ABSTRACT

The Second White Speckled Shale produces gas at widely spaced locations on the northern extension of the Sweetgrass Arch in Alberta and Saskatchewan, Canada. Evaluation of 335 gas analyses from the Second White Speckled Shale indicates that all but nine of the gases are biogenic in origin. Five of the nine analyses have a dry gas index less than 0.95. The other four are greater than 0.95 and less than 0.98, but contain concentrations of other hydrocarbon fractions (C2-C5) that preclude them from having a totally biogenic signature.

The ranges in composition of the various hydrocarbon and nonhydrocarbon gases along with other thermal maturity indicators together suggest that the study area is thermally immature. Methane and nitrogen show a consistent inverse correlation across the central part of the arch; thus samples high in methane are low in nitrogen and those lower in methane are associated with higher nitrogen content. Methane increases and nitrogen decreases from south to north in a pattern suggestive of groundwater recharge. The highest contents of methane in the gas analyses seem to follow the courses of the Red Deer and South Saskatchewan rivers, areas that currently have geothermal gradients somewhat higher than those of neighbouring uplands. The observed distribution of methane content is best explained by a later groundwater recharge over-printing an earlier, more uniformly distributed methane concentration predicted from a simple depositional model for the Second White Speckled Shale.