Quartz types and origins in the paleozoic Wufeng-Longmaxi Formations, Eastern Sichuan Basin, China: Implications for porosity preservation in shale reservoirs

Quartz in shales has been demonstrated to exert a significant effect on shale gas production by influencing rock mechanical behavior and reservoir porosity; however, quartz types and origins in shales are complicated and challenging to study due to their fine-grained nature. In this study, we propose combining thin section images, scanning electron microscopy (SEM) images, energy dispersive spectroscopy (EDS) maps, cathodoluminescence (CL) images and geochemical data to investigate quartz types and origins in shale formations. Shale samples from the Ordovician Wufeng-Silurian Longmaxi Formations, Eastern Sichuan Basin, China were used to study quartz types, origins and the effect of quartz cementation on porosity development. Samples from the Wufeng-Longmaxi Formations have TOC contents in the range of 0.5-4.7 wt%, averaging 2.7 wt%, and are currently in the thermally overmature stage. Five types of quartz, including silt-size detrital quartz, siliceous skeletal fragments, quartz overgrowth, clay matrix-dispersed microquartz, and aggregates of euhedral quartz, were identified. The ternary diagram of Fe-Al-Mn, the cross-plot of SiO2 versus Zr, and the cross-plot of Si versus Al suggest that the dissolution and reprecipitation of siliceous skeletons may provide the major source of silica for quartz cementation in the lower part of the Wufeng-Longmaxi Formations, and released silica as a result of smectite to illite transformation is also important for authigenic quartz formation in the upper part of the Wufeng-Longmaxi Formations. Our SEM images suggest that the rigid framework formed by quartz cementation not only prevents primary interparticle pores from compaction but also favors the generation of organic matter-associated pores.