干热岩勘查评价指标与形成条件

干热岩具有利用率高、无污染、储量巨大、分布较广、持续稳定、安全性好等特点，被全球公认为在21世纪能够取代化石能源的一种最优质的、可再生的新型清洁能源。目前对于干热岩的成因机理还没有定论，而对于干热岩的勘探寻找、远景区的圈定以及资源评价也有不同的观点，为了有利于干热岩勘探开发，本文在综述前人研究工作基础上，总结归纳了干热岩勘查评价指标，几个重要的指标及其特征如下。第一个指标是岩石圈厚度和莫霍面埋深，岩石圈厚度较小并且莫霍面埋深比较浅，是评价干热岩远景区的一个重要指标。莫霍面埋深较浅指示深部热源（幔源热）更加接近地表，故埋深较浅（较薄的厚度）并具有上隆的特征有利于深部的热量向上传导，为干热岩的孕热环境提供良好的条件。第二个指标是居里等温面，埋深较浅的居里等温面是干热岩远景区一个重要的评价指标。居里等温面是地球内部一个非常特殊的温度（热物质）界面，它不仅能指示地下温度场的分布特征，还可指示地壳深部热能分布特征，对干热岩及地震的成因研究具有十分重要的意义；如果居里面埋深较浅，则热量传导到地表的距离比较短，深部的热流活动更容易向地表传送，不仅是有利的高温干热岩孕热环境，也有利于储存的热能快速向上传导。第三个指标为地温梯度，地温梯度较大是寻找干热岩远景区的一个重要指标；如果一个地区具有较高的地温梯度（≥ 35℃/km），则随着深度的增加深部的地温增加较快，在相对较浅的地方就可以获得温度较高的岩石体。第四个指标是大地热流，大地热流值较大（≥ 75 mW/m2）就指示地球深部有存在高温岩石体的可能；大地热流是地温场的综合性热参数，能够准确地反映区域内的地温场特征。第五个指标是新构造运动，这也是人们寻找干热岩时容易忽略的一个重要指标。新构造运动包括火山、地震及活动断裂构造等。地震和火山是极具破坏性的自然灾害，两者的发生都表明了地球内部的热能汇聚到一定程度，从而打破了地球内部平衡状态而以地震或火山的形式把热量进行释放的地球系统行为。该指标中，如地震震级大（>3级）、震源深度浅（10~15 km）、频度大，火山活动时间新（活火山、休眠火山、中新世以来的死火山）规模大都说明地球深部存在不稳定的高热状态，易形成干热岩；如果能提前找到该区域的干热岩，可以先取出其中的热，那么地震和火山就有可能不会发生，这样可以达到取热减灾减排的作用。活动的深大断裂即能产生一部分热，也能将深部的热传输到浅部，尤其是活动性强的走滑拉张断裂，其深部具有韧性剪切特征，直接指示了深部的高温体的存在。第六个指标是高温温泉与气田等。温泉、气田的形成通常与深部的热储关系密切，一般认为地下水沿某个通道向下渗透接触到深部高温热储被加热后再沿某一通道流出地表而形成温泉；所以，温泉的出露指示了深部存在高温的岩石体（干热岩）；区域地温场异常明显，地表热泉等高温水热型地热田较密集的区域有望在深部寻找到干热岩，这也是一种就热（水热）找热（干热）的常规方法。作为固体矿产资源的干热岩，其形成具备四个必要条件:源、通、储、盖。第一个条件是要有丰富的动态热源如来源于深部地幔（幔源热），来源于晚新生代活动的控热构造系统-活动的韧性剪切带，来源于地壳内的低速低阻体（中下地壳热）以及来源于高放射性中新生代花岗岩体（壳源热）。第二个条件是要有优良的导热通道，如壳内15~25 km低速层不仅是热源，同时具有将深部地幔热能向上传导的作用；软流圈地幔上隆时具活动性的深大断裂（深部具韧性特征、浅部具脆性特征）常常具有很好的导热功能；地壳浅表层次的脆性断裂系统往往不是干热岩的热通道，而是水热型地热能的导水、释热构造。第三个条件是要有巨大的储热岩石体，除埋深要适中（3~6 km）并具有较高温度（≥150℃）外，其规模要大（蕴含丰富的热能），热导率大（>2 W/mK），裂隙少（不含水或含少量不流动的水）；当然热储层可以是变质岩、岩浆岩，也可以为沉积岩。第四个条件是要有良好的保热盖层，盖层（被子）导热率低（< 2 W/mK）、厚度适中（>1 km）（具有良好的保温效果），地温梯度高（≥40℃/km）、大地热流值高（≥70 mW/m2）（指示深部存在高温特征）是深部赋存有高温地热资源的必要条件。 

