NIELSEN, KARSTEN SCHJOEDT¹, CLAUDIA SCHRODER-ADAMS¹, and DALE LECKIE^2
1Carleton University, Ottawa, Canada
²Wascana Energy Inc. Calgary, Canada

Abstract: Drill the Flanks, Not the Crest: Structural Influence on Shelf Sand Reservoirs

This study illustrates the influence of a subtle structural high on sediment distribution patterns in a shelf environment. The Sweetgrass Arch, straddling the US/Canada border, is characterized by Late Cretaceous gas-bearing sandstones on the flanks, these gradually pinch-out towards the crest. During most of the Cretaceous, the arch was submerged by the Western Interior Seaway, however there is no evidence of significant erosion or winnowing of sediments on the crest of the arch.

Our results are based on analysis of the uppermost part of the Late Cretaceous Colorado Group, consisting of the Coniacian Unnamed Colorado shale, thin fine grained sandstones of the lower Santonian Medicine Hat Formation and the calcareous upper Santonian First White Speckled Shale. The widespread Medicine Hat sandstones were transported across a shelf environment by offshore currents either from the eastern part of the basin, where erosion is indicated by a major regional unconformity or from the western margin of the Basin. Sands were trapped and accumulated on the flanks of the arch when the current waned while moving up the arch. The Sweetgrass Arch itself was not the provenance of the sandstones.

For explorationists this case study emphazises the importance of a regional understanding of paleoenvironments. The unwary geologist may drill for structural traps over the crest of the arch, whereas the productive exploration targets are stratigraphic traps on the flanks of the structure.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas