(Chapter 3, Final Sweet Grass Hills Amendment and Environmental Impact Statement, April 1996)
John Smart Photo East Butte
East, Middle, and West Buttes of the Sweet Grass Hills are the exposed portions of igneous intrusions and the associated uplifted sedimentary rocks that extend over an area of approximately 68,605 acres. Of that acreage, 21,409 acres are Federal minerals that are currently leased for oil and gas, or are available for oil and gas leasing.
Middle Butte (Gold)
The sedimentary rocks found in the hills are generally faulted, slightly to highly metamorphosed sandstones, shales, and limestones. Composition of the intrusives varies from one butte to the next, but is generally classified as ranging from syenite to diorite porphyry. Monzonite, lamprophyres, and more mafic sills and dikes are known to occur throughout the buttes and in the surrounding sediments. The intrusives are believed to be laccoliths with their bases located in the Colorado group sediments or possibly the Mississippian age rocks. Laccoliths are relatively flat bottomed igneous intrusives that form as a result of magma piercing the overlying sediments and then spreading out from the original conduit into the surrounding sediments. Often times the conduit is much smaller than the surface expression. In vertical cross section these structures have a mushroom or umbrella like shape.
West Butte
John Smart Photo
The laccolith's shape is important in the occurrence of oil and gas traps. Like salt domes, these intrusives cause deformation of the overlying and surrounding sediments. This deformation can result in the formation of structural traps in which hydrocarbons can accumulate. Unlike salt domes, these intrusives also provide a heat source for thermal maturation of organic carbon contained within the surrounding and overlying rocks. In addition, the molten rock intrudes into fissures, faults, and weakened areas created by the warping and uplifting of the overlying sedimentary rocks thus, filling potential leak points and potentially forming trapping seals. Local metamorphism of sand to quartzite, shale to argillite and slate, and limestone to marble, can form permeability barriers that could trap hydrocarbons.
Sediments overlying and adjacent to the buttes are deformed as a result of the emplacement of the Sweet Grass Hills laccoliths. This deformation has created numerous smaller oil and gas fields in the surrounding sediments. However, because the extent of the known igneous intrusives at any particular depth is uncertain, and that the potential exists for unexposed laccoliths which may have caused similar structural traps, additional undiscovered oil and gas fields may underlie the sediments below and on the flanks of the buttes and in the surrounding sedimentary rocks. Accordingly, all of the Sweet Grass Hills study area and surrounding acreage is classified as high potential for oil and gas occurrence.
Existing production from the Sweet Grass Hills is primarily from Cretaceous and Mississippian age rocks. Virtually no deeper exploration of the Devonian age rocks has been conducted. Although the existing fields are not large by comparison, they are relatively shallow and easy to produce. These characteristics make such fields good exploration/development opportunities for small, independent operators.
The 13 townships adjacent to the three buttes contain numerous oil and gas fields. Production from these fields is quite significant and has played an important role in the local economy.