Biogenic Microcrystalline Quartz and Its Influence on Pore Development in Marine Shale Reservoirs
[Objective]Quartz is one of the most important minerals in marine shale and can be found in several forms with multiple origins. In addition,different types of quartz may contribute differently to the geomechanical pro-perties and pore evolution of marine shale,and related research is lacking. It restricts a deep understanding of shale reservoir characteristics.[Methods]This study briefly reviews recent progress in the classification of quartz in typical marine shales. In order to better understand quartz types,silica sources,and the influence of biogenic microcrystal-line quartz on geomechanical properties and pore evolution,marine shale samples were collected from the Upper Or-dovician Wufeng Formation and the Lower Silurian Gaojiabian Formation,specifically well Gudi-1 of the Lower Yang-tze Platform. These samples were analyzed using thin section-,X-ray diffraction-,and geochemical analysis,optical microscopy,nuclear magnetic resonance,and field-emission scanning electron microscopy combined with cathodolu-minescence. The aim was to further reveal the pore evolution pattern in the Lower Paleozoic marine shales of the Low-er Yangtze Platform from a new perspective and to provide a more reliable geological basis for the storage capacity of shale gas reservoirs.Optical microscope evidence was found of siliceous organism fragments,including biogenic silica accounting for approximately 45.17％ of the total silica content,as well as characteristics of major and trace ele-ments,such as a non-hydrothermal area located in the Al-Fe-Mn ternary diagram,negative correlations of Zr and SiO2,an Al/(Al+Fe+Mn) ratio of 0.65 to 0.76,and a cross-plot of Si versus Al. Overall,these indicate that siliceous organisms provided an important silica source for microcrystalline quartz. These siliceous organisms provide a rela-tively rich silica source for the precipitation of authigenic quartz during early diagenesis.[Results and Discussions]The quartz in well Gudi-1 is mainly detrital,microcrystalline,and organism skeletal quartz. Detrital quartz is of terrigenous origin and is characterized by bright luminescence. Most detrital quartz particles are silt-sized and range from 10-30 μm. Microcrystalline quartz is one of the most common authigenic quartz types and can be found in several marine shale formations. Microcrystalline quartz has no or low luminescence,indicative of authigenic origin. Furthermore,microcrystalline quartz can be divided into three categories:clay matrix-dispersed microcrystalline quartz,euhedral microcrystalline quartz,and amorphous microcrystalline quartz cements. Organism skeletal quartz is widely distributed in the studied shale samples and was mostly found in fragments of siliceous organisms,such as radiolarians and sponge spicules. Most radiolarians are oval and round with diameters of approximately 100 μm.[Conclusions]This study indicates that biogenic microcrystalline quartz may have implications for rock mechanics and the evolution of porosity. The positive correlation between the silica content and brittleness index of the studied shale reveals that the development of biogenic microcrystalline quartz enhances the brittleness of the shale to a certain extent. Biogenic microcrystalline quartz can be interconnected to form a rigid siliceous matrix framework,which largely promotes the geomechanical properties of the studied shale. Moreover,the positive correlation between the silica con-tent and porosity indicates that the enrichment of biogenic silica is conducive to pore evolution and preservation. Or-ganic matter and clay minerals in shale are susceptible to ductile deformation by compaction,while biogenic micro-crystalline quartz can form a rigid framework to enhance its resistance to compaction,which is conducive to the preser-vation of the internal pore space within shale,especially intergranular-and organic matter pores.
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