Shallow-marine reservoirs of the Upper Cretaceous Cardium Formation contain one of the largest light oil accumulations in Canada, which have been exploited since the 1950s. Since 2008, multistage fractured horizontal wells have revived this play. This study presents a reservoir characterization of the offshore to shoreface unconventional reservoirs of the Cardium Formation in the halo play along the western margin of the legacy Pembina oil pool in central Alberta. Although permeability of these facies is too low to achieve economic production from vertical wells, they were the main carrier beds forming the migration pathway for charging the updip conventional reservoirs in the Pembina field.
Three reservoir facies are present in the west Pembina study area: bioturbated muddy sandstones, interbedded sandstones and mudstones, and hummocky cross-stratified sandstones. Shoreface sandstones are relatively thin in the halo play area; thus, significant production in the horizontal wells is attributed to the thin sandstone beds and bioturbated muddy sandstone reservoir facies.
To characterize these unconventional, heterogenous reservoirs, traditional reservoir evaluation techniques are integrated with computerized tomography scanning to better evaluate reservoir properties and flow pathways. Cutoffs of 90 gamma-ray API units, 6% porosity, and 0.075-md permeability were used to create a series of reservoir property maps. These detailed maps show the most prospective area of the halo play is a 4-km (2.5-mi)-wide zone immediately to the west of the legacy west Pembina Pool boundary. This lateral heterogeneity with narrow sweet spot fairways explains the significant production variability in these shallow-marine reservoirs.