Thermal maturity evaluation on mildly artificially oxidised sporomorphs: A comprehensive calibration of palynomorph darkness index (PDI) with vitrinite reflectance

The assessment of thermal maturity of organic matter dispersed in sedimentary rocks is a fundamental task in geological studies, including the modelling of burial history of sedimentary basins, palaeotemperature evolution and the hydrocarbon generation assessment. Palynomorph Darkness Index (PDI) is a quantitative method for evaluating thermal maturity, based on the RGB image-analysis of palynomorphs. The growing use of this method has demonstrated the need for calibration against other maturity indicators such as vitrinite reflectance (Avg. VRo), Tmax, SCI (Spore Color Index) and TAI (Thermal Alteration Index). In the present study, PDI was determined on continental palynomorphs and compared with a large dataset of vitrinite reflectance measurements from the same samples, covering a thermal maturity range between the immature and overmature stages of hydrocarbon generation. The selected samples, belonging to twenty-five different wells, were previously oxidised with nitric acid (60%HNO3) for approximately 20 min to facilitate routine palynostratigraphic investigation. Calibration against vitrinite reflectance resulted in two linear correlation equations, one for Avg. VRo below 0.7% and another for values above 0.7%. Additionally, six samples from the Selli level of the Aptian Gorgo a Cerbara section (Umbria-Marche Apennine, Central Italy) were analysed to understand the differences in PDI values between artificially oxidised and unoxidised organic matter. The analysis showed a discrepancy of approximately 19% in PDI values for the same thermal maturity between unoxidised and oxidised sporomorphs. Our results confirm that PDI is a reliable and valid method for evaluating thermal maturity of organic matter, particularly when standard methods cannot be applied. Moreover, comparison with published data concerning other palynomorph groups (i.e., acritarchs) further supports the usefulness of this technique in geological investigations.