Crystal Chemistry and Formation of Authigenic Chlorite: Influence on Tight Sandstone Reservoir in the Yanchang Formation, Ordos Basin, China

Authigenic chlorite is a crucial diagenetic cement that significant impacts tight sandstone reservoirs. The occurrence, crystal chemistry, formation and influences on reservoirs of different authigenic chlorite types were studied in the Yanchang Formation tight sandstones from the Ordos Basin. This study identified two primary types of chlorites: alteration chlorite and autochthonous authigenic chlorite. The alteration chlorite, characterizing by high Fe, Mg, Ti and AlIV contents and low Si and AlVI contents, originated from biotite during diagenesis. The autochthonous authigenic chlorite can be further classified into grain-coating, pore-lining, and pore-filling chlorite. The grain-coating chlorite features thin layers and limited crystal form. During the syndiagenetic stage, Fe2+ and Mg2+ carried by water adsorbed onto the particle surfaces. Additionally, the grain-coating chlorite contains relatively high K content due to the hydrolysis of biotite during the eodiagenetic A stage. The pore-lining chlorite formed during the prolonged eodiagenetic B stage. The biotite and abundant intermediate-basic magmatic fragments released Fe2+ and Mg2+, contributing to the widespread development of euhedral pore-lining chlorite. Notably, the pore-lining chlorite displayed elemental composition variations from the grain edge to the pore center, with increased Fe, Mg and AlIV, and decreased K and AlVI. The pore-filling chlorite formed locally resulted from Fe2+ and Mg2+ crystallization due to the hydrolysis of intermediate-basic magmatic rocks and the dissolution early formed chlorite. The pore-filling chlorite contains the most Fe, Mg and Mn contents and the least K content. Overall, authigenic chlorite acts as a "double-edged sword" in tight sandstone reservoir. The grain-coating and grain-lining chlorite enhance the mechanical strength, resist compaction, and inhibit quartz overgrowth, while pore-filling chlorite trends to obstruct pore throats and break reservoir quality. This study provides valuable insights into the detailed control factors influencing reservoir quality and can serve as a reference for further research.