Lagoon and Tidal Flat Sedimentation of the Upper Devonian Nisku Formation in Southern Alberta

Abstract

Since 1985, 26 oil pools containing 64 million bbl of oil in place have been discovered in the Nisku Formation in southern Alberta. The thoroughly dolomitized Nisku Formation varies from 20 to 30 m thick in southern Alberta and northern Montana. It overlies anhydrites and shaly carbonates of the Southesk or Duperow formations and underlies anhydrites of the Stettler or Potlatch formation. Burrowed, nodular-bedded skeletal wackestone, deposited over a shallow marine shelf, forms the basal Nisku Formation. These strata are succedded diachronously and unconformably by several tidal-flat and lagoon facies that include (1) southeast-thinning washover fans of cross-bedded peloidal grainstone; (2) laminated mudstone to current-bedded peloidal and intraclastic grainstone sourced within the lagoon; (3) stromatolitic mudstones; (4) laminated anhydrite beds precipitated during salina episodes; (5) Amphipora and brachiopod wackestones and thrombolites containing Renalcis, serpulids, and ostracoes, marking a brief marine invasion; and (6) brackish or freshwater shale and mudstone containing fragmented lycopod leaves and antiarch fish remains. These sediments are overlain by cross-bedded, peloidal, and calcisiltite grainstone and stromatolitic mudstone deposited in tidal channels and over shoals. All facies have been subjected to periodic subareal exposure which has produced leaching, solution collapse brecciation, teepee structures, and nodular-mosaic and void-filling anhydrite.more » Permeable reservoirs exist where leached, dolomitized tidal flat and lagoon sediments contain intercrystalline and pelmoldic porosity and little anhydrite cement.« less