The uppermost Cretaceous and Tertiary in west-central Alberta are geologically unique gas exploration targets. New reserves typically occur at shallow depths between 175 and 1,000m and at pressures that are as much as two and a half times less than normal hydrostatic pressures. Under these circumstances, conventional testing and logging techniques are ineffective in evaluating these reservoirs. Subsequently, many accumulations of gas have been bypassed and gas resource estimates are overly conservative. These reserves are biogenic in origin, with large volumes of gas generated from in-situ organic sources early on in the depositional history of the reservoir units. Subsequently, these reservoirs were rapidly and deeply buried, and this gas was forced into solution in the interstitial water of the reservoirs and surrounding shales. In the late Tertiary, much of the overlying sediments were eroded and the entire stratigraphic section rebounded and de-compacted. This lowered the reservoir pressures and allowed the gas to come out of solution and migrate into the reservoirs. The uppermost Cretaceous and Tertiary are composed of nine formations.
These are, from oldest to youngest:
• Dinosaur Park
• Horseshoe Canyon
These prospective formations equate to a gross pay interval of up to 1,400m. All of these reservoirs are composed of sandstones deposited in fluvial to shallow marine settings. To calculate the exploration and resource potential in the study area, 3,172 wells were selected. From this dataset, the reservoir distribution was mapped. Based on this mapping and the hydrogeological analysis, the resource potential of each formation in the area was calculated and is shown in map view as BCF of potential gas-in-place per section. The calculation takes into account variations in reservoir quality and thickness, pressure and the potential of total gas saturation versus gas and water saturation for the different formations. The overall unrisked resource potential for all formations in the study area is 74.3 TCF of gas-in-place.