ABSTRACT

The Zama area of northwestern Alberta is in the Middle Devonian Black Creek basin. At the beginning of late Elk Point deposition, open-marine conditions prevailed and organic reefs of the Keg River Formation grew to thicknesses of 350 ft.

Reef growth was terminated abruptly during a period of uplift and marine regression, and the basin was partly filled with halite of the Black Creek Member of the Muskeg Formation. With the renewal of subsidence, the Black Creek basin ceased to exist and the Zama area became part of the more extensive Elk Point evaporite basin. With two exceptions evaporitic conditions represented by the cyclical anhydrite and dolomite of the Muskeg Formation prevailed through most of the remaining time of Elk Point deposition. During early Muskeg deposition, a well-laminated, nonreef calcarenite, the Zama Member, developed as a shoal over the dormant Keg River reefs, and a second, more widespread nonreef calcarenite, the Bistcho member, was deposited in a final transgressive phase.

Deposition of green, brackish-water shale of the Watt Mountain Formation was succeeded by the transgressive limestone sequence of the Slave Point Formation.

Contemporaneous solution of the Black Creek salt resulted in thickening of post-Keg River units in reef-flank and offreef positions. Structural draping of the Slave Point, Watt Mountain, and Muskeg Formations across the Keg River reefs reflects the continued removal of Black Creek salt during Late Devonian time.

The upper reef member of the Keg River Formation and the Zama Member of the Muskeg Formation are oil bearing. Above full-reef buildups there is no separation between the two reservoirs. In reef-flank positions the Zama Member is separated from the Keg River by a lower anhydrite member of the Muskeg Formation. The Bistcho member of the Muskeg Formation and thin calcarenite units in the Slave Point Formation are gas reservoirs.

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