Abstract

Within the past decade, the Big Valley and Exshaw formations in southern Alberta have received significant interest as a potential unconventional oil play because of the overwhelming exploration/exploitation success in the stratigraphically-equivalent Three Forks and Bakken formations in Saskatchewan, Manitoba and North Dakota. Because of continuous industry hype, this upper Devonian-lower Mississippian succession came to be known as the “Alberta Bakken”. Such a term suggests potentially prolific oil production from a reservoir similar to the Bakken in Saskatchewan. In fact the term “Alberta Bakken” is misleading since to date there has been only moderate oil production from localized carbonate reservoirs in the Big Valley Formation, and from very fine-grained sandstones and siltstones of the Mississippian Lower Banff Formation.

Reservoir development in the Big Valley Formation is stratigraphically and areally restricted by both depositional facies controls and post-depositional early diagenetic, subaerial and structural-collapse processes. Detailed lithofacies analysis indicates that the Big Valley Formation is divisible into two units: an upper shallow marine limestone and a lower hydrocarbon-bearing, partially dolomitized, peritidal carbonate. The peritidal unit, in turn, is divisible into four lithofacies: peloidal packstone-grainstone (the primary oil-bearing lithofacies), microbial laminite, laminated dolomudstone and intraclastic breccia-laminite. Regionally, each lithofacies is discontinuous and in the order of 0.5 to 2.0 m thick. Locally, however, the peloidal packstone-grainstone attains thicknesses of up to 8 m, forming isolated oil-producing reservoirs. These over-thickened zones correspond to specific areas of Big Valley Formation ‘thicks’, which tend to align with a NNW-SSE trending ‘basement’ lineament that underlies the study area. It is equivocal whether this structural trend is basement-controlled, or reflects dissolution of salt beds along the margin of an underlying evaporite basin. Successful exploration in the Big Valley Formation appears to depend on whether the over-thickened areas can be located.

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