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.