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Basinward Increase in Gas Prolificacy of Very Fine-Grained Sandstone Beds of the Alderson Member, Shackleton Area, Southwestern Saskatchewan

Pedersen, Per Kent
Saskatchewan Industry and Resources, Regina, SK

A large prolific gas pool was recently discovered within the Santonian–Campanian Alderson Member (Milk River Formation) in the Shackleton area of Southwestern Saskatchewan. The pool currently covers approximately 11 townships (396 miles2) with estimated 0.5-1 tcf of recoverable gas. Production is from thin siltstone and very fine-grained sandstone beds at shallow depths of 900-1400 ft, with initial production ranging from 200-780 mcf/d. The pool is located approximately 30 miles northeast of the Hatton Pool, a classic shallow, large-areal-extent, continuous-type, relatively low permeability, biogenic gas accumulation. Despite the more basinward location of the Shackleton gas accumulation and less sandy facies of thin sandstone beds, initial production rates from the new pool are significantly higher than they are sandier reservoirs within the Hatton Pool, averaging 300-350 mcf/d and 90-130 mcf/d respectively. Ongoing research of this prolific gas accumulation shows that the highest gas production rates coincide with the top of a structural high. The Alderson Member is composed of numerous, gas-charged, subtly upward-sanding parasequences of pro-delta deposits. The southwestern margin of the pool appears to be stratigraphically controlled by onlap of the main reservoir parasequence. Prolificacy appears also to be controlled by sedimentary facies, with thin-bedded facies being more prolific than more sandy but more intensively bioturbated facies. These and other factors will be discussed in context of their influence on gas trapping, reservoir characteristics and the potential for discovery of similar prolific gas accumulations within distal facies in southwestern Saskatchewan and within correlative upper Eagle deposits in northern Montana.