Incorporation of Core and Petrographic Data - Key to Improved Management and Exploitation of Middle Devonian Keg River Carbonate Pools in Rainbow Sub-Basin, Alberta, Western Canada

Iain D. Muir, Jeffrey J. Dravis

Middle Devonian Keg River pools in the Rainbow sub-basin have produced more than 1.1 billion barrels of oil from basinal pinnacle reefs and platform dolomites since the initial discovery well in the mid-1960's. Production from platform-interior dolomites along the northern margin of this sub-basin was initially related to thinner pinnacle reefs but never documented. When production rates drastically declined in 1988, an integrated geological and engineering study of these pools was initiated to define the traps, map reservoir units, and delineate controls on pool entrapment for further exploitation.

Detailed core and thin section studies quickly established that production was related to secondary porosity mostly associated with dolomitized subtidal facies at the bases of meter-scale depositional cycles. Porosity was formed by dissolution of replacement dolomites under deeper-burial conditions, however. Core-based stratigraphic and structural cross sections tied reservoir quality to depositional cycles and permitted correlation of larger 7-12 meter-thick composite cycles over distances of several kilometers or more, allowing reservoirs to be more effectively zoned and creating new opportunities for exploitation. As it turned out, pools were combination structural-stratigraphic-diagenetic traps influenced by reactivated basement faults, and not pinnacle reefs.

By 1989, daily cumulative oil production rates were increased from 500 m³/day (1988) to 1400 m³/day as a direct result of this study, which also led to improved conformance, sweep, and minimization of oil sandwich loss in these miscibly- and gas-flooded pools. Horizontal wells also were used more effectively to develop some of these pools.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995