Fluid tracing and evolution at micron-scale of shales in faulted lake basin: Evidence and constraints from in-situ analysis of analcime

The Huanghua Depression in the Bohai Bay Basin has experienced significant development of massive NNE-NE faults, influenced by the Eocene activation of the Lanliao Fault and its subsequent Oligocene dextral intensification. These faults serve as a conduit for connate fluid and magmatic hydrothermal fluid (MHF), thereby forming a complex fluid system. Gaining an understanding of these fluid activities is essential for research into mass balance and hydrocarbon migration within shales. This study explores the origin and distribution of analcime, aligns the fluid evolution stage with different analcime types, and constructs a comprehensive 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 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. Considering the tectonic context, we infer that the formation of analcime was primarily driven by high-frequency, low-intensity magmatism, governed by the activation of Lanliao deep fault. The study of analcime can aid in identifying and classifying fluid activity processes, offering a novel perspective for investigating sedimentation and diagenesis in continental shales.