Applicability of calculated vitrinite reflectance for assessment of source rock's organic matter maturity in hyperthermal basins (Banat Depression, Serbia)

Middle and Upper Miocene petroleum source rocks from the Banat Depression, SE Pannonian Basin (Serbia) were investigated. These rocks contain organic matter (OM) in a relatively wide range of maturity (from early to late stage of liquid hydrocarbons generation). OM consists predominantly of Type II kerogen and was deposited under variable redox conditions. This prolific oil and gas production area, as a part of the hyperthermal Pannonain Basin, is characterized by high geothermal gradients (4.0-7.5°C/100 m) and high heating rates (9-22°C/Ma) of the Miocene sediments. Considering complexity of the vitrinite reflection measurements in the source rocks containing Type I and/or Type II kerogen, in this study the values of the measured vitrinite reflectance were compared to calculated vitrinite reflectance, based on maximal palaeotemperature and numerous geochemical maturity parameters. The objective of the study was to assess the applicability of calculated vitrinite reflectance for the Type II kerogen source rocks from a hyperthermal basin. The correlation analysis was performed according to Spearman and Pearson tests. The obtained results showed that calculation of vitrinite reflectance and determination of thermal maturity of OM of the source rocks in hyperthermal basins with high heating rates is best done when based on geochemical parameters calculated from compositions of steranes and methyldibenzothiophenes, and as expected-on maximal palaeotemperature.