Origins of Siliceous Minerals and Their Influences on Organic Matter Enrichment and Reservoir Physical Properties of Deep Marine Shale in the Sichuan Basin, South China

Quartz is the main component of marine shale. However, the influence of quartz on shale gas accumulation is not yet fully understood, especially for deep shale reservoirs. In this paper, the deep Longmaxi shale recently drilled samples were taken from the Luzhou area of southern Sichuan, and their geochemical properties and pore characteristics were investigated, their quartz origin types were identified and quantified, and the effects of different kinds of quartz on the organic matter accumulation and physical properties of the shale were discussed. There are three types of quartz, namely, terrigenous quartz, clay-mineral-transformed quartz, and biogenic quartz in the shale. The biogenic quartz is concentrated in the siliceous shale, and the terrigenous quartz is dominant in the mixed shale. Different types of quartz have various effects on organic matter accumulation and physical properties. Biogenic quartz is positively related to total organic carbon (TOC), porosity, and brittleness. Clay-mineral-transformed quartz is not particularly conducive to the high TOC content and development of porosity. Although terrigenous quartz has a dilution effect on the TOC content, its effect on porosity is complex, with seeming dependence on lithofacies. Terrestrial quartz has a positive effect on the development and preservation of organic pores in the mixed shale, but it has no obvious impact on the siliceous shale. The high content of biogenic quartz in the studied shale indicates the enrichment of organic matter and excellent physical properties, which will be significant to conduct the exploration and development of deep and ultra-deep shale gas.