Geochemistry of Gas and Co-Produced Water in the Cretaceous Milk River Formation, Abbey and Lacadena Pools, Saskatchewan, Canada

J. L. Ridgley
U.S.Geological Survey, Lakewood, CO

The Cretaceous Milk River Formation in the contiguous Abbey and Lacadena (A-L) gas pools, Saskatchewan, is composed of very fine grained sandstone, siltstone, and mudstone that form stacked, upward-coarsening parasequences. Reservoir facies in the Milk River have a lobate pattern indicative of a distal deltaic or fan depositional system. The Milk River in the A-L pools has been folded into a composite "anticlinal" structure, which is elongated both northwest and northeast, and truncated along the eastern and southwestern margin.

Twenty-nine gas and 22 co-produced water samples were collected to determine (1) the origin of the gas and its relation to co-produced water, and (2) the relative importance of depositional pattern of reservoir strata, past hydrologic flow, and structure on the distribution of composition and isotope chemistry. The gas is of microbial origin (δ¹³C_methane (-69.4 to -71.6 ‰). A cross plot of δD_water versus δD_methane indicates that, for most samples, the gas is in disequilibrium with the co-produced water and did not form in the current aqueous environment. Contours of δD_water (-119 to -84 ‰) and chloride concentration (3370 to 6600 mg/l) have a northeastern orientation across the structure and are interpreted as indicating flow paths established prior to structural uplift. δ¹⁵N_nitrogen becomes heavier (-3.4 to -1.3 ‰) from west to east across the pools, reflecting fractionation during groundwater flow prior to uplift. Nitrogen content increases and methane content decreases off the crest of the structure; this stratification results from differences in solubility of methane and nitrogen influenced by uplift.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005