Reserves growth in a mature oil field: The Devonian Leduc Formation at Innisfail Field, south-central Alberta, Canada

Oil has been continuously produced from the Devonian Leduc Formation at Innisfail field since its discovery in 1957. To date, cumulative production at Innisfail is 98.9% of recoverable oil in place (84.3 MMBO) and indicates that the Leduc is near the end of its productive life. A sequence-stratigraphic interpretation produced from core, well logs, and three-dimensional (3-D) reflection seismic data, however, suggests that production may be extended through the development of two previously undetected scenarios of bypassed oil entrapment: (1) attic oil accumulations associated with small buildups atop the isolated Innisfail platform and (2) backstep-edge accumulations located in positions structurally low to currently producing wells. In both cases, oil accumulations are related to a Leduc depositional history that was characterized by rates of long-term carbonate sediment accumulation exceeded by the rate of sea level rise. Positive bathymetric relief created at the time of deposition and, subsequently, present-day stratigraphic traps were produced during the final phase of stratal retrogradation that immediately preceded the drowning of the isolated Innisfail platform. High-quality 3-D seismic data are essential in the exploration for both attic and backstep-edge oil accumulations at Innisfail. Integration of 3-D seismic data, well-log, and core data indicates that although depositional relief on both types of features may be low, they are nonetheless seismically resolvable. The small platform-top buildups are up to 0.16 km(2) (40 ac) in diameter and have up to 10 m (33 ft) of independent closure above spill point. Backstep-edge oil accumulations occur within the structurally highest parts of the Leduc high-frequency sequence G downlap limit. Depositional relief along the downlap limit and associated trap closure ranges from approximately 5 to 15 m (16 to 49 ft). During 2003 and 2004, two platform-top buildups were directionally drilled, and one existing well located near a backstep-edge position and suspended since 1988 was reactivated. All three wells are successful and, to date, have cumulative production of 71 thousand bbl of oil. A total of 17 platform-top buildups and backstep-edge development drilling prospects exist across Innisfail and have expected-case recoverable reserves of approximately 960 thousand bbl of oil. These reserve additions suggest the potential for 1.14% field growth at Innisfail and are representative of the potential value that may yet exist within all mature Leduc fields in western Canada.