AAPG Hedberg Conference
Vail, Colorado
April 24-29, 2005
Gas
-Saturated Sands
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.
Copyright ©2005. The American Association of Petroleum Geologists. All Rights Reserved.