Petroleum geochemistry data from thirty-one oil samples from southern Alberta and northern Montana are used to distinguish oil families in this region. The highest quality oils (API gravity generally >35degrees, sulphur content <1.5 wt %) are grouped together as Family M. The second oil family displays lower oil gravity and higher sulphur values, and is correlated with Family E (as initially described by Karavas et al., 1998). Biodegradation of oils in the study area is relatively minor, and is restricted to only a few pools. The source rock for the Family E oils appears to be the Devonian-Mississippian Exshaw Formation, whereas calcareous shales of the Mississippian Banff/Madison Group are postulated as the source for the Family M oils.