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

岩溶是用来描述石灰岩溶解形成的一系列特征的术语。它包括表面特征，如石灰石路面，漏斗（下沉洞）和塔岩溶，以及地下洞穴和化学（碳酸盐）沉淀物。岩溶是由石灰岩溶解形成的，可以形成连通良好的洞穴、裂缝和通道，控制地下水的地下流动。洞穴的形成贯穿地质时代，是由于地下水位之上、之下和之下的弱酸性地下水的溶解作用以及不同化学性质的沃茨的混合（例如在海洋-大气混合带）。最常见和最好理解的地下洞穴是后生的-它们由于地下水的溶解而在近地表形成，包括水平或近水平的洞穴，并且与石灰岩地层大致一致。这种表生岩溶的成因已相当清楚。深成岩溶是另一种研究较少的地下洞穴类型，尽管它是地球上许多最大洞穴网络的组成部分。它的特点是垂直或近垂直的洞穴，横切地层，并能连接层控洞穴，形成迷宫洞穴系统。它们被解释为通过从向上流动的流体中溶解而形成，并且也可能与化学反应有关，例如与CO2或H2S富集的水有关的氧化和/或溶解。它们具有潜在的重要性，因为：1）它们可以控制地下水流的方向和速度，以及热水沃茨的供应（例如，用于温泉或地热生产）2）在地热能和油气生产的钻井过程中，它们可能导致高速率的流体损失和压力扰动3）它们可能在地下产生地质力学不稳定区，这可能导致地面不稳定和塌陷（即天坑的形成）。该项目涉及石炭纪内一系列非层控、垂直和近垂直洞穴的特征（中密西西比）石灰岩的德比郡平台，英国，并评估其成因的因素。由于至少有四种类型的非层控洞穴发生在研究区，他们可能作为一个范例的范围内的深成过程。它们是由方铅矿-萤石-重晶石-方解石组合和/或沉积物填充的，但不清楚它们是否是在同一时间和同一过程中形成的。该项目将利用实地测绘、低温地球化学和反应性运输建模来确定不同类型洞穴的形成方式和时间，以改进对其分布的预测。该项目汇集了通常不相互作用的社区，将a）碳酸盐沉积学和地球化学方面的专门知识与B）洞穴学、水文地质学和建模方面的深厚知识以及c）英国地质调查局拥有的广泛的地球科学数据库结合起来。它将提供洞穴形态和填充的数据库以及一套解释其形成的模型。这是特别相关的，因为密西西比石灰岩是英国和北方欧洲地热能生产的主要目标。研究结果还将广泛适用于全球水文地质学家和洞穴学家，他们对物理和化学地下水建模和地面稳定性感兴趣，以及对碳酸盐岩储层中的碳氢化合物生产、天然气储存和碳固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.