ABSTRACT

Oils in Red River, Winnipegosis and Bakken reservoirs have distinctive compositions belonging to oil families defined in Canada but now recognized throughout the Williston Basin. These oils occur generally in the stratigraphic group from which they were sourced. Only in rare instances, such as at the Winnipegosis Minton Pool, have oils migrated across strata. Compositional distinctions defined by biomarkers extend into lighter hydrocarbons, particularly the saturate and gasoline range compositional fractions. These distinguish the non-degraded oil families A, B and D in Red River, Bakken and Winnipegosis reservoirs, respectively, as well as degraded oil family E occurring in Bakken reservoirs. In the Bakken reservoirs of Saskatchewan, Family B oils are water washed and Family E oils are both water washed and biodegraded. Family E gasoline range compositions differ from Family B, but overlap the compositional range of Family D. Whether or not the difference between gasoline range compositions in families B and E is due to source or biodegradation remains undetermined since the pristine composition of Family E is unknown. The saturate and gasoline range compositional fractions persist to high maturities. Their use as indicators of familial affinity and oil alteration processes allows correlation between high and low maturity oils, making it possible to describe the changes in biomarker compositions resulting from thermal cracking. Although such changes can be related to thermal maturity, many compositional ratios commonly used to indicate thermal maturity levels are affected by source rock composition. This suggests a need for caution when comparing such levels between oil families.