ABSTRACT

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

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

Deposition of the green, brackish-water shales 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 off-reef positions. Structural draping of the Slave Point, Watt Mountain, and Muskeg Formations over 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. Over full-reef build-ups 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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