The use of free radicals of organic matter to determine paleogeothermal gradient

Free radicals in 60 kerogen samples from 5 wells in the Qiongdongnan, Tarim and Liaohe basins in China were analyzed by Electron Spin Resonance (ESR) for use in geothermometry. Based on ESR data for 44 kerogen samples from the Tertiary Qiongdongnan basin, a regression curve was shown to fit free radical densities (Ng) with burial depth of samples for use in geothermal gradient estimation. Using this curve, the highest geothermal gradients experienced by Carboniferous-Permian strata (Well TMC1 in the Tarim basin) and Oligocene-Eocene strata (Well LR16 in the Liaohe Basin) were estimated. Concurrently, five samples from Liaohe Basin were examined by apatite fission track analysis to assess the reliability of the ESR-kerogen method for geothermal gradient determination. Results of ESR analyses match those of vitrinite reflectance and apatite fission track analysis, indicating that maximum geothermal gradients obtained from the ESR curve are reliable. This paper presents a useful ESR-kerogen method for estimating paleogeothermal gradients, which is especially useful where vitrinite and apatite are not present.