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<br/>analogue for offshore reservoirs. Previous studies have used relative cross-cutting<br/>relationships and correlations with adjacent sedimentary basins to constrain its<br/>development. For the first time, in-situ U-Pb carbonate geochronology has dated<br/>calcite slickenfibres within normal, thrust, and strike-slip faults in the East<br/>Quantoxhead and Kilve region of Somerset.<br/>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<br/>Cenozoic contraction, σ2 and σ3 were periodically interchangeable due to local stress<br/>variations and fluid pressure changes.<br/>Geochemical analyses indicate evolving fluid sources, with more saline waters during<br/>extension transitioning to more freshwater during contraction. Larger normal faults<br/>facilitated the upward migration of saline fluids from the Mercia Mudstone Group<br/>(MMG), altering pore fluid composition and raising formation water temperatures in<br/>the Blue Lias Group. These fluids also influenced fault mechanics, particularly along the<br/>East Quantoxhead Fault (EQHF).<br/>Microstructural analysis of vein generations highlights fault weakening through<br/>progressive deformation. Celestine (SrSO₄), sourced from the MMG, localised strain<br/>and promoted reactivation during contraction, as evidenced by reverse-sense S-C<br/>Introduction<br/>viii | P a g e<br/>fabrics. In contrast, smaller faults lack celestine and exhibit simpler fluid histories,<br/>although crystal-plastic textures (e.g., GBM, SGR) are observed.<br/>Understanding fluid-driven changes in fault core composition and strength is critical<br/>for evaluating fluid flow partitioning along fractures. Constraining fault population<br/>evolution within the BCB sheds light on regional stress dynamics and can inform<br/>studies of nearby basins. These insights have practical applications for managing fluid<br/>flow in modern reservoirs, such as Enhanced Geothermal Systems (EGS).

Author dc:creator

• Connolly, Joseph

Contributors dc:contributor

• Mark Anderson, Gregory Price, David Peacock, Catherine Mottram

Language dc:language

eng

Repository record dc:identifier

https://pearl.plymouth.ac.uk/gees-theses/472

OAI identifier oai:identifier

oai:pearl.plymouth.ac.uk:gees-theses-1914