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

Abstract

The sulfur isotopic composition (δ 34S) of petroleum is believed to be affected mainly by sulfur incorporation reactions into the sedimentary organic matter during the early diagenesis. However, secondary processes could affect the original δ 34S of oil under the effect of thermal maturity or of the microbial activity of biodegraded reservoirs. In this study, the different processes that may affect the δ 34S of in-reservoir oils were assessed based on the sulfur content and isotopes of a series of oil and core samples coming from various reservoirs of the Lower Cretaceous Mannville Group, Western Canada Sedimentary Basin (WCSB). Based on the molecular study, these samples appear to have reached various levels of maturity and biodegradation, ranging from 0 to 6.5 on the biodegradation scale of Peters and Moldowan. In addition, mixing of organic matter coming from different source rocks was identified based on the comparison with extensive correlation studies performed in the WCSB. Investigation of the δ 34S 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 δ 34S variations have rather to be ascribed to contributions of oils generated by various source rocks. Alternatively, variations of δ 34S 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, δ 34S 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.