Middle Member of Minnelusa Formation (Middle and Upper Pennsylvanian)--Implications for Stratigraphic-Trap Oil Accumulations in Powder River Basin, Wyoming: ABSTRACT

Abstract

The middle member of the Minnelusa Formation (Middle to Upper Pennsylvanian) thins northward across the Powder River basin from about 150 m thick in the Hartville uplift to less than 50 m near the Wyoming-Montana border. Much of the thinning occurs beneath a regional, pre-Permian unconformity, which is identified in the south half of the basin by a mudstone commonly identified as the red shale marker. Cyclically arranged units up to 10 m thick, composed in ascending order of black organic-rich shale, mud-supported dolomite, anhydrite, and quartzose sandstone, characterize most of the middle Minnelusa. The majority of the sandstone units (informally designated in the subsurface as the Leo sandstones) are less than 3 m thick, tabular shaped, and commonly cemented with anhydrite and dolomite. Locally, however, the sandstones, particularly in the first Leo interval, are lenticular and linear, very porous, and attain thicknesses of more than 15 m. Several of these 10 to 15-m thick, linear first Leo sandstone bodies trend northwest across the southern Powder River basin. They probably represent wadi-type channels that have cut across sabkha and associated peritidal deposits during low st nds of sea level. The source of the Leo sandstones is presently uncertain, but at least the lowermost ones appear to be distal equivalents of the Tensleep Sandstone (Desmoinesian) to the northwest. Where the thicker sandstone units of the first Leo cross anticlinal noses such as at Red Bird, Pine Lodge, and Little Buck Creek, they commonly contain stratigraphically trapped oil. The oil in these fields was probably locally derived from the thin (0.5 to 2 m), widespread, black organic-rich shale units in the middle member. The lenticularity, proposed northwest trend, thickness, porosity, and associated probable source rocks make these sandstone units prime targets for oil and gas exploration in the sparsely tested, deep, southern Powder River basin. End_of_Article - Last_Page 778------------