Abstract: Disparate Roles of Unconformity Surfaces in Porosity Generation: An Example from the Pekisko Formation, Western Canadian Sedimentary Basin

Kent C. Kirkby, J. A. Toni Simo

Subaerial exposure can significantly affect the diagenesis of carbonate strata, but porosity generation and loss beneath an unconformity may also be due to the surface's subsequent influence on burial fluid migration and mixing. Distinguishing these disparate roles is important, because dissolution by migrating burial fluids can enhance and create reservoirs preferentially within oil migration fairways. The Lower Carboniferous Pekisko Formation furnishes an example of secondary porosity that, although associated with a karst surface, may have a late burial origin.

The Pekisko Formation in west-central Alberta consists of carbonate ramp and mound strata encased in shale deposits. This Carboniferous package tilts gently towards the southwest and is truncated to the northeast by an extensive pre-Cretaceous karst surface. Impermeable shales overlie this surface and, in conjunction with the Carboniferous shales, act as confining beds for fluid migration through the Pekisko carbonates. Secondary porosity occurs in limestone basin, slope, and mound strata up to 100 km from the subcrop edge, although it is best developed within 40 km, where reservoirs locally exceed 25% porosity and 10 darcys permeability. Secondary porosity resulted from fabric-selective dissolution of micritic matrix and some skeletal grains, creating textures that closely mimic pres rved primary porosity.

Although the distribution of secondary porosity along the subcrop edge suggests a genetic association, the distance, depth, and manner of dissolution appear to be incompatible with a meteoric/karst origin. Dissolution affected grain types that are typically resistant to meteoric diagenesis, and the extreme fabric selectivity of dissolution suggests fluids only slightly undersaturated with respect to calcium carbonate.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994