The Upper Devonian Nisku Formation in Alberta, Canada: From Sour Gas Production to Acid Gas Sequestration

Hans G. Machel
University of Alberta, Edmonton, AB

The Nisku Formation in central Alberta, Canada, produces oil, sweet and sour gas condensate from Upper Devonian reefs that range in depth of about 2300m to 4200m. In the early years of exploration of this play, which was discovered in 1979, the distribution of sour gas was poorly known, which contributed to a nasty sour gas blowout in 1982. Subsequent petrographic data, stable isotopes of several solids (calcite, dolomite, anhydrite, elemental sulfur, pyrite, marcasite), isotope and bulk composition of gas condensates, and brine compositions of several sour gas pools, have shown that thermochemical sulfate reduction was responsible for the formation of the sour gas, and that its distribution is predicable.
A unique feature of the Nisku play is that the hydrocarbons are contained in numerous closely spaced pools that have been essentially isolated hydrodynamically from one another since hydrocarbon entrapment about 50 - 60 million years ago. This hydrodynamic isolation renders the Nisku play suitable for carbon dioxide and/or acid gas (hydrogen sulfide + carbon dioxide) sequestration. The Brazeau Nisku Q-Pool now is a depleted sour gas pool, and one of more than forty acid gas injection operations currently active in western Canada. The available data suggest that the injected acid gas will remain in the structure that contains the Nisku Q-Pool on a geological time scale. However, upward leakage of acid gas rapid enough to be of human concern is possible through wells that were improperly completed and/or are abandoned and are not monitored.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece