Anatomy of a paleokarst reservoir: Sedimentation and diagenesis of the Pekisko Formation in Twining oil field, Alberta, Canada

The relative importance of depositional facies and diagenetic alteration in the development of reservoir characteristics in Twining oil field is examined. Twining field lies at the subcrop edge of the Pekisko Formation where hydrocarbons are trapped in porous grainstones at its updip termination. A basinwide unconformity separates the Pekisko Formation from overlying Cretaceous clastics. During formation of the pre-Cretaceous unconformity, the shallow-water carbonates of the Pekisko Formation were subaerially exposed, resulting in the development of a karst terrain. Subsequent burial has preserved this karst in the subsurface. Post-depositional alteration had a significant effect on the creation of the reservoir at Twining field. Pervasive calcite cementation destroyed all primary porosity in the Pekisko Formation limestones, but later exhumation allowed for the creation of extensive secondary porosity. Macroscopic solution voids also developed in the exposed limestones and are often filled with eluviated Cretaceous sediment carried downward by percolating meteoric waters.