The Medicine Lake field of Sheridan County, Montana, was discovered in March 1979 by the drilling of a seismic anomaly. Production is obtained from Paleozoic carbonate reservoirs ranging in age from Ordovician to Mississippian. Cumulative production from the field, as of March 1982, is 1.2 million bbl.

A mini-3D seismic survey was acquired in October 1981 to facilitate development drilling. The survey covered 2.4 mi² (6.2 km²), encompassing the field's seven producing wells and two dry holes. The purpose of this survey was to provide an accurate image of the subsurface structure and delineate the extent of the producing formations.

The areal coverage and improved subsurface imaging of the 3D survey provided a detailed view of the Medicine Lake anomaly. The seismic data reveals that the structure results from a local basement (Precambrian) high. Mapping of the Ordovician Winnipeg Formation revealed a domal structure covering approximately 0.6 mi² (1.5 km²) with closure in excess of 180 ft (55 m).

Although all producing wells are located on the Medicine Lake structure, stratigraphic variations within the reservoirs may localize production within structural closure. Porosity in several producing formations is diagenetic; prediction of reservoir trends from well data alone is difficult. Inversion and interactive modeling were used to study these stratigraphic variations. A correlation between relative acoustic impedance and porosity was established for several formations. Vertical and horizontal relative acoustic impedance sections were then employed to locate zones of possible porosity. This information, combined with the improved structural data, should aid in further development of the Medicine Lake field.

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