The reservoirs of the Mannville Group and associated horizons in southern Alberta contain anomalously high concentrations of carbon dioxide. To determine the distribution and causes of this CO2 anomaly, the hydrogeology and geochemistry of formation fluids in the Mannville Group and associated horizons in southern Alberta (49 degrees to 51 degrees N and 111 degrees to 115 degrees W) were examined using a comprehensive set of published water and gas chemical and stable isotope data. The formation waters of the Mannville Group have been modified by gravity driven fluid flow with a decrease in the hydrodynamic head, an increase in salinity and a progressive isotopic enrichment from south to north across the study area. Low salinity waters relatively depleted in O-18, consistent with meteoric recharge of the aquifer, are found in the southern part of the study area with higher salinity and waters relatively enriched in O-18 to the north. The gases in Mannville and stratigraphically lower reservoirs show an abrupt increase in mole fraction of carbon dioxide and a decrease in the ratio of methane to other hydrocarbon gases at 1000-m depth and 35 degrees C, concomitant with the top of the oil window as determined from vitrinite reflectance data. This CO2 anomaly does not extend into reservoirs above the Joli Fou shale, which overlies the Mannville Group. The juxtaposition of low salinity and sulphate-rich waters in a temperature setting optimal for microbe growth is favorable to the development of anomalous concentrations of carbon dioxide through bacterial sulphate reduction (BSR).