Origins and Evolution Mechanisms of Shale Formation Water in the Sichuan Basin: Insights from Hydrochemistry, Isotope Tracing, and Structural Analysis

Abstract: High water saturation in shale reservoirs represents a critical challenge for hydraulic fracturing efficiency and gas productivity. Understanding the genesis and evolution of shale formation water is essential for predicting "sweet spots" and managing water production. In this study, we established a systematic theoretical framework for the multigenetic origins of shale water by analyzing the hydrochemical characteristics, Hydrogen-Oxygen-Strontium (H-O-Sr) isotopes, and fluid inclusion data derived from fracturing flowback fluids and associated calcite veins in the Sichuan Basin. Our results classify shale formation water into two distinct genetic categories: Native Water and Exogenous Water. Native water comprises sedimentary residual water, characterized by high salinity and paleo-seawater isotopic signatures, and mineral transformation water released during clay diagenesis and hydrocarbon generation. Conversely, Exogenous water is injected into the reservoir via fracture networks. By integrating structural analysis, we identified three exogenous subtypes: (1) Deep hydrothermal fluids, evidenced by radiogenic Sr isotopes and high-temperature mineral assemblages along strike-slip faults; (2) Meteoric water infiltration facilitated by shallow "open" faults; and (3) Inter-layer formation water migrating through vertical fault conduits. We propose that the actual shale water system is a dynamic product of fluid mixing and fluid-rock interactions controlled by tectonic styles. Structural deformation not only drives the vertical injection of external fluids but also regulates the lateral migration of fluids along bedding planes, resulting in significant heterogeneity in water saturation (e.g., fluid enrichment in structural lows). Case studies in the Weiyuan and Dingshan blocks demonstrate how deep hydrothermal upwelling and atmospheric precipitation alter the primordial connate water, creating complex fluid systems containing magmatic or metamorphic signals. This study elucidates the macro-background of pore-surface fluid-rock interactions and provides a geochemical basis for evaluating the pore water distribution in high water-bearing shale gas plays.

Keywords: Shale Formation Water; Isotopes; Flowback Fluids; Fluid-Rock Interaction; High Water-Bearing Reservoirs