Abstract

The occurrence of the non-hydrocarbon gases hydrogen sulphide (H 2 S), carbon dioxide (CO 2 ), helium (He) and nitrogen (N 2 ) is ubiquitous in strata of all ages in the Alberta Basin. The patterns of occurrence at a regional scale show many similarities. Hydrogen sulfide and CO 2 tend to increase to the west and with depth in Devonian and Mississippian strata, and concentrations are higher than in Cretaceous units. In the Cretaceous Mannville Group, H 2 S and CO 2 tend to be highest in concentration along the Jurassic subcrop edge. In the Cretaceous Colorado Group H 2 S is not present in significant amounts and CO 2 tends to increase with depth to the west. Nitrogen, and to some degree helium, tend to show an antipathetic relationship with high H 2 S and CO 2 in Devonian and Mississippian strata. Helium shows relatively high concentrations in north central Alberta in the Devonian, and in southern Alberta in the Mannville. Nitrogen in southern Alberta in the Mannville and the Colorado groups is higher at shallow depths. Isotopic data, cross-plots of compositions, and trends with depth and temperature all suggest that H 2 S and CO 2 in Devonian rocks is largely the product of thermal reduction of sulphate, most of which is probably derived from Devonian anhydrite. In Mississippian and Triassic strata these gases show a combination of both a thermal origin and bacterial reduction of sulphate. In the Mannville Group, H 2 S is the product of bacterial reduction of sulphate. The bacterial reduction process is driven by incursion of meteoric water from the south that causes mixing, at the Jurassic subcrop edge, of waters from Mississippian carbonate rocks and waters from Mannville clastic rocks. Some areas of high bicarbonate (and presumably relatively high CO 2 ) in southern Alberta, on the basis of isotopic data, apparently originate from methanogenesis. Carbon dioxide in the Colorado Group is probably formed by oxidation of organic matter, presumably by bacterial processes. The trend to less negative isotopic compositions with depth suggests that organic sources are mixed with inorganic sources (calcite) of carbon. Interpretation of isotopic data for helium suggests that the helium observed in Devonian units originates from deep crustal sources, probably the Precambrian basement, that mix with atmospheric sources. In contrast, high helium and nitrogen concentrations in the Mannville are the result of entry of meteoric water. A similar pattern is observed for nitrogen in the Colorado Group and correlations with hydrocarbon gases suggest the high nitrogen content is related to bacterial accumulations of natural gas.

You do not currently have access to this article.