Silica diagenesis in the Lower Paleozoic Wufeng and Longmaxi Formations in the Sichuan Basin, South China: Implications for reservoir properties and paleoproductivity

Silica diagenesis is one of the most important aspects of black shale diagenesis and critically controls the reservoirs properties of black shales. Three stratigraphic sections of the Wufeng and Longmaxi Formations in the Sichuan Basin along a proximal to distal transect were investigated to study quartz types and their silica sources, as well as their influences on reservoirs properties including rock mechanical properties and porosity preservation. Six types of quartz including detrital quartz, recrystallized radiolaria, siliceous fossil fragments, microcrystalline quartz, quartz overgrowth, and hydrothermal quartz veins were identified based on their occurrences and morphologies under the petrographic microscope and scanning electron microscope and cathodoluminescence color and intensity. Original biogenic silica now exists as recrystallized radiolaria and microcrystalline silica in the mudstone matrix and follows systematic distribution patterns both laterally and stratigraphically. The biological origin of this silica component is supported by the presence of recrystallized radiolaria with preserved spines, preservation of “ghosts” of the central capsule, serrated edges reflecting the spherical lattices of the outer shell, and in addition preserved sponge spicules. A further source of silica for microcrystalline quartz formation was silica released in the course of clay mineral (smectite to illite) transformation. The different types of quartz make different contributions to the reservoir properties of shales, with microcrystalline quartz having the most important impact. Microcrystalline quartz enhances the rock mechanical properties of shales and protects initial porosity from mechanical compaction through a rigid framework of interconnected diagenetic silica grains. Pores between microcrystalline quartz grains are shielded from complete mechanical compaction and provide space for accumulation of bitumen and oil later in burial history. Subsequently, these pore-filling organic matter develop secondary organic nanopores at higher thermal maturity. Although biogenic silica content has been used as a paleoproductivity proxy, the method used to calculate it is based on questionable assumptions, and model parameters need to be fine-tuned for a given shale succession on the basis of petrographic observations. In addition, the commonly observed positive correlation between total organic carbon (TOC) and biogenic silica is invalid for very low sedimentation rates and extremely high concentrations of biogenic silica. From low to extremely high biogenic silica content in organic-rich sediments, a parabolic relationship between TOC and biogenic silica should be common.