THERMAL MATURITY EVALUATION OF SULFUR-RICH SOURCE ROCKS AND OILS USING THE DI- AND TRI-METHYLDIBENZOTHIOPHENES

Constraining the thermal maturity of source rocks according to the properties of their generated hydrocarbons is important to develop accurately calibrated basin models for exploration. Organosulfur-rich marine source rocks (“Type II-S” kerogen) for example are found in many petroleum-prolific basins around the world. The sulfur in the organic matter changes the kinetics of kerogen transformation, which lowers the activation energy for thermally breaking bonds. As a result, Type II-S source rocks can generate oil at lower temperatures than organosulfur-poor source rocks (Type II). Geochemical indicators (e.g., vitrinite reflectance, T_max, hydrogen index, methylphenanthrenes, etc.) that are commonly used to measure maturity for Type II source rocks will therefore have a high uncertainty with applied to Type II-S source rocks. To resolve this issue, designing source-specific indicators for Type II-S are often needed for predicting the timing of oil and gas generation for a given basin.

This current study uses gas chromatography mass spectrometry techniques to measure the concentrations of dimethyldibenzothiophene (DMDBT) and trimethyldibenzothiophene (TMDBT) isomers in extracted bitumen and produced oils to show how these sulfur-bearing alkylated compounds can be used to assess and constrain maturity in Type II-S sources. Similar to the methyldibenzothiophenes (MDBT), both DMDBT and TMDBT are also thermodynamically stable and their isomeric changes make them useful maturity indicators from the immature window to the wet gas window. The results presented from this study will be corroborated with other ancillary geochemical indicators (methylphenanthrenes, T_max, hydrogen index, biomarkers) to demonstrate the reliability of numerous DMDBT and TMDBT isomer ratios over a large maturity range. Results from this study can assist in maturity verification as well as sweet-spot identification for basins that contain high sulfur content.