Abstract

Author's Note: The following is an abstract of a talk given before the Tulsa Geological Society February 12, 1962. The full paper is to be published under the title "Subsurface Geology of the North Gotebo Area, Kiowa and Washita Counties, Oklahoma.

The producing area is in Townships 7 and 9 North, Ranges 16 and 17 West, in Kiowa and Washita Counties, Oklahoma. Several field names were given because of irregular development but it is now convenient to use the name North Gotebo for the whole producing trend which is approximately nine miles long by one-half mile wide. (See )

Located regionally on the north flank of the subsurface expression of the Wichita Mountain complex, this area is characterized by thrust faulting, over-turned folds, and upside-down producing sands.

The principal thrust fault, here called the Gotebo Fault, dips to the south at the rate of 5000 feet per mile. The oldest rocks so far encountered in the overthrust block are Hunton, of Siluro-Devonian age, resting on Springer, of probable Mississippian age.

The entire area is overlain by Permian rocks, at the base of which occurs the Pontotoc conglomerate. The erosion prior to the deposition of this conglomerate has erased much of the record so that the maximum extent of the thrusting cannot be accurately determined. The present trace of the Gotebo Thrust where it meets the unconformity is a convenient boundary to use between the Wichita Mountains and the Anadarko Basin.

North Gotebo is principally a gas field but oil is also produced from Springer sands beneath the thrust. There are four sand zones identifiable and in most wells they are upside-down due to overturn folding. Springer sands also occur in normal attitude, in the overthrust block, but are not productive.

Inconsistent bottom-hole pressures and oil production structurally higher than gas in correlative sands, indicate discontinuous reservoirs. This may be due to permeability barriers or minor faulting but in either case, the per-well gas reserves are therefore rather low, and variable, ranging from one billion to six billion cubic feet. Average porosity of the sands is twelve to fifteen per cent and permeability is estimated to be 100 millidarcies since most of the wells flow naturally, without fracture treatment.

The formations involved in the thrust faulting are primarily the Morrow-Springer-Chester section whose rocks are relatively incompetent. They lend themselves readily to overturned folding which ultimately results in thrust faulting. The thrusting is considered to have occurred originally at the end of Morrow time and to have been rejuventated in late Pennsylvanian time.

The Gotebo thrust, or one like it, can be traced for some miles in either direction from the North Gotebo production. It is probable that similar geologic conditions exist adjacent to any mountain complex, particularly if relatively incompetent beds are present.