| Abstract |
A cap layer with a certain thickness and good thermal insulation performance is a necessary condition for the enrichment of geothermal resources. The thermal conductivity of rocks in the crust varies not only vertically but also horizontally due to the change of caprock thickness. In this case, the heat flow will refract and redistribute at the strata interface, which seriously affects the enrichment of geothermal resources. In this paper, a series of ideal geological models are established based on the caprock thickness conditions in the igneous rock area in Southern China (caprock thickness from 0-8000m, including thick caprock area with thickness greater than 3000m, medium caprock area with thickness of 100-3000m, and thin caprock area with thickness less than 100m). Then, this research simulates the geothermal field evolution process under different caprock properties (thickness, thermal conductivity and dip angle), and the influence of caprock properties on the geothermal field at the interface between uplift and sag is also analyzed. The simulation results show that the vertical geothermal distribution presents a mirror image reflection relationship with depth, the isotherm in the upper part of the elevated basement is an upwardly convex curve, while the isotherm at the lower part of the elevated basement is a concave curve. There is a temperature equilibrium line between the convex and concave isotherms. Under the temperature equilibrium line, heat flow accumulates from depression to bulge. Above the temperature equilibrium line, heat flow accumulates from uplift area to depressed area. On the basis of the above simulation results, the typical thin-cap area, medium-cap area and thick-cap area are selected for geothermal field inversion. The results show that: 1) The coastal igneous belt of Fujian belongs to the thin-cap area, and mainly develops medium and low temperature geothermal resources of hydrothermal type. 2) The Yuezhong depression belongs to the medium-cap area. The temperature in the burial depth range of 3000-6000 m is 100-200 °C, and it is possible to develop superimposed medium-high temperature hydrothermal and hot dry rock geothermal resources. The uplift of the bedrock in the basin will affect the geothermal temperature of the depression area. 3) The continental area of Beibuwan Basin belongs to thick-cap area, with the maximum thickness of cover rock up to 8000m. The thermal storage temperature range of 3000~6000m buried depth is 120~220℃. There are two types of geothermal resources in the depression area, which are medium-high temperature hydrothermal and hot dry rock. The uplift area develops medium-low temperature hydrothermal and hot dry rock geothermal resources. Exploration and site selection can be carried out according to the buried depth of the temperature balance line and in combination with specific geothermal resource requirements. |