Fluid overpressure and diagenetic evolution recorded by antitaxial fibrous illite veins in deep coal-bearing strata: Insights from sub-micron O-PTIR and in-situ geochemistry

In deep sedimentary basins, the formation and evolution of fracture-vein systems are critical for understanding fluid migration and overpressure history. This study investigates antitaxial fibrous illite veins in the Upper Triassic Xujiahe Formation of the Wubaochang area, Sichuan Basin, to decipher the mechanisms of fluid overpressure and diagenetic evolution. A multi-proxy approach was employed, combining detailed petrography, SEM, and micro-XRD. Crucially, we applied Optical Photothermal Infrared Spectroscopy (O-PTIR)  to achieve sub-micron resolution mapping of organic functional groups within single illite fibers, alongside in-situ REE and C-O isotope analysis. Detailed petrography and SEM analysis reveal that these veins exhibit typical antitaxial growth characteristics, where mineral fibers grow from a median plane toward the host rock, recording the continuous opening and synchronous filling of fractures. In-situ rare earth element (REE) and carbon-oxygen (C-O) isotope analyses identify two distinct fluid evolution stages: Stage I reflects an external, deep-circulating basin fluid system driven by regional tectonic stress, characterized by significant water-rock interaction with host rocks. Conversely, Stage II represents localized diagenetic and hydrocarbon-generated fluids, where isotopic signatures shift toward organic-derived carbon sources, indicating a transition to hydrocarbon-generation-induced overpressure. To definitively address the timing of fluid injection, sub-micron resolution O-PTIR (Optical-Photothermal Infrared) analysis was conducted, revealing the simultaneous presence of organic acid (1720cm^-1), aromatic (1600cm^-1), and aliphatic (1450cm^-1) functional groups coexisting with the illite lattice vibration (1034 cm^-1) within single fibrous crystals. The high ratio of organic acids to mineral signals indicates that organic acids directly mediated water-rock reactions and mineral precipitation rather than being late-stage infiltrations. Our findings demonstrate that these fibrous veins are coupled products of tectonic-induced fracturing and organic-acid-mediated mineral growth. This study highlights the power of O-PTIR as a novel tracer for deciphering organic-inorganic interplays, offering new insights into the mechanisms of fluid overpressure and hydrocarbon expulsion in deep, complex basin systems.