Underpressuring Mechanism for Gas-Saturated Sands

Jim Letourneau
Big Picture Geoscience Inc., Calgary, AB, Canada

In Western Canada, many tight gas sand accumulations are underpressured and can be categorized as Basin Centered-Gas Accumulations (BCGAs). First documented in the late 1970s in the area known as the “Deep Basin”, these systems have been identified in a near continuous band from Southern Alberta to Northeast British Columbia. A series of examples from Western Canada illustrate long distance gas-phase migration in gas-saturated strata.

Regional hydrogeological investigations often fail to recognize continuous gas accumulations because of a lack of pressure data and the use of conventional hydrogeological mapping tools. Recharge and discharge areas defined by the elevation of the water table are not present in a BCGA. Instead, there is a gas-charged petroleum system with little water present. Gas migration in BCGAs is completely independent of hydrogeological boundary conditions once the system becomes gas-saturated.

In Western Canada it is common to observe the presence of a regionally extensive continuous gas phase with sub-hydrostatic formation pressures. Continuous gas-charged petroleum systems have decreasing gas pressure gradients towards outcrop. This implies that in some cases sub-hydrostatic pressures are due to gas seepage to atmosphere.