The Elk Basin field is in the north end of the Big Horn basin, on the Wyoming-Montana state line. The structure is a NW-SE-trending asymmetric anticline approximately 8 mi long, 4 mi wide, and has about 5,000 ft of structural closure.

Oil production from the Mississippian Madison Group was discovered in 1946. Cumulative production is now more than 75 million bbl of oil from 5,100 acres; productive closure is about 1,400 ft. A recent core study of the Madison reservoir shows it can be divided into several separate, distinct geologic and production units.

The Madison carbonate sequence has been greatly altered and distorted by groundwater erosion, by the formation of karst topography and subsequent solution brecciation in Late Mississippian-Early Pennsylvanian time, and by selected remineralization in certain areas of the field. The overall effect of karst activity was the collapse of sections of the upper Madison, up to 300 ft thick, into brecciated rubble zones. As a result of such collapse, entire blocks of the upper Madison have no effective communication with each other.

There are areas of remineralization which, because of redeposition of dissolved carbonates, silica, and anhydrite into pore space and fractures by the downward-percolating groundwater, have local, relatively impermeable zones. These zones form local stratigraphic traps. Zones of insoluble residue of clay and rock fragments form an effective barrier between the A and B producing zones, and explain the different reservoir characteristics of each zone.

Groundwater action removed the more soluble limestone but left the less soluble dolomite, and formed the good secondary porosity now found in the Elk Basin Madison. The secondary porosity zones can be correlated and subdivided into readily recognizable and distinct units. The fact that such subdivision is possible demonstrates a certain degree of continuity of solution action. The understanding of this continuity is necessary to the evolution of efficient drilling and flooding programs.

Electric-log and core evaluations of the Madison in other Big Horn basin fields indicate reservoirs similar to the Elk Basin Madison reservoir. However, most of the other Madison fields do not show the same degree of karst development.

From the above-mentioned variations, a multiple working hypothesis can be developed for reservoir engineering analysis. The overall hypothesis provides a good vertical and areal model of the Elk Basin Madison reservoir. Practical application of the hypothesis has resulted in a rather dramatic production response. The exploration implications are that geologists need to understand the characteristics of known reservoirs before they can conduct effective exploration for new reservoirs.