Fluid flow, mixing and the origin of CO2 and H2S by bacterial sulphate reduction in the Mannville Group, southern Alberta, Canada

Sandstone reservoirs of the Mannville Group in southern Alberta host petrol eum and natural gas deposits with abnormally high amounts of CO2 and H2S. P roduced water and gas chemistry and isotopic composition suggests that bact erial sulphate reduction has taken place. Limited isotopic data for gas sam ples indicate that CO2 Is organic, with calcite dissolution accounting for intermediate delta(13)C values for bicarbonate. CO2 and H2S are thought to be formed by bacterial sulphate reduction (BSR), the rate of which is depen dent on the rate of supply of sulphate. H2S has been precipitated as pyrite and removed from the system. CO2, from both bacterial sulphate reduction a nd resulting water-rock reactions, has accumulated in quartz-rich sandstone reservoir rocks causing the amount of CO2 in the gas to be much higher tha n H2S. Anomalously high CO2 and H2S concentrations are coincident with the Jurassic subcrop edge and are related to cross formational fluid flow. Sulp hate-rich waters from Mississippian aquifer units that underlie the Jurassi c aquitard mix with waters in Mannville sandstone at the subcrop edge. Orga nic acids ratios in Mannville waters suggest that coaly material, not petro leum hydrocarbons, may provide the organic food source required for sulphat e reducing bacteria. (C) 1999 Elsevier Science Ltd. All rights reserved.