Case History for a Major Alberta Deep Basin Gas Trap: The Cadomin Formation

Abundant, high-quality geological data from the gas productive Elmworth region of the Alberta Deep basin reveal the existence of enormous gas accumulations found in an unconventional form of trap. This special form of "deep basin" gas trap defies conventional concepts of gas entrapment by turning them virtually upside-down. Gas is trapped in the deepest part of the basin, rather than on the flank of the basin. The gas/water contact occurs at the updip end of the accumulation, rather than the downdip end. Original gas accumulation pressures lie below the regional formation water pressure gradient, rather than above it as in conventional traps. Gas in at least one giant accumulation is in a dynamic state of updip migration, rather than in static equilibrium as found in conv ntional traps. The Lower Cretaceous Cadomin formation in the Alberta Deep basin provides some of the best available information on basic characteristics of deep basin gas traps. Physical principles behind this form of gas entrapment were confirmed in fluid flow models designed to simulate the Cadomin gas-trapping conditions.