The Origin And Significance Of Quartz In Shales

The quartz content is positively related to TOC in the Wufeng-Longmaxi Formation shale in the Sichuan Basin,and the origin of quartz is indicative of the gas-bearing nature of shale.In this paper,petrography,quartz morphology,and mineral composition of the field profiles or cores of Changning,QQ1#,and Shizhu,etc.,are combined with shale geochemical characteristics to investigate the origin of quartz in shale and the shale gasification.The impact of sexuality has achieved the following preliminary understanding:1.The Wufeng-Longmaxi Formation shales mainly consist of four lithologic sections of siliceous shale,carbonaceous shale,black shale,and silty shale.The lithology and mineral composition in different sections did not change much.The content of quartz in the vertical direction at the bottom was high and the upward content was reduced.The content of quartz was closely related to TOC and gas content.The development of siliceous and micro-organism fossils in the siliceous and carbonaceous shales of the bottom segment is an advantageous layer for the development of shale gas.2.The type of quartz include terrigenous detrital quartz,biologically associated quartz,and authigenic quartz.Among them,authigenic quartz has different causes: in the siliceous shale of the Wufeng Formation and in the carbonaceous shale at the bottom of the Longmaxi Formation,the main mechanism for the formation of excess silicon is the dissolution of amorphous silicon(biological skeleton),and the organic matter content is high;Longmaxi The TOC content in the black and silty shale of the upper part of the group is low,and the siliceous matter is mainly related to the conversion of clay minerals.3.The pores of biological associated quartz and the pores in the authigenic quartz crystallite accumulation control the distribution space and range of the transported organic matter,Occurred in the intragranular pores(interlayers of clay mineral interlayers,intracellular pores of strawberry-like pyrite particles),and together they may form a three-dimensional organic pore network,increasing the storage space of shale gas,the pore network are one of the main factors controlling the gas-bearing properties of shale.