Depositional and Diagenetic Controls on Devonian Nisku Reservoirs, Sweetgrass Arch, Montana: Implications for Exploration and Development: ABSTRACT

Abstract

The Upper Devonian Nisku (Birdbear) Formation has been the focus of a shallow oil and gas play on the northern and eastern flanks of the Kevin-Sunburst dome but potentially embraces a far broader region. The composite dolomite-evaporite sequence on the Sweetgrass arch, deposited within an inner-shelf regime proximal to coastal lands of central Montana, incorporates (1) pyritic, argillaceous, dolomicrite mudstone and nodular anhydrite, forming uppermost Duperow strata deposited in density-stratified, hypersaline water; (2) laminate to nodular mudstone and brachiopod-crinoid wackestone, deposited during passive formation of broad shoals; (3) Amphipora wackestone deposited exclusively along outer margins of shoals; (4) outer to inner shoal complex of Renalcis boundstone, peloid-oncolite packstone and grainstone, stromatolitic mudstone, and laminated dolomicrite mudstone; and (5) nodular-mosaic anhydrite forming the top of the Nisku. Seaward progradation of shoal-margin facies took place under sea level drawdown, leading eventually to shelfwide restriction. Correlative zones of detrital clay and quartz, underlain by clay-filled solution-collapse breccias, provide evidence for subaerial exposure of shoals twice during deposition of the Nisku. Leaching and dolomitization in distinct vadose, meteoric phreatic, and mixed meteoric-marine phreatic zones led to development of reservoirs within algal-rich outer shoal facies and Amphipora-rich skeletal bank facies. Areal coincidence of these depositionalmore » facies with shallow-burial environments defines Nisku exploration trends. On a more local scale, primary carbonate textures, burial depth during diagenesis, and resultant dolomite fabric determine porosity and permeability characteristics for reservoir development trends.« less