Deep Basin Gas Trap, Western Canada

Abstract

Gas accumulations are distributed in a fashion similar to most other natural resources. The high-grade deposits are comparatively small. In general, as the grade decreases the size increases. Three of the largest sandstone gas fields in western North America are in low porosity-low permeability Cretaceous sandstone, in downdip structural locations, with porous water-filled reservoir rock updip. Examination of the details of these fields sets the stage for recognizing an enormous tight-sand gas trap in western Canada. The Mesozoic rock section, only 1,000 ft (300 m) thick on the shelf in eastern Alberta, thickens westward to over 15,000 ft (4,570 m) in the Deep Basin in front of the Foothills overthrusts. Most of the developed sandstone gas fields are in updip porosity traps, or minor structural traps, on the shelf. The porous, generally water-saturated sands of the shelf become less porous and permeable westward and downdip, passing from the water-bearing area with local gas traps through a transition zone to a gas-bearing area. This change is demonstrated by electrical resistivity logs and confirmed by drill-stem tests. Recent exploratory drilling in the Deep Basin has resulted in numerous discoveries in the area. Several hundred log analyses provide reliable data for measuring potential gas resources in the range of 400 Tcf. Recoverable gas at $2.00/Mcf net after royalty may reach 150 Tcf. The quantities of gas apparently present would be a major addition to the North American energy supply.