and mineral composition between marine and marine continental shale reservoirs are analyzed;based on the field emission scanning electron microscope,energy spectrum,cathodoluminescence and other analysis techniques,combined with the main,micro and rare earth elements test results,the morphology,genetic type and content of quartz of the marine / marine and continental shale are identified and divided,and the genetic differences of quartz between the marine /Marine and continental shale are revealed.Based on the qualitative analysis of the brittleness characteristics and contribution rate of different quartz and other single minerals in shale reservoirs,the brittleness evaluation method of mineral composition based on the different contribution rate of single mineral is established;the confining pressure-bedding effect of mechanical properties(stress-strain characteristics,failure mode,strength-elastic parameters)of shale is analyzed,and the brittleness index calculation model of elastic parameter method is modified;the weight coefficient of brittle minerals relative to quartz is proposed,and a comprehensive evaluation model of shale brittleness index is constructed by mineral composition method and normalized elastic parameter method,which reveals the difference of brittleness characteristics between marine shale and marine continental shale.The results show that the content of terrigenous quartz in marine shale is low,and the content of authigenic quartz is high(biological quartz + chemical quartz),the mainly mineral of marine continental shale is terrigenous quartz,basically without biological quartz,and contains a small amount of quartz from clay mineral transformation;There are great differences in mineral content,Young's modulus and Poisson's ratio between marine shale and marine continental shale.The content of brittle minerals in marine shale is high,with high Young's modulus and low Poisson's ratio.The content of brittle minerals in marine continental shale is low,with low Young's modulus and Poisson's ratio;The brittleness index of marine shale is obviously higher than that of marine continental shale.The research results provide a theoretical basis for brittleness evaluation and fracturing of shale reservoir.