Western Cordillera feels that two possible stratigraphic plays in the evaluation area have a strong potential for hydrocarbon accumulation. We have termed these stratigraphic possibilities the Pinon shelf margin and the Butte basin.

The Lower Paleozoic Pinon shelf margin is a well-defined north-northeast south-southwest-trending zone that represents the Lower Paleozoic shelf-slope break. This zone has oscillated only very slightly, perhaps 10 to 20 miles in an east-west direction, from Middle Ordovician through the Late Devonian (Matti and others, 1974; Smith and Ketner, 1977; Matti and McKee, 1977; Kendall and others, 1984). The carbonates deposited along this shelf margin are generally diagenetically enhanced and porous. Rapid lateral facies changes and lithologic variation provide an ideal situation for a stratigraphic play based upon porosity or permeability pinchouts.

Shelf margin carbonates such as the vuggy and coarse-grained, biohermal Silurian Lone Mountain Dolomite, and the Devonian Devils Gate, Sadler Ranch, and Telegraph Canyon Formations, and the Oxyoke Canyon Sandstone appear to have well developed intercrystaline and vugular porosity in local surface exposures. The upper part of the Nevada Group is producing in the Blackburn field in Pine Valley, and facies-equivalent shelf carbonates such as the Simonson produce in fractured reservoirs in the Grant Canyon Field in Railroad Valley.

Surface geochemical data suggest a possibility of source potential within the biohermal shelf margin. Several of the lower Paleozoic carbonates may have source potential as well as the Ordovician Vinini Formation or Devonian Woodruff Formation and perhaps locally the Mississippian Chainman Formation.

Questions to be answered in detailing the Pinon shelf margin play include the continuance of surface porosity, which may in part represent surface leaching, into the subsurface; the availability of adequate source rocks beneath, within, as well as in juxtaposition with the shelf-margin units, and the careful delineation of the favorable reservoir facies within the limestone-dolomite transition along or behind the shelf.

The Permian Butte basin along the eastern margin of the evaluation area contains several thousand feet of shallow-marine shelf and lagoonal carbonates, along with fine-grained clastics and evaporites. Both potential source (Phosphoria and Arcturus) and reservoir (Loray, Pequop, Plympton, and others) are present within the Butte basin. These Permian sediments are lithologically and depositionally similar to the highly productive sediments in the Permian basin of Texas.

The central portion of the Butte Basin suffers from the overprint of structural dismemberment by low-angle normal faults. The major effects of low-angle deformation however are seen in the Pennsylvanian and younger rocks and it may be possible that although shuffled and locally attenuated, as well as offset by high-angle normal faults, these Permian sediments may locally contain intact hydrocarbon traps. The northern and southern portions of the basin are less affected by extensional deformation and may provide better opportunities for intact stratigraphic traps.