ABSTRACT: Porosity in Sour Gas Reservoirs Associated with Thermochemical Sulfate Reduction

ELIUK, LESLIE S., and CHRISTIAN A. VIAU, Shell Canada Ltd, Calgary, Alberta, Canada

The creation of porosity or its destruction with preservation of remnant porosity is a complex and unique sequence of events that defy detailed generalization except in specific examples. Nevertheless, the application of thermochemical sulfate reduction (TSR) to deep subsurface porosity creation/modification due to anhydrite (and dolomite!) removal seemed like a straightforward opportunity to generalize in a useful and perhaps predictable manner. Western Canada, with its sour (H2S-rich) dolomite reservoirs, is clearly a place where TSR is occurring and where porosity creation/modification ought to be evident and give encouragement in exploration for deeply buried reservoirs. This is exemplified by good reservoir porosities at 3600 m in the Swan Hills Caroline discovery with 35% H2S. B t Shell Canada's TSR research was focused on a comparatively simple case. The Devonian Crossfield Member was selected based on its assumed simplicity of history and facies. It underscored the need for pre-existing porosity to allow TSR, that is secondary, not primary-bedded, anhydrite is involved. This "simple" reservoir can be contrasted to Mississippian subcrop dolomites cut by multiple unconformities with potential to dissolve evaporites or redeposit more. Or consider the Leduc and Winterburn reefs, which have significant lateral facies changes that along with intraformational exposures again go into the subsurface with a complicated charge of secondary anhydrites. During burial, there are various periods when sulfates may be supplied by groundwaters and along faults. In fact, seconda y anhydrite is commonly linked with dolomite so that understanding TSR requires understanding dolomitization. Sulfate in solution may be the source of H2S so that the removal of local reservoir solids may be a bad assumption. These are just the beginning of the complications. So with our imperfect knowledge there is room for both research and humility.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)