Lower Cretaceous Oil and Gas in Western Canada

Abstract

The Western Canada sedimentary basin contains tar deposits which exceed by three times the known recoverable oil reserves of the entire world. The tar was originally liquid oil which has been degraded by aerobic bacteria. It was generated principally from Lower Cretaceous but also from Jurassic and Triassic shales. The oil is contained in only 900 ft (275 m) of stratigraphic section above and below the Paleozoic unconformity. Oil migration paths were northeasterly, directly updip from the oil thermal window. The Athabasca anticline, a drape structure caused by Devonian salt removal, connects southward with the Sweetgrass Arch to form a 600-mi (965-km) long structural barrier on the eastern, updip rim of the basin. Most of the tar deposits are along the anticline or in a giant stratigraphic trap on the Paleozoic unconformity surface on the west flank of the anticline. There is no oil or gas east of the anticline. In the deepest part of the basin the Mesozoic section generated gas in comparably large volumes. Most of the gas has escaped to the outcrop, a small amount is contained in thousands of conventional stratigraphic pools on the east side, and an enormous volume is contained in tight sands on the west side, or the Deep basin. Most of the reservoirs are Lower Cretaceous sandstones. The tight, gas-saturated sands grade updip into porous water-saturated sands. The trap is not tightly sealed but leaks off at a steady rate. Continuing gas generation keeps the trap pumped full. This bottleneck trap contains 1,750 tcf of gas in place. Commercial gas accumulations are present in the Deep basin where coarser-grained marine shoreline sands occur. These are most numerous in the Elmworth area but are also present in five specific trends to the south. The gigantic oil and gas accumulations of the Lower Cretaceous make the Western Canada sedimentary basin the richest hydrocarbon province in the world.