Abstract

Clark's Fork field is at the northern edge of the Bighorn basin (T9S, R22E) in Carbon County, Montana. Production was first established in 1944 by General Petroleum Corporation in the Upper Cretaceous Peay Sandstone Member of the Formation Frontier (basal Frontier), and was later extended to the Lower Cretaceous Greybull (1949) and Lakota (1956) sandstones by British American. Total cumulative hydrocarbons from this field (as of 9-85) are 1,179,193 bbl of oil and 3,061,522 mcf of gas, with the Lakota sandstones being the most productive.

Lakota production occurs from a structural-stratigraphic trap in an easterly-trending channel on the axis of Clark's Fork anticline near the center of the township. Production from the Lakota also occurs in a structural trap on Clark's Fork anticline in the northwest corner of the township. Greybull sandstones in this area are characterized by distributary channels originating from a major channel in the northwest corner of the township. Accumulation of hydrocarbons in the Greybull occurs in a distributary channel-sandstone pinchout on the Elk Basin anticline. Greybull production also occurs in the structural trap on Clark's Fork anticline. Production from the ubiquitous Peay sandstones occurs from a structural trap on Elk Basin anticline.

Our structural reinterpretation of Clark's Fork field suggests that Elk Basin anticline is a northwest extension of the Elk Basin field anticline. The Elk Basin thrust truncates the north limb of this fold and does not strike to the northwest as shown by earlier interpretations. We interpret a northwest-striking thrust in the center of the township as a splay off the Elk Basin thrust, and call it the Clark's Fork thrust. The Clark's Fork anticline is located in the hanging wall of Clark's Fork thrust.

Our subsurface maps indicate that the Clark's Fork area has not been fully developed by additional drilling. Stratigraphic traps in the Lakota and Greybull sandstones are present in several areas of the township. Structural traps in the center and northwest portions of the township may also exist.