Evaluation indexes and formation conditions of hot dry rock exploration

Hot Dry Rock has the characteristics of high utilization rate, pollution-free, huge reserves, wide distribution, sustained stability, good security, and so on. It is recognized globally as a new type of high-quality, renewable and clean energy that can replace fossil energy in the 21st century. At present, the genetic mechanism of Hot Dry Rock has not yet been concluded, and different opinions emerge on the exploration and search of Hot Dry Rock, the delineation of the prospective area, and the evaluation of resources. To facilitate the exploration and development of Hot Dry Rock, this paper summarizes the evaluation indexes of Hot Dry Rock exploration based on previous researches. Several important indexes and their characteristics are described as follows.The first index is the thickness of the Lithosphere and the depth of the Moho. The thickness of the Lithosphere is small and the depth of the Moho is shallow, which is an important index to evaluate the Hot Dry Rock prospect. The shallow buried depth of the Moho indicates that the deep heat source (mantle heat) is closer to the surface, so the shallow buried depth (thin thickness) and uplifting feature are conducive to the upward conduction of deep heat and provide advantageous conditions for the thermal environment of Hot Dry Rock. The second index is the Curie isotherm surface. The shallower buried Curie isotherm surface is an important evaluation index for the Hot Dry Rock prospective area. The Curie isotherm surface is a very special temperature (thermal material) interface inside the earth. It can not only reflect the distribution characteristics of the underground temperature field, but also the heat energy in the deep crust, which is of great significance to the study of Hot Dry Rock and earthquake genesis. If the Curie isotherm surface is buried shallowly, the distance of heat conduction to the surface is relatively short. The deep heat flow is easier to transfer to the surface, which is not only a favorable environment for heat generation of high-temperature Hot Dry Rock thermal environment, but also conducive to the rapid upward conduction of stored heat. The third index is geothermal gradient. A larger geothermal gradient is an important index to hunt Hot Dry Rock prospect. If an area has a high geothermal gradient (≥35℃/km), so the deep geothermal temperature will increase faster as the depth increases. A higher temperature rock mass can be obtained in a relatively shallow place. The fourth index is terrestrial heat flow. A large value of terrestrial heat flow (≥ 75mW/m2) indicates the possibility of a high-temperature rock body in the deep part of the earth. Terrestrial heat flow is a comprehensive thermal parameter of the geothermal field, which can accurately reflect the characteristics of the regional geothermal field. The fifth index is neotectonic movement, which is also an important index while people tend to ignore when searching for Hot Dry Rock. Neotectonics include volcanoes, earthquakes, and active faults. Earthquakes and volcanoes are extremely destructive natural disasters. The occurrence of both exhibits that the heat energy inside the earth has accumulated to a certain extent, thus breaks the earth's internal equilibrium and releases the heat in the form of earthquakes or volcanoes. Among the indicators, the earthquake with the characteristics of large magnitude (> 3 m), shallow focal depth (10-15 km), and the volcanic with the characteristics of high frequency, new activity time (such as active volcano, dormant volcano, and dead volcano since Miocene) mostly indicate an unstable state of high heat in the deep part of the earth, where is easy to form Hot Dry Rock. If we can find the Hot Dry Rock in this area in advance, we can utilize the heat first, then earthquakes and volcanoes may not occur, which can achieve the goal of heat extraction, disaster reduction, and emission reduction. Active deep faults can not only generate part of the heat, but also transfer the deep heat to the shallow parts. Especially the highly active strike-slip tensile faults, featuring ductile shear characteristics in the deep part directly forecast the existence of a high-temperature body in the deep Earth. The sixth index is high-temperature hot springs and gas fields. The formation of hot springs and gas fields is usually closely related to the deep thermal reservoir. It is generally believed that the groundwater penetrates downward along a certain channel, contacts the deep high-temperature thermal reservoir, and then flows out of the surface along a certain channel to form hot springs. Therefore, the exposure of hot springs indicates that high-temperature rocks (Hot Dry Rocks) exist in the deep. It is expected that Hot Dry Rock can be found in the field with the characteristics of abnormally regional geothermal, intensive hot springs, and other high-temperature hydrothermal geothermal fields, which is also a convent method of finding dry heat by hydrothermal heat.As a solid mineral resource, the formation of Hot Dry Rock in deep Earth has four necessary conditions: Source, Transportation, Storage, and Cap. The first condition is abundant dynamic heat sources, such as the deep mantle (mantle-derived heat), the heat-controlled tectonic system-an active ductile shear zone that is active in Late Cenozoic, and the low-velocity and low-resistance body in the crust (middle-lower crust heat), and high-radioactive Mesozoic-Cenozoic granite body (crust-derived heat). The second condition is a quality heat conduction channel, such as the low-velocity layer of 15-25 km in the crust. The low-velocity layer is not only a heat source but also access conducting the thermal energy of deep mantle upwards. The active deep faults (ductile in deep and brittle in shallow) also conduct heat excellently when the asthenospheric mantle uplifts. The brittle faults at the shallow level of the crust are not the heat channel of Hot Dry Rock, while they can transport water and release heat for hydrothermal geothermal energy utilization. The third condition is a huge thermal storage rock body with moderate burial depth (3-6 km), high temperature (≥150℃), large scale(containing rich thermal energy), which has high thermal conductivity (>2 W/mK) and few fractures (containing no water or a small amount of immobile water). Moreover, the thermal reservoir can be metamorphic rock, magmatic rock, or sedimentary rock. The fourth necessary condition is a favorable thermal insulation cover layer, which has the characteristics of low thermal conductivity (< 2 W/mK), moderate thickness (>1 km) (good heat preservation effect), high geothermal gradient (≥40 ℃/ km) and high geothermal flow (≥70 mW/m2) (indicating high-temperature characteristics in the deep) the existence of high-temperature geothermal resources.