Insights into the formation and growth of authigenic chlorite in sandstone: Analysis of mineralogical and geochemical characteristics from Shaximiao Formation, Sichuan Basin, SW China

Chlorite is a widely distributed clay mineral in sandstone reservoirs, attracting significant attention due to its complex physicochemical properties and unique reservoir significance. The Shaximiao Formation in the Sichuan Basin, SW China, features abundant diagenetic chlorite, offering valuable insights into chlorite growth theories. Petrographic observations and electron probe analyses reveal that the formation of authigenic chlorite resembles a continuous, staged growth process, akin to tree growth. This process encompasses stages from depositional smectite clay (seeding) to inner-layer chlorite formation (germination), outer-layer chlorite development (branching), and pore-filling chlorite precipitation (blooming). During the initial burial, depositional smectite (seeds) transforms into low-Fe, low-Mg inner-layer chlorite (germination) under the influence of Fe and Mg ions in the original formation water. In the early diagenetic stage, volcanic detritus and biotite alteration release substantial Fe²⁺ and Mg²⁺, fostering the growth of outer-layer chlorite (branching) on the inner-layer chlorite. In the middle diagenetic stage, volcanic detritus releases Ca²⁺, Fe²⁺, and Mg²⁺ under the influence of organic acids, leading to supersaturation of Si⁴⁺, Al³⁺, Fe²⁺, and Mg²⁺ in pore water, which directly crystallizes into highly euhedral pore-filling chlorite (blooming). The presence of precursor smectite is fundamental to this process, while sufficient Fe and Mg sources are crucial for chlorite formation.