Abstract: Isotopic and Compositional Characterization of Gas Sources and Migration in the Lower and Middle Triassic System, Alberta Basin

The Triassic system of the Alberta Basin contains an estimated 2.78 X 10{11} m{3} of gas. Despite abundant reserves, little attempt has been made to characterize source and migration-related factors affecting gas composition in these units. This study will integrate compositional analyses from over 700 wells in the Montney, Doig, and Halfway Formations with isotopic analyses from approximately 40 wells in order to characterize regional variations in gas composition and stable isotopic signature.

Dry gas indices range from 0.566 to 0.999. Gases in the deepest central part of the study area show the lowest DGls, becoming increasingly dry with decreasing depth away from the center. Preliminary data indicate that delta{13}C of CH[4] shows a strong formational control, becoming increasingly negative upwards in the stratigraphic succession. These variations in delta{13}C are not correlative with depth. Furthermore, coexisting C[2+] fractions do not show similar formational control over delta{13}C. In combination, compositional and isotopic data suggest mixing of thermogenic gas with methane, possibly of bacterial origin, from overlying units.

Occurrences of the non-hydrocarbon gases He, N[2], CO[2] and H[2]S are observed in Lower and Middle Triassic pools. H[2]S concentrations as high as 29.53 mole % are present, with relative H[2]S and CO[2] highs being coincident. This coincidence, coupled with delta{34}S signatures between 6 and 14 o/oo CDT, suggests that a combination of bacterial and thermochemical sulfate reduction is responsible for the occurrence of H[2]S highs in the study area.  

AAPG Search and Discovery Article #90940©1997 AAPG Foundation Grants-in-Aid