The Muddy Sandstone, Bighorn Basin, Wyoming: An Underexplored, Stratigraphic Gas Play

Mason, Robert M.
Andex Resources, LLC, Denver, CO

The Lower Cretaceous Muddy Sandstone is an underexplored target in the Bighorn Basin of northwest Wyoming and southwest Montana. Historically, the focus of most drilling in the basin has been on prolific structural traps identified both from surface and seismic data. The Muddy Sandstone produces primarily gas and associated liquids from many of these features both on the east and west flanks as well as within the basin proper. Production ranges from 3500' to 19,600'and is both normal and overpressured. Field studies of the principal Muddy Sandstone accumulations in the basin show that these are stratigraphic traps on structure closure.

The Muddy Sandstone in the Bighorn Basin is a complex transgressive systems tract deposited on a widespread erosional sequence boundary formed at the top of the underlying Skull Creek Shale. Virtually all Muddy Sandstone production in the basin is from either estaurine/fluvial or nearshore marine sandstones found within or on the flanks of paleo-valleys. A basinwide map of this paleo-valley system integrating surface and subsurface data shows a series of east to west trending valleys on the east side of the basin draining into a main north south system located on the west side of the basin. This is an important local modification of the regional picture so well documented by earlier studies. By incorporating shows and productive trends several paleovalleys have been identified that have a high probability of containing hydrocarbon charged reservoirs in large stratigraphic traps not associated with structural closure.