Geological controls on upper crustal heat flow for deep geothermal energy in Cornwall

英格兰西南部有局部高地表热流，这与科尔努比亚基花岗岩中放射性元素（U、Th和K）含量升高有关。因此，它是英国深层地热能最有前途的地区之一。以前对高温地热能生产区域潜力的调查包括由坎伯恩矿业学院（1975-1991）承担的4200万磅地热热干岩研究计划。有新的战略动力确保康沃尔两个获得深层地热规划许可的地点中至少有一个得到资助。耗资1800万英镑的联合唐斯深层地热发电项目位于康沃尔郡的雷德拉斯附近，将于2018年10月开始钻探两口井，一口深2.5公里，另一口深4.5公里；这项活动将产生英国最深的陆上地热钻孔，为博士学位提供框架。该项目的目的是解决有关康沃尔地壳热生产和传导模型的不确定性，其中包括：(1)放射性元素U、Th和K在先前研究的花岗岩上部的含量不够高，不足以解释观测到的热流；(2)历史上深地热井的He-4产量高于预期；(3)地球物理模拟逐渐减少了Cornubian花岗岩的解释体积。这些不一致意味着实质性的热源可能出现在岩基较深部分的内部或下方。该项目将进一步了解康沃尔的上地壳温度控制和深部地热能开采，以及这与以下因素的关系：(i)复合Cornubian基内的非均质放射性成因产热（目前未采样于2.6 km以下），（ii）中/下地壳和地幔源的热量贡献，以及（iii）裂缝控制的深部地下水循环的改造。carmenellis花岗岩的钻探深度约为1.5-4.5公里。这些上升将允许花岗岩的矿物学和全岩石地球化学特征，包括U、Th、K收支及其矿物学宿主，以及高温蚀变和浸出的证据。分析技术包括光学显微镜，QEMSCAN（自动扫描电镜），电子探针和XRF/ICP-MS)。QEMSCAN的使用是新颖的，因为它允许从细粒度的钻井井口进行快速矿物学分析和宿主岩石解释。这些数据将通过井下测井（伽马射线、地层流体成分/温度）得到补充。将模拟辐射源产热，并与实测热流数据进行比较。利用现有的深部地球物理数据和对英格兰西南部后瓦里斯坎构造演化中可接受的情景的理解，根据上地壳对流流体流动和/或中/下地壳和地幔热贡献的潜在作用来评估差异。深层热源的确认将对区域地热和地壳模式以及全球花岗岩地形深层地热工程产生深远的影响。该学生将在坎伯恩矿业学院接受以下方面的培训：(1)矿物学和地球化学分析和解释技能，(2)岩石学和热流建模技能，(3)区域地壳演化。地学有限公司将提供井下测井解释和裂缝控制流体流动模型方面的培训。

SW England has locally high surface heat flows associated with elevated levels of radioactive elements (U, Th and K) within the granites of the Cornubian Batholith. As such, it is one of the most prospective areas in the UK for deep geothermal energy. Previous investigations into the regional potential for high temperature geothermal energy production included the 42 million pound geothermal Hot Dry Rock research programme, undertaken by Camborne School of Mines (1975-1991). There has been renewed strategic impetus to ensure that at least one of the two sites in Cornwall with planning permission for deep geothermal is funded. The £18M United Downs Deep Geothermal Power project, near Redruth in Cornwall, will commence drilling two wells in October 2018, one 2.5 km deep and the other 4.5 km deep; this activity, which will result in the UK's deepest onshore geothermal borehole, provides the framework for the PhD. The purpose of the project is to address uncertainties regarding heat production and conduction models in the Cornish crust which include: (i) radioactive elements U, Th and K are not present in sufficiently high quantities within the previously investigated upper parts of the granite to account for observed heat flow, (ii) He-4 production from historical deep geothermal wells is higher than anticipated, (iii) geophysical modelling has progressively reduced the interpreted volume of the Cornubian granites. These inconsistences imply substantive heat source(s) may occur within, or below, the deeper parts of the batholith.The project will develop further understanding of the controls on upper crustal temperatures and deep geothermal energy exploitation in Cornwall and how this might relate to: (i) heterogeneous radiogenic heat production within the composite Cornubian Batholith (presently unsampled below 2.6 km), (ii) heat contributions from mid/lower crustal and mantle sources, and (iii) modification by fracture-controlled deep groundwater circulation. Drilling within the Carnmenellis Granite will occur over a depth range of approximately 1.5-4.5 km. The arisings will permit mineralogical and whole rock geochemical characterisation of the granites, including the U, Th, K budget and their mineralogical hosts and evidence for high-temperature alteration and leaching. Analytical techniques include optical microscopy, QEMSCAN (automated SEM), electron microprobe and XRF/ICP-MS). The use of QEMSCAN is novel as it permits rapid mineralogical analysis and host rock interpretation from fine-grained drilling arisings. These data will be complemented by those obtained during downhole logging (gamma ray, formation fluid composition / temperature). Radiogenic heat production will be modelled and compared with measured heat flow data. Discrepancies will be evaluated in terms of the potential role of upper crustal convective fluid flow and/or mid / lower crustal and mantle heat contributions using existing deep geophysical data and an understanding of admissible scenarios during the post-Variscan tectonic evolution of SW England. Confirmation of deeper heat sources would have profound implications for regional geothermal and crustal models and deep geothermal projects in granitic terrains globally. The student will receive training at Camborne School of Mines in: (i) mineralogical and geochemical analytical and interpretation skills, (ii) petrological and heat flow modelling skills, and (iii) regional crustal evolution. Training in downhole log interpretation and fracture-controlled fluid flow models will be provided by GeoScience Limited.