ABSTRACT: Thermochemical Sulfate Reduction in the Devonian Nisku Formation, Alberta, Canada - a unique sour gas play with generally applicable characteristics

Thermochemical sulfate reduction (TSR) occurs in diagenetic environments with 100°C < T < 150 - 200°C, corresponding to depths of about 2000 to 6000 m. TSR does not have a sharply defined, generally valid minimum temperature because the onset and rate of TSR are governed by several, locally variable factors.

The Devonian Nisku Formation in the subsurface of Alberta, Canada, produces oil, gas condensate and sour gas. A unique feature of this play is that hydrocarbons and sour gas are contained in numerous closely spaced pools that have been essentially isolated hydrodynamically since hydrocarbon entrapment, as shown by initial reservoir pressures and gas compositions. Each pool constitutes a small natural laboratory with respect to deep burial diagenesis.

Petrographic features, stable and radiogenic isotopes of several solids (calcite, dolomite, anhydrite, pyrite), as well as geochemical data from the gas condensates and reservoir brines, leave no doubt that TSR formed the sour gas that is presently pooled in the Nisku. The data also show that TSR can take place in extremely narrow reaction zones, i.e., only a few meters thick, and they define a sharp temperature limit for the onset of TSR in this particular play. The Nisku also contains biogenic pyrite, indicating that it was affected by bacterial sulfate reduction earlier in its evolution.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia