Determination of processes governing hypogene dissolution and collapse of carbonate rocks

The susceptibility of carbonate and evaporite successions to dissolution means that they are prone to collapse. This creates complex zones of brecciation, and in the subsurface, these zones might result in borehole instability, poor core recovery, erratic wireline log responses and possibly high flow rates. They might also be recognised on seismic data as localised, non-stratabound, chaotic responses that are associated with vertical zones of disruption, indicative of fluid escape. Developing a predictive understanding of these features is important for effective development and management of resources, from groundwater aquifers, geothermal and hydrocarbon reservoirs, to subsurface storage of hydrogen and CO2.
Often, brecciated zones in limestone and evaporites are assumed to form as a result of stratal collapse by near-surface dissolution from surface recharge of meteoric waters1,2 i.e. they are typically assumed to be the product of groundwater flow. However, dissolution from upward-rising meteoric or formational fluids, for example along faults and fractures, can also occur3. The processes by which this so-called hypogene karst forms are less well understood, and dissolution as a result of cooling and/or mildly acidic groundwater, oxidation of H2S, fluid mixing and CO2 ingress have all been invoked4-8. What has yet to be fully achieved is a holistic interpretation of the structural, sedimentological and diagenetic processes that govern the location of collapse features, coupled with process-based understanding of interactions between fluid flow and water-rock interaction to assess rates, patterns and distributions of dissolution under contrasting conditions. This PhD project aims to address these issues by:
• Data review and characterisation of documented collapse features from published case studies based on outcrop, core and seismic studies
• Characterization of subsurface collapse features in core, using subsurface data from two areas in the West Canada Sedimentary Basin (WCSB) within Devonian strata.
• Construction of a series of 1D, 2D and 3D reactive transport models to assess the processes governing cavern formation from rising fluids and the viability of a range of existing conceptual models for a suite of rock types
• Delineation of the structural controls on collapse breccia distribution
Comparison of model outputs with natural geological systems, and evaluation of critical feedbacks between permeability evolution, fluid flow, rock properties and dissolution.
This project has the potential to deliver new and exciting results that will inform how fluids of variable compositions flow and react within sedimentary basins. It will also have direct application to the improved recovery of oil and gas from mature fields, as well as the safe and secure storage of CO2 in carbonate and evaporite reservoirs, cap-rock integrity, and water management in geothermal heat production.
This project requires an upper second class or first class degree BSc or MEarthSci in geological sciences and/or an MSc at merit level or above in a geological discipline. Students should be numerate, have some knowledge of coding and experience of using models in Earth Sciences as well as be willing to conduct geological description and petrographical analysis. Experience of carbonate sedimentology, diagenesis and structural geology is an advantage.
This is a split-site PhD project between University of Manchester and University of Bristol. The successful applicant will be expected to spend time working on site at both universities, but the lead university will be decided in consultation with the applicant based on their skill sets and personal circumstances. It is expected that periods of data collection will be spent in Canada (Calgary and Edmonton) and visits to Norway to look at subsurface data is also anticipated.