“Uniform geothermal gradient” and heat flow in the Qiongdongnan and Pearl River Mouth Basins of the South China Sea

The Pearl River Mouth Basin (PRMB) and Qiongdongnan Basin (QDNB) are oil and gas bearing basins in the northern margin of the South China Sea (SCS). Geothermal survey is an important tool in petroleum exploration. A large data set comprised of 199 thermal conductivities, 40 radioactive heat productions, 543 measured geothermal gradient values, and 224 heat flow values has been obtained from the two basins. However, the measured geothermal gradient data originated from diverse depth range make spatial comparison a challenging task. Taking into account the variation of conductivity and heat production of rocks, we use a “uniform geothermal gradient” to characterize the geothermal gradient distribution of the PRMB and QDNB. Results show that, in the depth interval of 0–5 km, the “uniform geothermal gradient” in the PRMB varies from 17.8 °C/km to 50.2 °C/km, with an average of 32.1 ± 6.0 °C/km. In comparison, the QDNB has an average “uniform geothermal gradient” of 31.9 ± 5.6 °C/km and a range between 19.7 °C/km and 39.5 °C/km. Heat flows in the PRMB and QDNB are 71.3 ± 13.5 mW/m2 and 72.9 ± 14.2 mW/m2, respectively. The heat flow and geothermal gradient of the PRMB and QDNB tend to increase from the continental shelf to continental slope owing to the lithosphereic/crustal thinning in the Cenozoic.