Origin and controlling factors of chlorite coatings—an example from the reservoir of T3x Group of the Baojie area, Sichuan Basin, China

Abstract

The study of the chlorite coatings always attracts scholars in China and other countries because the chlorite coatings play an important role in the preservation of residual primary pores in sandstone reservoirs. At present, the study of the origin and the controlling factors is relatively few. The occurrence, time of formation, genesis, controlling factors, and the mechanism of chlorite coatings inhibiting quartz overgrowths were studied in detail with thin section and SEM analysis. Samples were from the sandstone reservoirs of the T3x Group in the Baojie area, the transitional zone from the middle to the south of Sichuan Basin. The results indicate that the chlorite coatings on the walls of the pore spaces are oriented perpendicular to grain surfaces in the form of isopachous (even-thickness) grain-coating, while the chlorite coatings at the contacts between adjacent detrital grains are arranged with a preferred orientation tangential to the surface of detrital grains. The chlorite coatings were formed in the eogenetic stage. They were formed by recrystallization of Fe-rich clay films during the syndepositional period, and chlorite cements would be recrystallized after the coatings’ formation. The formation of chlorite coatings was mainly controlled by the depositional environment, provenance conditions, and diagenetic environment. The presence of chlorite coatings could result in the preservation of primary pores in deeply buried sandstone reservoirs by effectively inhibiting quartz overgrowths and the development of compaction and pressure solution.