The impact of faults on fluid flow in microbial and hydrothermal carbonates

Faulting may provide conduits or baffles in subsurface hydraulic systems. Despite substantial research characterising faults, heterogeneities within carbonates means that there are no current empirical relationships which can be used as a predictive tool in fault zone characterisation. This is especially true for non-marine carbonates which include a range of complex microbial and hydrothermal facies. Despite substantial interest surrounding oil-bearing non-marine carbonates of the South Atlantic pre-salt, data to provide analogues for deformation within non-marine carbonates remains scarce. This thesis aims to improve understanding of the controls on fault formation and resultant fluid-flow impacts within nonmarine carbonates. This is carried out through the integration of fieldwork, petrophysical analysis, geomechanical experimentation and optical, and CT scan microstructural analysis, with existing work on carbonate faults. Faulted, non-marine carbonate exposures are limited due to the under-representation of ancient non-marine carbonates due to alteration and poor preservation. However, observations show that shallow deformation varies greatly within different non-marine carbonate deposits and is dependent on the stress, lithology and rock strength as a product of diagenesis. The Ballık travertine (Turkey) hosts hybrid dilational faulting at low stresses which can act as fluid conduits. This contrasts the Crato Formation (NE Brazil) where poorly cemented, fine-grained laminated limestone results in semi-brittle deformation. Triaxial mechanical tests and ultra-sonic velocity measurements, show that depending on principal stress relationships, fault heterogeneity can be related to the primary depositional fabrics. Confining pressures typical of 200 – 1500 m burial depth likely produce deformation within travertines dominated by initial brittle deformation, localised in moderate porosity, micrite-dominated facies resulting in enhanced permeability. Permeability reduction may then occur in more mature fault zones from progressive microcracking and brecciation processes. Conversely, shrub travertine facies deform by compactional processes resulting in localised reductions in grain-size and porosity, impeding permeability. The study provides quantitative data to be integrated into future global datasets on non-marine carbonate rock properties and demonstrates the potential for conceptual models for characterising uncertainty in the prediction of deformation and subsurface fluid flow.