Geological Study of the Middle Devonian Gilwood Arkoses in the Nipisi Area, Alberta

Abstract

This paper is a compendium of a geological study of the Devonian Watt Mountain sedimentary sequence in the Nipisi field. An understanding of the genesis and distribution of the Gilwood sandstones is of prime importance in feasibility considerations with regard to secondary recovery. The Gilwood sandstone member of the Watt Mountain formation was deposited as a complex of interbedded arkoses and shales along the shoreline of a broad, shallow sea that bordered the eastern flank of the Peace River Precambrian High; this positive element was the source of arkosic detritus deposited in Nipisi. A shallow salt-created depression localized this shoreline, resulting in the deposition of a thin clastic dump or fan-like delta, containing about 50 feet of sandstone. The Nipisi clastics were divided, on the basis of shale units, into four zones, each zone containing one or more individual sandstone beds. The shale units were deposited during repeated periods of delta inundation and/or as clay drape on the upper deltaic plain. The alluvial sandstones were deposited as sheet-like sand bodies by anastomosing streams flowing over a flat delta surface. The sheet-like nature of these deposits is significant from a waterflooding point of view, since lateral reservoir communication is interpreted to occur in most of the alluvial sandstone zones which form the major portion of the reservoir. Small prodeltaic bars occur along the eastern side of the field; these deposits, unlike the alluvial sands, appear to be small in areal extent and could be isolated reservoir elements having no significant lateral reservoir communicability.