Abstract: Abstract: Nugget Oil Accumulations at Dry Piney, Tip Top, and Hogsback Fields, Sublette County, Wyoming

John B. Dunnewald, Kenneth A. Gorton

Until recently, oil production from the Jurassic Nugget Sandstone in western Wyoming was confined to three fields on the LaBarge platform in Sublette County. The Tip Top, Hogsback, and Dry Piney fields are all located on generally north-south-trending or slightly northwest-trending anticlines beneath the overlying Darby thrust. Tip Top and Dry Piney anticlines are associated with west-dipping reverse faults. Limited available seismic and subsurface control does not document reverse faulting at the depth of the Nugget in Hogsback field; however, it may be present.

The first of these fields to be discovered was Tip Top which was drilled by Mobil (General Petroleum) in early 1951 after detailed surface mapping and limited seismic work indicated the presence of a faulted anticline. Ten wells in the 9,500 to 10,000-ft (2,900 to 3,000 m) range have defined a 280-acre (113 ha.) field which has produced 2.5 million bbl of oil and is currently producing 75 BOPD.

Subsurface mapping of the top of the Frontier Formation in development wells led Mobil, in 1960, to drill the Nugget discovery on Hogsback anticline. The discovery well GP-13-31 penetrated the Nugget at 11,021 ft (3,359.2 m) and was completed for 285 bbl of oil and 9 bbl of water per day. The discovery well plus five additional wells have produced 5.3 million bbl of oil and the field currently is making 245 BOPD from three wells.

The Dry Piney field was discovered in the fall of 1969 when the Mountain Fuel Dry Piney 17 penetrated the Nugget at a depth of 10,998 ft (3,352.5 m) and was completed for 368 BOPD. Seven additional wells have been completed with initial production as high as 2,275 BOPD. The field has produced 7.5 million bbl of oil and currently is making 3,900 BOPD.

The producing Nugget Sandstone in these fields is fine to very fine grained, well sorted, and rounded, with local fracturing and calcite and anhydrite infilling. Average porosities range from 12.2% at Dry Piney to 17% at Hogsback, and the maximum net pay ranges from 36 ft (11 m) at Tip Top to 141 ft (43 m) at Dry Piney. All three fields produce 50+ API gravity oil, have less than 200/1 initial GOR, and have an active water drive.

The drilling associated with these fields, as well as for the overlying and adjacent Frontier gas accumulations, provides a unique opportunity to study the relation of Paleozoic rocks in the Darby thrust to the underlying Cretaceous and older rocks. Well control shows that the Darby is a bedding-plain thrust for 6 to 8 mi (9.6 to 12.8 km) west of its leading edge with respect to both the overlying Ordovician and the underlying Cretaceous (lower Mesaverde), and that there is a striking similarity between the structure at the top of the Frontier or Nugget and at the surface, suggesting that the faulting preceded folding.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado