GEOCHEMICAL ANALYSIS OF MARINE SOURCE ROCKS TO ASSESS THERMAL MATURITY IN SULFUR-RICH ENVIRONMENTS

Temperature is the main driving force in the transformation of organic matter from source rocks (kerogen) to hydrocarbons. Various geochemical indicators are often utilized to constrain thermal maturity, however, not all can be applied universally. Historically, vitrinite reflectance (%R_o) has been used to constrain maturity in source rocks, but this method was designed around terrestrial sources (‘Type III’). The application of %R_o leads to high uncertainties in marine sources (‘Type II’) due to the absence of vitrinite. Furthermore, the presence of organosulfur changes the kinetics of kerogen transformation with respect to time and temperature. As a result, organosulfur-rich marine source rocks (Type II-S) can generate oil at lower temperatures than Type II source rocks. This variability in sulfur content can lead to poor constraints on basin models, which are designed to predict maturity and fluid properties for exploration. Therefore, it is important to design a robust method for predicting maturity that is specific to sulfur-rich environments to calibrate basin models.

The distributions and concentrations of sulfur-bearing, aromatic thiophene compounds are used as an alternative method for predicting maturity. The examination of the dibenzothiophene series, which are analyzed using gas chromatography mass spectrometry, indicates thermal changes can be quantified using these compounds. In this study, methyldibenzothiophene isomers are explored to develop a method for predicting maturity that are strongly correlated to the maximum generative temperature obtained through pyrolysis and empirical calculations. The chemical differences (e.g., isomerization, demethylation, etc.) in these compounds are not only indicative of thermal variations related to the transformation of kerogen into hydrocarbons, but also indicative of the source rock organofacies (e.g., carbonate vs. clastic) and the properties of their derived fluids (e.g., GOR, API gravity, etc.), all of which are essential parameters needed for accurate basin modeling.