The Exshaw Formation: a Devonian/Mississippian Hydrocarbon Source in the Western Canada Basin

Abstract

The Devonian-Mississippian period marks a time of extensive development of black shale deposition within epeiric seaways. These source systems have been attributed with generating a large portion of the proven reserves around the world. One of these source systems, the Exshaw Formation of the Western Canada basin, is examined in this chapter. The Exshaw Formation consists of two members, a lower shale-dominated member and an upper siltstone to silty limestone member. The lower member contains the organic-rich, oil-prone, black shale facies of the Exshaw Formation. The Exshaw black shales were deposited beneath an upwelling zone that extended along the western margin of the Devonian/Mississippian craton. During deposition of the Exshaw Formation shales bottom waters apparently varied between dysaerobic to anoxic and the seaway may at times have been stratified. Variations in the preservation potential appears to have been the primary influence on the degree of organic enrichment and the oil-proneness of the preserved organic matter. Maximum burial depths in the Western Canada basin were reached in the Eocene and subsequent uplift and erosion indicates that the Exshaw black shales are at present not generating and expelling hydrocarbons. The Exshaw Formation has been suggested as one of the primary sources for the Early Cretaceous heavy oil deposits in the Mannville Group. The Exshaw may also have contributed locally to several Devonian and Mississippian reservoired oils. The volumetric significance of the generated and trapped hydrocarbons from the Exshaw Formation is uncertain as a result of mixing with hydrocarbons from other source systems in each accumulation. It is clear, however, that the Exshaw Formation black shales are one of the primary sources in the Western Canada basin.