MACHEL, HANS G., Department of Geology, Univ. of Alberta, Edmonton; H. ROY KROUSE, Department of Physics, Univ. of Calgary, Calgary; ELIZABETH A. BURTON, Department of Geology, Northern Illinois Univ., De Kalb, IL; JULIA M. FOGHT, Department of Microbiology, Univ. of Alberta, Edmonton; and LEE R. RICIPUTI and DAVID R. COLE, Chemistry Division, Oak Ridge National Laboratory, TN

ABSTRACT: The Nisku Sour Gas Play: A Unique Natural Laboratory for Study of Thermochemical Sulfate Reduction

The Upper Devonian Nisku Formation in the subsurface of central 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. Hence, 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), stable and radiogenic isotopes, bulk composition and biodegradability of gas condensates, and brine composition of several sour gas pools leave no doubt that thermochemical sulfate reduction was responsible for the formation of sour gas. Anhydrite, H2S and pyrite have positive (isotope){34}S values, saddle dolomite and some late calcite spar have negative (isotope){13}C values. Present formation waters, saddle dolomite and late calcite spar have highly radiogenic {87}Sr/{86}Sr-ratios. The gas condensates are biodegradable, yet they are not biodegraded. The data also show that thermochemical sulfate reduction can take place in extremely narrow reaction zones, i.e., zones that are only a few meters thick.

This study provides several new clues regarding the process of thermochemical sulfate reduction. Furthermore, some aspects of this study can be used as an exploration tool in sour gas provinces.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.