Abstract

The Deep Basin type gas trap in the Cadomin Formation is a combination of stratigraphic, diagenetic and dynamic factors. Textural variations developed in a braided-river depositional environment and an increasing diagenetic modification with depth has resulted in a downdip decrease in porosity and permeability from conventional to unconventional (<0.5 md) reservoir rocks. Gas sourced from coal beds within the surrounding strata was injected into the deepest portions of the Cadomin which resulted in the initial displacement of water and the creation of a deep gas reservoir. The pressures in the reservoir are a function of the rates of gas migration through low permeability strata, both to and away from any given point in the reservoir.

Gas generally migrates from the deeper portions of the Cadomin to the gas/water contact where it is lost due to buoyancy to the transition and water zones. The Cadomin is a dynamic gas reservoir that responds to rates of gas generation and addition, gas migration, and gas loss to the updip transition and water zones. These parameters control whether the gas accumulation is in equilibrium with the updip water section, or increasing or decreasing in size. The transition cone presently cuts across structural contours and is characterized by water pressures approximately 800 to 1200 kPa lower than the water pressure at a corresponding depth in the water zone. Both the water and transition zones have water pressure gradients of 9.9 kPa/m which is lower than the calculated hydrostatic gradient of 10.37 ± 0.18 kPa/m for Cadomin water. This is an indication of a hydrodynamic flow of water downdip through the water and transition zone to balance a net loss of gas from the gas zone. Darcy’s law has been used to approximate a water influx rate of 128 to 12 800 m³/day; and a rate of total gas loss of 109 to 28 200 m³/day. These calculations suggest that the rate of total gas loss approximates or exceeds the rate of water influx, and therefore gas generation and addition to the Cadomin may be minimal or range up to tens of thousands of cubic metres per day. Low rates of gas migration (leakage) from the gas accumulation resulting from stratigraphic, diagenetic and dynamic factors is the critical trapping mechanism for the Cadomin Deep Basin type gas reservoir.