A stratigraphic and geochemical investigation of Upper Devonian shale and marl aquitards, west-central Alberta, Canada

A regional investigation of Devonian aquitards in west-central Alberta has identified the aquitards as effective barriers to (paleo-)fluids since the Late Devonian/Early Carboniferous. Aquitards thin progressively towards the disturbed belt, and in areas such as the Obed and Simonette fields, aquitards are absent, permitting interconnection between the four major Devonian carbonate aquifers (i.e., D-1 through D-4).; Aquitard lithofacies vary in depositional environments from basin, via slope (or ramp), to near-reef. Paragenetic sequences have been established for both near-reef and near-basinal lithofacies.; Stable isotopic analyses (δ18O and δ 13C) reveal that aquitards have been relatively impermeable to aqueous formation fluids since the Late Devonian/Early Carboniferous. Mercury injection data show that the aquitards are relatively effective seals to hydrocarbons. The near-marine 87Sr/86Sr ratios of the carbonate fractions within the aquitards differ significantly from highly radiogenic (87Sr enriched), late-stage, carbonate cements within adjacent carbonate aquifers. This finding suggests the latter were precipitated from fluids containing an extrabasinal fluid component. Extrabasinal fluids likely entered the Devonian system through tectonic expulsion during the Laramide orogeny. These fluids are confined to Devonian carbonate aquifers and have probably moved in a predominantly northeast direction. In the vicinity of the disturbed belt region, cross formational fluid flow is likely, due to the absence of aquitards or in areas where the D-1 to the D-4 is hydrologically connected.