Relationship between vitrinite reflectance, fluoresence red/green quotients, apatite fission tracks and temperature by joint inversion of three wells

Thermal maturity assessment in sedimentary basins often relies on thermal indicators of varying quality and sensitivity, frequently available in an uneven distribution across wells. To improve interpretative reliability and reduce the influence of local statistical variability, we apply a joint inversion approach to thermal maturity data from three wells situated in contrasting geological settings. One well, Aars-1, located in the Danish Basin of the eastern North Sea, is constrained by high-quality data including present-day temperature, vitrinite reflectance (VRo), red-to-green (R/G) fluorescence quotients, and apatite fission track (AFT) ages. The other two wells, from the northwestern Black Sea Basin, contain VRo and R/G data but lack temperature and AFT records.

All wells contribute to constraining a kinetic model for the R/G quotient and the thermal evolutiuon of the Black Sea wells. The resulting models yield predicted present-day bottom hole temperatures of approximately 100 °C at 4000 m depth, consistent with values from independent studies in the region. The thermal immaturity of the Black Sea wells is attributed to low background heat flow and high sedimentation rates. In the immature to early mature maturity window, a linear relationship is established between VRo and R/G, described by the equation: VRo = 1.3239 × R/G – 0.2352, valid within the calibration range of 0.43 < R/G < 0.73 and 0.33 < VRo < 0.73. Based on this relationship, the previously reported VRo discontinuities at the base of the Black Sea wells are likely artefactual.

This study demonstrates the value of combining independent thermal indicators across multiple wells and depositional settings. The approach enhances the robustness of thermal history reconstructions and offers a practical framework for evaluating relative heat flow and maturity in basins with incomplete or uneven datasets. Particularly, the R/G quotient, derived from autochthonous alginite deposited within the basin, is less affected by transport-related degradation than VRo and may offer a more representative measure of in-place thermal maturity, particularly in high-sedimentation environments.