Partitioning of fluid flow along faults and fractures in the Bristol Channel Basin: implications for low carbon geoscience projects in SW England
The Bristol Channel Basin (BCB) is a Mesozoic continental rift basin and an important analogue for offshore reservoirs. Previous studies have used relative cross-cutting relationships and correlations with adjacent sedimentary basins to constrain its development. For the first time, in-situ U-Pb carbonate geochronology has dated calcite slickenfibres within normal, thrust, and strike-slip faults in the East Quantoxhead and Kilve region of Somerset. Protracted N-S extension (ca. 150–120 Ma) formed normal faults, while subsequent NS shortening (ca. 50–20 Ma) involved (i) cross-cutting strike-slip faults, (ii) minor E-Wstriking thrust faults, and (iii) reactivation of pre-existing normal faults. During Cenozoic contraction, σ2 and σ3 were periodically interchangeable due to local stress variations and fluid pressure changes. Geochemical analyses indicate evolving fluid sources, with more saline waters during extension transitioning to more freshwater during contraction. Larger normal faults facilitated the upward migration of saline fluids from the Mercia Mudstone Group (MMG), altering pore fluid composition and raising formation water temperatures in the Blue Lias Group. These fluids also influenced fault mechanics, particularly along the East Quantoxhead Fault (EQHF). Microstructural analysis of vein generations highlights fault weakening through progressive deformation. Celestine (SrSO₄), sourced from the MMG, localised strain and promoted reactivation during contraction, as evidenced by reverse-sense S-C Introduction viii | P a g e fabrics. In contrast, smaller faults lack celestine and exhibit simpler fluid histories, although crystal-plastic textures (e.g., GBM, SGR) are observed. Understanding fluid-driven changes in fault core composition and strength is critical for evaluating fluid flow partitioning along fractures. Constraining fault population evolution within the BCB sheds light on regional stress dynamics and can inform studies of nearby basins. These insights have practical applications for managing fluid flow in modern reservoirs, such as Enhanced Geothermal Systems (EGS).
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