A novel VRo, Tmax, and S indices conversion formulae on data from the fold-and-thrust belt of the Western Outer Carpathians (Poland)

Results are presented on the relationships between the vitrinite reflectance VRo, the T-max temperatures determined with Rock-Eval pyrolysis, and the smectite (S) indices representing the percentage of smectite in the mixed-layer illite-smectite (I/S) from claystones. Samples locations are recovered from the Polish part of the Outer Carpathian fold-and-thrust belt. Organic indices show thermal maturities of most outcrop samples in the range of 0.47 to 1.49% R-o and 421 to 485 degrees C T-max. Inorganic S indices values range from 8 to 60%. Lateral variability of the maturity measured with both organic and inorganic indices reflects mainly differences of maximum tectonic burial in the accretionary prism and subsequent exhumation and denudation. For the Western Outer Carpathians the calculated formulae for T-max to VRo and S to VRo conversion are VRo = 0.0152 * T-max 5.938 (r(2) = 0.87) and VRo = 1.1432-0.01295 * S (r(2) = 0.65), respectively. Maximum paleotemperatures were calculated for individual samples from S indices by applying the novel formula T = ln (1.1432-0.01295 * S) + 1.68/0.0124. The proposed new conversion formulae might be useful in other regions with a similar geological setting, i.e. for claystone formations dominated by kerogen type III in fold-and-thrust belts, characterized by rapid tectonic burial accompanied by a low geothermal gradient, as well as maturities of the sub-bituminous to the medium volatile bituminous rank (0.5 to 1.50% R-o).

Automated summary

The study presents relationships between vitrinite reflectance VRo, T-max temperatures determined with Rock-Eval pyrolysis, and smectite indices in claystones, unveiling lateral variability of maturity driven by tectonic burial and subsequent exhumation. Additionally, new conversion formulae for T-max to VRo and S to VRo calculations are proposed, with potential applications in regions with similar geological settings and maturities.