ABSTRACT: Depositional and Early Diagenetic Setting for Upper Devonian Nisku Reservoirs in Southern Alberta and Northwestern Montana

Don L. Kissling

Markedly different reservoirs are hosted by the Nisku Formation in southern Alberta and on the Kevin-Sunburst Dome in Montana. East Kevin field was defined during 1983-1986 as a broad stratigraphic trap of low porosity and permeability. Since 1986 Nisku exploration has focused on the Enchant area of Alberta where fields are defined by highly permeable Nisku pay over prominent structures formed by multistage salt solution and pre-Nisku depositional compensation. In both areas reservoirs exist in dolomitized lagoon-fill facies that accumulated leeward of a barrier bank complex trending northeast-southwest through southern Alberta. Although generally restricted, the lagoon experienced salinity extremes marked by laterally persistent estuarine and marine facies and by recurre t salina facies. Nearest to barrier banks, including the Enchant area, lagoon settings were dominated by flood tide washover fans of current-bedded calcisiltite and peloid-intraclast calcarenite. Over its southern reaches, including East Kevin, subtidal to supratidal mudflats comprised of laminated mudstone bound by stromatolites dominated. These prograded over Renalcis-serpulid hardgrounds and Amphipora shoals. Repeated subaerial exposure and leaching during deposition is evidenced by bleached reticulate vugs, solution-collapse brecciation, nodular-mosaic anhydrite, and detrital clay and quartz residue. Microkarst features are most pronounced where facies parameters indicate elevated paleotopography. Subsequent dolomitization may have been the product of post-burial seepage reflux of br nes derived from adjacent massive gypsum deposits. Thick anhydrite forming the overlying Potlatch and Graminia formations serve as reservoir seals.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990