Although the term "petrophysics" was defined by G. E. Archie in 1949, application of petrophysical approaches and techniques to exploration projects has been minimal during the past 30 years. This paper documents the application of varied petrophysical techniques to a field study and illustrates the integration of data to form a detailed interpretation of the reservoir.

Cut Bank field is a giant oil and gas field located on the west flank of the Sweetgrass arch in north-central Montana. Production is from the Lower Cretaceous Cut Bank Sandstone, which was deposited in a braided fluvial system.

Porosity versus permeability cross-plots of core data indicate a wide range of different rock types within the Cut Bank Sandstone interval. Capillary pressure curves, X-ray diffraction analyses, scanning electron micrographs, and thin section evaluation further define the rock types and document pore geometry differences. An Sw versus height-above-sea-level plot indicates field-wide pressure communication with a common water-oil

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contact. These data explain downdip shows and water production, poor oil production within the field, and gas production updip.

Integration of all information indicates that (1) Cut Bank field is a continuous reservoir which displays the classical updip progression of water to oil to gas, (2) local rock type variation accounts for the varying quality of production within the field; and (3) there is a common oil-water contact on the downdip edge of the field and its position is controlled by rock type differences.

Drilling subsequent to this study has confirmed the interpretation reported here.

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