Point-Bar Origin of Fall River Sandstone Reservoirs, Northeastern Wyoming

Abstract

Proved oil reserves of more than 40 million bbl have been found in the Lower Cretaceous Fall River Sandstone on the northeast flank of the Powder River basin. Most of the oil is in stratigraphic traps on gentle regional dip of about 2° SW. Fall River reservoir sandstone formerly was believed to have been deposited as barrier bars with the updip permeability barriers provided by lagoonal shale. A new interpretation of West Moorcroft field (Mettler, 1966) suggested an origin as point-bar sand deposited in a meandering stream channel with the updip permeability barrier provided by a clay-filled, abandoned channel. This interpretation is applicable to other Fall River oil fields, as shown by distribution of sandstone at Coyote Creek and Miller Creek fields. On the basis f thickness of permeable sandstone, a pattern is present which is remarkably similar to that of point-bar, swale, and channel deposits of recent stream-meander belts. Recognition of these distinct facies will aid in exploration for new fields, in development drilling within fields, and in planning for secondary recovery projects.