Formation mechanism of authigenic chlorite in sandstones of the Middle Jurassic Sha 1 Member in the northeastern central Sichuan Basin, China

Using thin sections, scanning electron microscopy, electron probe, X-ray diffraction, and isotopic techniques, the formation mechanisms of authigenic chlorite rims in sandstones of the Middle Jurassic Sha 1 Member in the northeastern central Sichuan Basin are analyzed. Authigenic chlorite has three forms in Sha 1 Member sandstones, grain-coating, pore-lining, and pore-filling; pore-lining chlorite with a double-layer structure is the main type. The grain-coating and inner pore-lining chlorites formed in stage A of eodiagenesis, originating from the hydration and hydrolysis of silicate minerals and the transformation of montmorillonite to chlorite in Fe2+- and Mg2+-rich pore fluids. The outer pore-lining chlorite was deposited from stage B of eodiagenesis to stage A1 of mesodiagenesis and was related mainly to the transformation of montmorillonite to illite and the dissolution of silicate minerals with organic and carbonic acids releasing sufficient Fe2+ and Mg2+. The pore-filling chlorite, which was related to the dissolution of silicate minerals by carbonic acid from decarboxylation of organic acid and transformation of clay minerals (such as illite) to chlorite, formed mainly in phase A2 of mesodiagenesis. Fe and Mg for the authigenic chlorites mainly originated from dissolution of magmatic lithic debris and biotite rich in these elements and compaction fluids from the surrounding mudstones. The authigenic chlorites are mainly distributed in medium- and fine-grained sandstones of shallow lake beach bar, estuary bar, and subaqueous distributary channel microfacies.