Sedimentology and source-rock potential of the lower Kaskapau Formation (cenomanian-lowermost turonian), northwestern Alberta

Abstract

The Cenomanian to earliest Turonian Kaskapau Formation in northwestern Alberta and adjacent British Columbia contains three sandstone bodies, the Doe Creek, Pouce Coupe, and Howard Creek members. The three sandbodies are encased in shale. The sedimentary structures, microfauna, megafauna and trace fossils in the Doe Creek, Pouce Coupe, and Howard Creek members and surrounding Kaskapau shale indicate deposition on a shallow, marine shelf with normal to near normal salinity. The members represent a series of relatively thin, shallow water marine sandstones, deposited approximately parallel to the shoreline of the correlative Dunvegan Formation during brief episodes of progradation, within an overall transgressive phase represented by the thick shales of the Kaskapau Formation. Each of the sandbodies comprising the Doe Creek, Pouce Coupe, and Howard Creek members, as well as those sandbodies within the individual members, is bound by unconformities. The lower bounding surface represents rapid deltaic progradation during a drop in relative sea level. The upper bounding surface is a flooding surface, transgressed and abandoned as relative sea level subsequently rose. Within the Doe Creek Member, the overlapping I, N, and A reservoir sandbodies of the Valhalla Field suggest high-frequency, fluctuating sea levels and an oscillatory shoreline during an overall retrogradational interval. The sea-level changes need not be eustatic, but may be relative in nature, possibly caused by autocyclic processes such as delta switching. The shale of the lower to middle Kaskapau Formation (Cenomanian) from the top of the Dunvegan Formation to the base of the Howard Creek Member was deposited in a cool, shallow water, nearshore marine environment between fairweather and storm wave base. Sedimentation was generally slow and continuous, permitting intense bioturbation and thorough mixing of the sediments, but was periodically interrupted by storms during which numerous thin sandstone lenses were deposited. The overlying Second White Speckled Shale (Turonian) was deposited during the peak of a marine transgression, which corresponds to a global eustatic sea-level rise. The Second White Speckled Shale contains in excess of 11 per cent dispersed total organic carbon and has very good hydrocarbon generation capacity.