Pinnacle Reefs of Devonian Age in the Nisku Formation of the West Pembina region, Alberta, were discovered by Chevron Standard in 1977. To date, more than 50 such reefs have been located in the play trend area, which runs in a NE-SW direction and extends for more than 150 km.

The Nisku pinnacle reefs are characterized by rugose corals and stromatoporoids. The reefs are encased in variably argillaceous carbonate ramp sediments that make up the bulk of the Nisku Formation in the West Pembina area.

The pinnacle reefs have been altered by a variety of diagenetic processes since burial. In terma of economic impact, the most significant of these events has been dolomitization, which has resulted in increased porosity and permeability, hence better reservoir quality. Constructive dolomitization is believed to be a late stage or deep burial diagenetic event. The majority of the pinnacle reefs have been wholly dolomitized, but some are only partly altered and limestone reefs also occur, especially toward the northeast end of the play trend.

Total reserves in the Niksu reef trend amount to some 500 MMSTB of oil and 500 BCF of gas. Enhanced oil recovery (E.O.R.) schemes have been initiated in many of the pools in order to maximize hydrocarbon recovery. Waterflood drives have been implemented in smaller reefs or in reefs with relatively poor reservoir quality. Vertical-displacement miscible-slug schemes have been put into effect in a number of the higher reservoir quality dolostone pools.

Reservoir geology studies of individual Nisku reefs have proven to be useful in several ways to Texaco’s Reservoir Engineering Group, in particular the performance modelling simulation studies of miscible slug schemes: Firstly, the studies locate and delineate areally significant barriers to fluid flow which may be present in a given reservoir. Secondly, the studies are generally able to split up or subdivide a reservoir into two or more reservoir layers, each differing from the others in reservoir quality. Reservoir layer boundaries may follow changes in lithofacies and hence arestratigraphic. Equally, the contact between two reservoir layers may only reflect a change in the style and/or degree of dolomitization within the reservoir.

The results obtained from reservoir geology studies of the Bigoray Nisku “C” (limestone reef) and Pembina Nisku “L” (dolostone reef) pools are shown in this display.