A Tight Carbonate Reservoir in the Mississippian Pekisko Formation: The Role of Pervasive Microvuggy Porosity

Abstract

The Lower Mississippian (Tournaisian) Pekisko Formation in the Hawk Hills area of northern Alberta is a 25 to 40 m thick carbonate ramp succession which hosts a large, medium to heavy-gravity oil resource that is presently being evaluated. The formation has been subdivided into three informal units: the lower carbonate, middle shaly, and upper carbonate. The units are composed of three lithofacies associations: (1) basinal – characterized by argillaceous lime mudstones and crinoid wackestones; (2) outer ramp – consisting of crinoid-brachiopod wackestones and packstones; and (3) middle to inner ramp – composed of peloidal-skeletal packstones to grainstones. The Pekisko Formation has been significantly affected by marine, burial, and meteoric diagenesis. Marine diagenesis was typified by significant micritization and microboring of allochems, as well as limited radial-fibrous calcite cementation. Burial diagenesis was dominated by neomorphism of the matrix and allochems, dissolution of gastropods and other allochems, significant syntaxial and coarse mosaic calcite cementation, minor dolomitization, and limited quartz and gypsum cementation. Late meteoric diagenesis included dedolomitization and precipitation of chert and fluorite. Partial dissolution of the matrix, interpreted to have occurred during late burial to meteoric diagenesis, resulted in significant, pervasive microvuggy porosity in packstones. This style of porosity development has not been commonly observed in Mississippian strata. Oil-saturated reservoir packages, 0.1 to 13.1 m thick, are dominantly composed of mid-inner ramp lithofacies, with thin intervals of outer ramp facies. These packages are typified by 10-20% microvuggy porosity. Permeability is typically low, ranging from 0.5 to 20 mD on average, but is up to 420 mD locally. The Pekisko Formation in the Hawk Hills area is commonly considered an unconformity-related play, but this study demonstrates that the development of microvuggy porosity is not restricted to the subcrop area, and extends down-dip into the subsurface. Although there are development issues to be addressed, the Pekisko Formation in the Hawk Hills area is an excellent reservoir, with significant potential for future exploitation.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016