REGIONAL WATER AND GAS GEOCHEMISTRY OF THE MANNVILLE-GROUP AND ASSOCIATED HORIZONS, SOUTHERN ALBERTA

The reservoirs of the Mannville Group and associated horizons in south ern Alberta contain anomalously high concentrations of carbon dioxide. To determine the distribution and causes of this CO2 anomaly, the hyd rogeology 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 comprehensiv e 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 increa se in salinity and a progressive isotopic enrichment from south to nor th across the study area. Low salinity waters relatively depleted in O -18, consistent with meteoric recharge of the aquifer, are found in th e southern part of the study area with higher salinity and waters rela tively enriched in O-18 to the north. The gases in Mannville and strat igraphically lower reservoirs show an abrupt increase in mole fraction of carbon dioxide and a decrease in the ratio of methane to other hyd rocarbon gases at 1000-m depth and 35 degrees C, concomitant with the top of the oil window as determined from vitrinite reflectance data. T his CO2 anomaly does not extend into reservoirs above the Joli Fou sha le, which overlies the Mannville Group. The juxtaposition of low salin ity and sulphate-rich waters in a temperature setting optimal for micr obe growth is favorable to the development of anomalous concentrations of carbon dioxide through bacterial sulphate reduction (BSR).