The basic model for a Deep Basin gas trap is characterized by laterally extensive, low-porosity and low-permeability subsurface strata that contain gas downdip and water updip. No permeability barrier separates the two phases in the transition zone at the updip limit of the gas accumulation. Two other features of the model are significant: (1) the original gas and water phase pressures are about equal at the updip limit, and (2) there is no downdip gas/water contact.

In many respects, the Deep Basin trap is just the reverse of a conventional gas over water-type trap. In the conventional trap, gas has migrated to the highest structural position in the reservoir owing to its buoyancy in the ambient formation water phase; there is a downdip gas/water contact where original pressures in both phases are nearly equal and no permeability barrier is necessary to separate the two phases.

The physical principles controlling the Deep Basin water over gas trap are just as simple and straightforward as those long recognized for conventional traps. Because there is no downdip water phase or gas/water contact, the Deep Basin gas accumulation is not subjected to buoyancy forces as in the contentional trap. As long as pressures remain equal between both phases at the updip contact or transition zone, there will be no unbalanced forces present. As a result, the gas accumulation will remain in a state of static equilibrium. Similarly, in the conventional trap, equal pressures at the gas/water contact maintain the gas accumulation in a state of static equilibrium.

Evidence for these basin principles of Deep Basin trapping of hydrocarbons result from the study of abundant, high quality reservoir data derived from an extensive, ongoing development drilling program pursued by Canadian Hunter in the Elmworth gas field of northwestern Alberta.

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