Fluid evolution and tracing at micron-scale of shales in faulted lake basin: A new perspective based on analcime

Focusing on the shale oil reservoirs of the second member of the Kongdian Formation in Cangdong Sag, eastern China, this study explores the origin and distribution of analcime, aligns the fluid evolution stage with different analcime types, and constructs a new micron-scale water-rock reaction sequence. The study identifies six types of analcime based on occurrence characteristics (occurring as laminae, lens, fracture filling, bioshell filling, vein marginal crystal and cement). The above six types of analcime is further classified into hydrothermal fluid origin analcime (HFOA: include analcime cement, vein marginal crystal, lens, fracture filling and bioshell filling) and connate fluid origin analcime (CFOA: analcime laminae) based on major elemental indicators (Si/Al and 10K/(10K + Na)). HFOA has lower ∑REE (rare earth element) and strong positive correlation between ∑REE and LILEs (large ionic lithophilic elements); while CFOA has higher ∑REE and weak positive correlation between ∑REE and LILEs. Different analcime types correlate with varying fluid properties and transport stages. HFOA forms during magmatic hydrothermal fluid (MHF) upwelling. After the MHF entering and mixing with the lake water, thermal repulsions between the crystal particles made it move to form fine grained sedimentary layer, the connate fluid trapped in pores directly precipitate to form CFOA, or form CFOA by modifying clay minerals and feldspars. Through this study, we systematically analyzed the fluid evolution and activity characteristics of the faulted lake basin by using analcime distributed in micron-scale laminae and fractures, hoping to provide new perspectives for the study of diagenetic processes in sedimentary basins.