Evaluating the sulfur isotopic composition of biodegraded petroleum: The case of the Western Canada Sedimentary Basin

Investigation of the δ34S shows a trend that seems a priori correlated to the level of biodegradation. However, a careful interpretation of molecular and sulfur isotope data leads to the conclusion that the observed δ34S variations have rather to be ascribed to contributions of oils generated by various source rocks. Alternatively, variations of δ34S could neither be related to maturity differences nor to kinetic effects during organic sulfur compounds biodegradation. In the case of some specific core samples showing a common origin based on biomarker study, δ34S variations might not be related to different sources but to secondary sulfur incorporation/exchange processes occurring within the reservoir. These processes would involve reduced sulfur species from bacterial sulfate reduction formed in situ or migrated into Mannville reservoirs. This hypothesis is supported by laboratory experiments showing sulfur exchange/incorporation under plausible conditions for shallow reservoirs.