Ultra-Shallow Gas, Northwest Alberta, Canada: Origin and Detection

Hans H. von der Dick¹, Peter Bauman², and Dane A. Bosman^1
1 ChemTerra International (CTI), Calgary, AB
² Crew Energy, Calgary, AB

In recent years, several ultra-shallow gas pools in the depth range of 35-300m (100-900ft) that occur in Pleistocene glacio-fluvial deposits have been discovered in NW-Alberta and NE-British Columbia. The shallow reservoirs represent economic natural gas accumulations but also drilling hazards in the form of blow-outs and artesian flow. The sand reservoirs were deposited in valleys formed by the inundation of the Laurentide ice sheet into the area, 10,000 to 43,000 b.p. and sediment laden melt water eroding local bedrock.

Reservoir gas composition and methane isotope values indicate a dry gas of predominantly thermal origin. Gas charge into these young reservoirs had to be recent and from re-migrating thermal gas of late Cretaceous/Early Tertiary origin.

There are gas seeps active in the area, and it is proposed that active methane seepage since the time of tectonic uplift and overburden erosion is gas feeding mechanism for Pleistocene reservoir gas charge. Since untrapped, vagabond natural gas is quickly dispersed after the time of generation it appears likely that the gas seepage-feeding mechanism is from continued deep reservoir breaching and gas exosolution out of pressure fluids as the basin continues tectonic uplift. Using rates of tectonic uplift coupled with declining fluid pressure tentative volumes of gas release are calculated.

The active seepage patterns observed in surface gas readings are a useful indicator for gas leakage points and ultra-shallow gas reservoir potential where thermal gas release coincides with shallow reservoir formation.