ABSTRACT

Fluorescence microspectrometry of hydrocarbon fluid inclusions and entrapped oil globules has been used to evaluate hydrocarbon migration within the Upper Devonian Birdbear Formation of southeastern Saskatchewan. Based upon visible light region fluorescence properties three distinct types of oil inclusions (A, D1A and DIB) have been identified within a potential reservoir shallow marine carbonate unit of the lower Birdbear member and within overlying anhydrite and algal dolostone of the upper member. The estimated gravity of type D1A and DIB oil inclusions ranges from 30 to 40° API. These oils were the first to migrate through the formation and occur mainly within overgrowths, cements, and fracture-fill anhydrites. Type A oil inclusions have a higher gravity, estimated at 40 to 50° API, post-date type Dl oils and occur mainly within latest stage allochem and dolomite overgrowths and carbonate cements infilling microfractures. Type Dl oils are the most widely distributed and show evidence for southwest to northeast migration. Both lateral and vertical migration of Dl oils appears to be related to a disconformity between a carbonate unit of the lower member and an anhydrite unit of the upper member. A key factor to consider when exploring for crude oil in the Birdbear Formation in southeastern Saskatchewan is that oils probably migrated from the carbonate shoal unit, across the leaky anhydrite seal into an algal dolostone unit. Type A oil inclusions also display a southwesterly to northeasterly trending migration pathway but are areally restricted compared with the Dl oil inclusions. Discoveries of pools, such as the Hummingbird, Star Valley and Walpole, in the Birdbear Formation coincide with regions through which the high gravity type A oils have migrated.