Hypogene karst: genesis and implications to optimisation of low enthalpy energy resources

Karst is the term used to describe a suite of features formed by dissolution of limestone. It includes surface features such as limestone pavements, dolines (sink holes) and tower karst, as well as subterranean caverns and chemical (carbonate) precipitates. Karst forms by dissolution of limestone and can create well connected caverns, fissures and passages that control the subterranean flow of groundwater. Caves form through geological time as a result of dissolution above, at and below the water table from mildly acidic groundwater and mixing of waters of different chemistry (e.g. at the marine - meteoric mixing zone). The most common, and best understood, subterranean caverns are epigenetic - they form in the near surface due to dissolution by groundwater and include caves that are horizontal or sub-horizontal and broadly conformable with limestone strata. The genesis of this epigene karst, is fairly well understood. Hypogene karst is another type of subterranean cavern which is less well studied, even though it is a component of many of Earth's largest cavern networks. It is characterised by vertical or sub-vertical caverns that cross-cut strata and can connect stratabound caverns to form maze cave systems. They are interpreted to form by dissolution from upward-flowing fluids and may also be associated with chemical reactions such as oxidation and/or dissolution associated with CO2 or H2S- rich water. They are potentially significant because:
1) They can control the direction and rate of groundwater flow, as well as the supply of thermal waters (e.g. to hot springs or for geothermal heat production)
2) They might result in high rates of fluid loss, and pressure perturbations, during drilling for geothermal energy and hydrocarbon production
3) They can create zones of geomechanical instability in the subsurface, which could lead to ground instability and collapse (i.e. formation of sinkholes).
This project is concerned with the characterization of a range of non-stratabound, vertical and sub-vertical caverns within Carboniferous (mid-Mississippian) limestone of the Derbyshire Platform, UK, and an assessment of the factors governing their genesis. Since at least four types of non-stratabound cavern occur in the study area, they potentially serve as an exemplar for a range of hypogene processes. They are variably open, calcite-cemented, mineralised by a galena-fluorite-barite-calcite assemblage and/or sediment filled, but it is unclear if they were formed at the same time and by the same process. The project will use field mapping, low temperature geochemistry and reactive transport modelling to determine how and when the different types of caverns formed, in order to improve prediction of their distribution. The project brings together communities who do not commonly interact by coupling a) expertise in carbonate sedimentology and geochemistry with b) a deep knowledge of speleology, hydrogeology and modelling, and c) extensive geoscience databases held at the British Geological Survey. It will deliver databases of cavern morphology and fill and a suite of models to explain their formation. This is of particular relevance because Mississippian limestone is a primary target for geothermal energy production within the UK and northern Europe. The results will also be widely applicable to hydrogeologists and speleologists globally, who are interested in physical and chemical groundwater modelling and ground stability, as well as to subsurface geoscientists interested in hydrocarbon production, gas storage and carbon sequestration in carbonate reservoirs.