The Sanish pool of the Antelope field has a number of unusual facets which make it almost unique in the Williston basin. Some of these are: (1) high productivity of a number of wells from a nebulous, ill-defined reservoir; (2) association with the steepest dip in the central portion of the basin; (3) very high initial reservoir pressure; and (4) almost complete absence of water production.

Analysis of these factors indicates that Sanish productivity is a function of tension fracturing associated with the relatively sharp Antelope structure. A relationship between fracture permeability and structural

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curvature (the second derivative of structure) can be expressed mathematically and it is found that permeability varies as the third power of curvature. A map of values of structural curvature shows a remarkable coincidence between areas of maximum curvature and areas of best productivity.

Volumetric considerations show that the quantities of oil being produced cannot be coming from the Sanish zone. It is concluded that the immediately overlying, highly petroliferous, Bakken Shale is the immediate as well as the ultimate source of this production. The role of the Sanish fracture is primarily that of a gathering system for many increments of production in the Bakken.

The extremely high initial reservoir pressure indicates that the Sanish-Bakken accumulation is in an isolated, completely oil-saturated reservoir and, hence, is independent of structure in the normal sense. Similar accumulations should exist anywhere in the Williston basin where a permeable bed, of limited areal extent, is indirect contact with either of the Bakken shales.

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