Abstract

The Mannville Group sediments contain more hydrocarbons, by far, than any other formation in the Western Canada Sedimentary Basin. Yet, convincing evidence points to the conclusion that not all of the hydrocarbons found in the Mannville Group originated there. Significant cross-formational migration of fluids including oil, water, and gas must have occurred into the Mannville Group from other formations over time.

In west-central Alberta, Mannville Group sediments form part of, and are influenced by, two regional-scale hydrogeologic flow regimes. The lower Paleozoic regime is comprised of the Upper Devonian Hydrogeologic Group (UDHG) and the Mississippian Hydrogeologic Group (MHG). The upper Jurassic-Lower Cretaceous flow regime consists of the Mannville Group Aquifer (MGA) and the Viking Group Aquifer (VGA).

Flow patterns in the Mannville Group are complex. On a regional scale, fluid flow in the MGA is directed laterally up-dip to the northeast. Superimposed on the large-scale system are numerous local areas of vertical flow and down-dip flow attributed to the presence of geologic heterogeneities. A boundary line in the MGA separates the hydrocarbon-saturated Deep Basin region from the water-bearing section of the aquifer located up-dip. In the northeast corner of the study area, reduced lateral hydraulic gradients reflect a more vertical flow-regime.

A large plume of saline water with concentrations of total dissolved solids (TDS) in excess of 100,000 mg/l was identified by mapping water chemistry of the Mannville Group aquifer. The position of the plume coincides with the subcrop of the Wabamun Group beneath the Mannville Group, where erosion has thinned the Wabamun aquitard (generally more than 220 metres thick) to less than 40 metres. This plume results from the mixing of the ambient Mannville waters (#40,000 mg/l TDS) with ascending Devonian brines (>150,000 mg/l TDS) above the Bashaw reef complex. A smaller plume is evident in the southwest corner of the study area where the Elkton-Shunda aquifer intersects the MGA east of the limit of the Fernie Group aquitard.

Geologic, hydrochemical, and hydraulic evidence confirms that cross-formational fluid migration out of the Mannville Group is occurring (or has occurred), in west-central Alberta. Geologic mapping of the thickness of the Joli Fou Formation shale reveals that this aquitard is absent in the southwest corner of the study area. Furthermore, in areas where the aquitard is present, flow rates on the order of 2.3 to 5.3 × 10^-4 metres/year across the shale Joli Fou are calculated, based on measured hydraulic gradients and published values of permeability from similar shales. No hydraulic or hydrochemical evidence supports the postulation that the Joli Fou Formation acts as an impermeable seal for formation fluids.

The Mannville Group acts as a collector of formation fluids by intercepting the vertically flowing Devonian and Mississippian fluids and re-directing them laterally up-dip. Analysis of flow systems and fluid chemistries of the Mannville Group may help detect other areas of vertically moving water, oil and gas of Paleozoic origin, and help identify other hydrocarbon exploration targets.