Haybarn Field, Fremont County, Wyoming: An Upper Fort Union (Paleocene) Stratigraphic Trap

Haybarn field was discovered in the fall of 1981 and produces stratigraphically trapped 43.7° API gravity oil from shallow reservoir sandstones in the upper member of the Fort Union Formation. The sandstones were deposited in a marginal lacustrine delta front setting in the Wind River Basin, Wyoming. The interfingering lacustrine Waltman Shale has provided the trapping mechanism for the field. The Waltman Shale also appears to be the petroleum source; this primary source, however, was probably deposited in deeper portions of the basin north of the field rather than immediately adjacent to the reservoir. The reservoir sandstones are arkosic and have excellent porosity averaging about 20%. Clays in the reservoir appear to be entirely secondary suggesting that the sandstones were deposited in a high energy, wave-dominated depositional system. Electrical log evaluation of the Upper Fort Union reservoirs is complicated by variations in formation water resistivities between and within the sandstone tongues. The uppermost section of the Upper Fort Union member tends to be fresh water-bearing, while the producing zones in the lower portion of the member have much lower, more saline Rw's.