Integration of methyldibenzothiophene and pyrolysis techniques to determine thermal maturity in sulfur-rich Type II-S source rocks and oils

Abstract

Isomer ratios between certain polycyclic aromatic compounds in produced oils and bitumen extracts change according to the maturity of the source rock, similar to results obtained from visual vitrinite reflectance and pyrolysis. For example, empirical relationships between methyldibenzothiophene (MDBT) and methylphenanthrene compounds correlate to these parameters. However, isomer ratios and their corresponding empirical relationships are not universally applicable. Their concentrations are source-specific, influenced by maceral type, and change with the kinetics of the source rock that is generating the hydrocarbon. The organosulfur content in kerogen can strongly affect the kinetics by lowering the activation energy, resulting in the generation of hydrocarbons at lower temperatures than other source rocks. This can cause uncertainty in the calibration of basin models for predicting the timing of hydrocarbon generation, expulsion, and migration, especially for Type II-S sources. Presented are the results of a study conducted on Type II-S carbonate organofacies from the United States and the Middle East that integrates the MDBTs extracted from bitumen and measured in produced oil with other ancillary maturity indicators. The MDBT isomers show a correlation between Tmax and hydrogen index that allows for the development of maturity models to accurately predict a %Ro equivalent from immature to condensate/wet gas maturity. Results indicate the often-utilized empirical equations developed from the methylphenanthrenes are not applicable for determining maturity of marine Type II-S organofacies. Instead, the MDBT-HI and MDBT-Tmax methods provide more reliable estimates of maturity for the Type II-S, marine carbonate organofacies to aid in validating basin model predictions for hydrocarbon generation.