ABSTRACT: Anomalously High pCO₂ in Quartzose and Lithic Mannville Group Sandstones

CODY, JOHN D., IAN HUTCHEON, and H. ROY KROUSE, University of Calgary, Calgary, Alberta, Canada

Initial pool pressure data for oil and gas reservoirs within the Lower Cretaceous Mannville Group in southern Alberta show values of pCO₂ that are two or three orders of magnitude greater than those reported for similar reservoir rocks from the Texas Gulf Coast and North Sea. To determine the cause of high pCO₂, water, gas, and rock samples have been obtained from reservoirs in southern Alberta that straddle the area and producing horizons in which high pCO₂ values are observed.

In the study area there is low salinity meteoric water, with less than 10,000 ppm TDS and (d)8O approximately -15o/oo (SMOW), probably from the Sweetgrass Arch, a topographic high that lies to

the southeast. This low salinity water contacts, and may displace, more evolved highly saline formation waters with greater than 50,000 ppm TDS and 18 Oxygen approximately -4.0^o/oo (SMOW), that lie to the west. Reservoirs with anomalously high pCO₂ are close to this abrupt change in formation water chemical and isotopic composition. Some reservoirs have high H₂S solution gas and these are irregularly distributed. The chemical and isotopic data suggest that the H₂S is produced by bacterial sulfate reduction (BSR) of sulfate in the high salinity waters (SO₄ > 7000 mg/L). Isotopic compositions of CO₂ in equilibrium with HCO₃(aq) range from 0 to -15o/oo Carbon 13 (PDB), with the most depleted values consistent with BSR. Values of Carbon 13 close to 0.0o/oo probably reflect dissolution f calcite.

Reaction path models show that BSR is a net porosity producing process if sulfate is derived from waters; however, if sulfate is derived from anhydrite, porosity increases are limited and demand unrealistically high volumes of sulfate minerals.

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