Fault-controlled paleofluid flow in shale caprocks: Structural, geochemical, and mineralogical evidence from the Bristol Channel Basin (UK)

Understanding the development of fault zones in caprock shales and their impact on permeability is critical when considering underground CO₂ storage in aquifers and depleted reservoirs. To enhance this knowledge, we characterize the paleofluid system in seismic-scale faults cutting through low-permeability shale formations, assesing whether fluids recorded in the fault core and damage zone record large-scale migration and enhanced effective permeability. The Somerset coast (UK), along the southern margin of the Bristol Channel Basin, exposes shale and marl dominated Mesozoic caprock successions dissected by seismic-scale normal and inverted faults, of which the timing of initial activity is well constrained by radiochronology and which exhibit well-preserved fault cores and damage zones as well. In this study, we present field-based structural analyses, petrographic investigations, stable isotope geochemistry (δ¹³C, δ¹⁸O) and clay mineralogy (XRD) from six selected outcrops. The vein-rich damage zones exhibit calcite and gypsum precipitation, recording transient fluid-flow episodes during reactivation. Stable isotope data, combined with fluid inclusion petrography, indicate that these episodes were dominated by meteoric fluids (δ¹⁸O<-10‰ VPDB) from which synkinematic calcite precipitated in faults at geothermal conditions (<60°C). When considering published radiogenic ages for the extensional development (150-120 Ma) and subsequent inversion (50-20 Ma) of the considered faults, the recharge of meteoric fluids in the fault at depth is consistent with regional paleogeographic reconstructions showing fluctuating emergence of landmasses in the area during the Late Triassic to Cretaceous. Within this framework of episodic fluid circulation, most fault cores are mechanically sealed rather than swelling-sealed, with permeability reduction controlled by grain-size reduction and the development of aligned clay fabric. Nevertheless, mineralized fault cores demonstrate that fault sealing is not static, during episodes of elevated fluid pressure or reactivation, permeability may be locally and temporarily enhanced along discrete slip surfaces. This behavior is strongly controlled by fault-zone architecture, with increasing displacement promoting gouge thickening and fabric development, ultimately leading to more effective long-term sealing. Beyond regional implications, our study reconstructs kilometer-scale downward fluid-flow along faults, supporting the significant impact of the damage zone on the long-term integrity of clay-rich caprocks.