Geochemical Characteristics of Bentonite and Its Influence on Shale Reservoir Quality in Wufeng-Longmaxi Formation, South Sichuan Basin, China

On the basis of X-ray diffraction analysis, trace element test, rare earth element test, total organic carbon (TOC) analysis, total gas content, and porosity test, the characteristics of bentonite and its influence on shale reservoir quality in Wufeng-Longmaxi Formation were studied in the South Sichuan Basin. The results show the following: (1) Bentonite is K-bentonite, mainly composed of clay mineral (illite-montmorillonite mixed layer and illite) and amorphous body, containing a certain content of quartz, feldspar, pyrite, and carbonate minerals in Wufeng-Longmaxi Formation. (2) Bentonite is rhyodacite dacite-andesite, and its protomagma is basic-neutral. (3) Bentonite is rich in Th, U, Cs, and Co but low in Cr, Ni, and Ta. Its partition diagram of rare earth elements leans to the right, rich in light rare earth elements but low in heavy rare earth elements, presenting a slight Eu negative anomaly. (4) Primordial magma eruption of bentonite occurred in a volcanic arc environment, speculated to be related to the volcanic eruptions on the edge of the continent after post-collision proliferation between Cathaysia and Yangtze Plates during the Late Ordovician to the Early Silurian period. (5) The main influence of bentonite on the shale reservoir is its controlling effect on organic matter enrichment in the Wufeng-Longmaxi Formation. Moderate volcanic activity (thickness of bentonite is 1-2 cm) can promote algal blooming to improve the paleo-ocean productivity and lead to the formation of high TOC. On the contrary, if the volcano is less active (thickness of bentonite is <1 cm) or overactive (thickness of bentonite is >2 cm), the productivity improvement of the paleo-ocean would be restrained and TOC would drop.