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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 53 (1969)

Issue: 3. (March)

First Page: 731

Last Page: 731

Title: Geological Reservoir Analysis, Madison Formation, Elk Basin Field, Wyoming-Montana: ABSTRACT

Author(s): J. A. McCaleb, D. A. Wayhan

Article Type: Meeting abstract

Abstract:

The Elk Basin field is in the northeast end of the Big Horn basin, on the Wyoming-Montana state line. The structure is a NW-SE-trending asymmetrical anticline, approximately 8 mi long and 4 mi wide, with about 5,000 ft of structural closure. Oil production from the Madison Formation was discovered in 1946, and the Madison has supplied more than 75 MM bbl of oil within 5,100 productive acres from a closure of about 1,400 ft. A recent core study of the Madison reservoir shows that it can be divided into several separate, distinct, geologic and production units.

The Madison carbonate sequence has been altered greatly and distorted by groundwater erosion as a result of the formation of karst topography, by subsequent solution brecciation in Late Mississippian-Early Pennsylvanian time, and by selective remineralization in some areas of the field. The overall effects are the collapse of sections of the upper Madison--up to 300 ft thick--into brecciated rubble zones, thereby removing blocks from 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 caused local, relatively tighter zones, forming, in effect, local stratigraphic traps. Zones of insoluble reside of clay and ro k fragments from an effective barrier between the "A" and "B" producing zones, and account for the different reservoir characteristics of these zones.

An important effect of the groundwater action has been the removal of the more soluble limestone, leaving the less soluble dolomite and thereby forming the good secondary porosity found in the Elk basin Madison. The development of this secondary porosity can be correlated and subdivided into readily recognizable and distinct zones. This shows a certain degree of continuity, which is necessary in evolving an efficient drilling and flooding program. Electric-log and core evaluation of other Big Horn basin fields which penetrate the Madison indicate the existence of a situation similar to that in the Elk Basin reservoir, except that the Madison in other fields generally does not have such pronounced karst-solution development.

The above-mentioned variations can be unified into a practical working hypothesis for reservoir engineering analysis; the hypothesis so developed provides a useful three-dimensional reconstruction of the Elk basin Madison reservoir. Application of the hypothesis has led to a dramatic production response within the reservoir. The practical success of the hypothesis has important exploration implications; specifically, the exploration geologist must understand known producing reservoirs before effective exploration for new reservoirs can be carried out successfully.

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Copyright 1997 American Association of Petroleum Geologists