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

喀斯特是用来描述石灰岩溶解形成的一系列特征的术语。它的表面特征包括石灰岩路面、石灰岩（沉洞）和塔式岩溶，以及地下洞穴和化学（碳酸盐）沉淀。喀斯特是由石灰岩溶解形成的，可以形成连接良好的洞穴、裂缝和通道，控制地下水的地下流动。溶洞是在地质年代中形成的，它是在地下水位之上、之下以及在温和酸性地下水中溶解的结果，也是不同化学成分的水混合的结果（例如在海洋-大气混合带）。最常见和最容易理解的是，地下洞穴是后生的——它们是由于地下水的溶解而在近地表形成的，包括水平或次水平的洞穴，与石灰岩地层基本一致。这种表生岩溶的成因已相当清楚。下生岩溶是另一种类型的地下洞穴，虽然它是地球上许多最大的洞穴网络的组成部分，但研究得较少。其特征为垂直或亚垂直洞室，洞室横切地层，可连接层控洞室，形成迷宫洞室系统。它们被解释为由向上流动的流体溶解形成，也可能与化学反应有关，如氧化和/或与富含CO2或H2S的水溶解有关。它们具有潜在的重要意义，因为：1)它们可以控制地下水流动的方向和速度，以及热水的供应（例如，用于温泉或地热生产）2)它们可能导致高速率的流体损失率，以及在地热能源和碳氢化合物开采期间的压力扰动3)它们可以在地下形成地质力学不稳定区，这可能导致地面不稳定和塌陷（即形成天坑）。该项目涉及英国德比郡台地石炭纪（中-密西比世）石灰岩中一系列非层控、垂直和亚垂直洞穴的特征，并对其成因进行了评估。由于研究区内至少有四种类型的非层控溶洞，它们可能是一系列地下作用的范例。它们是不同的开放，方解石胶结，由方铅矿-萤石-重晶石-方解石组合矿化和/或沉积物填充，但不清楚它们是否在同一时间和同一过程中形成。该项目将使用野外测绘、低温地球化学和反应输运模型来确定不同类型洞穴形成的方式和时间，以改进对其分布的预测。该项目将通常不互动的社区聚集在一起，将a)碳酸盐沉积学和地球化学方面的专业知识与b)洞穴学、水文地质学和建模方面的深厚知识以及c)英国地质调查局（British Geological Survey）拥有的广泛的地球科学数据库结合在一起。它将提供洞穴形态和填充的数据库，以及一套解释洞穴形成的模型。这是特别相关的，因为密西西比石灰岩是英国和北欧地热能源生产的主要目标。研究结果也将广泛适用于全球的水文地质学家和洞穴学家，他们对物理和化学地下水模型和地面稳定性感兴趣，以及对碳酸盐储层的碳氢化合物生产、天然气储存和碳封存感兴趣的地下地球科学家。

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 production3) 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.