David M. Wheeler¹, Edmund R. Gustason²

(1) Ensign Oil & Gas, Inc, Denver, CO
(2) Schlumberger Holditch-Reservoir Technologies, Denver, CO

ABSTRACT: Comparison of Stratigraphy and Sedimentology of the Shannon Sandstone in Jepson-Holler Draw and Pine Tree Fields, Powder River Basin, Wyoming

Previous work on the Shannon Sandstone of the Powder River Basin has resulted in three divergent depositional models: shelf ridge, lowstand shoreface and tidal bars within incised valleys. Cores and well-logs in the Jepson-Holler Draw and Pine Tree fields demonstrate important stratigraphic relationships that must be accounted for in any of these models. Although the Shannon comprises comparable facies in these two fields, reservoir architecture is dramatically different and provides insight on how the unit was deposited.

In both field areas the lithofacies above and below the reservoir interval is thinly interbedded, intensely burrowed sandstone and shale. The reservoir lithofacies is primarily planar bedded to ripple laminated sandstone with minor sets of small scale cross bedding, scattered shale drapes and burrows. Planolites, Terebellina, Palaeophycus and Skolithos dominate the trace fossil assemblage in the reservoir and non-reservoir lithofacies.

The major difference in the two field areas is apparent in stratigraphic architecture. In Pine Tree Field the reservoir interval drops in the section from west to east (seaward) relative to chronostratigraphic bentonite markers. There is a scour surface at the base of the reservoir interval that truncates up to 30 feet of the underlying section. In contrast, the reservoir interval at Jepson-Holler Draw climbs section from west to east (seaward) relative to the same markers.

The facies associations observed in core and stratigraphic relationships established from well-logs have been utilized to improve reservoir predictability in the ongoing development program at Jepson-Holler Draw. The origin of these relationships, and those observed at Pine Tree, are being evaluated in an effort to reinterpret the depositional origin of the Shannon Sandstone.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado