Evaluation of the development potential of hot dry rock in Hainan Island

Hainan Island is located in the northwestern South China Sea at the boundary of two tectonic plates. Surface features and actual drilling have confirmed that there are rich geothermal resources in the area, with predicted resources of 480 million GWh. On the basis of the 1:200,000 gravity and 1:10 aeromagnetic data and constrained by the well-explored Fushan Depression and Baocheng Rock Mass, we conducted a systematic study of hot dry rocks and evaluated the development potential of the geothermal resources of the medium-depth and deep hot dry rocks in Hainan Island for the first time. The island is structurally active, with extensive granites, strong volcanic and earthquake activities, and many hot springs, indicating that the geothermal geological conditions are favorable. There are two types of hot dry rocks, sedimentary basin type and uplifted mountain type, on the island. The sedimentary basin-type hot dry rocks are mainly found in the north of the island and have an area of 2657 km², an average depth of 2480 m, and an average heat reservoir temperature of 230 °C at 6000 m depth. Uplifted mountain-type hot dry rocks with a total area of 13,000 km² mainly occur in Danzhou in the north of the island and Baocheng in the south. The heat reservoirs of these rocks are Yanshanian intrusive plutons and the average temperature is 236 °C at 6000 m depth. Nine favorable hot dry rock targets are identified, of which five are sedimentary basin types and four are uplifted mountain types. The Fushan and Eman depressions in north Hainan Island and the Baocheng Rock Mass in the south of the island are the preferred exploration targets.