Quantitative Fracture Study--Sanish Pool, Mckenzie County, North Dakota

Abstract

The Devonian Sanish pool of the Antelope field has several unusual characteristics which make it almost unique in the Williston basin. Some of these are: (1) high productivity of several wells from a nebulous, ill-defined reservoir; (2) association with the steepest dip in the central part 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. Fracture porosity and fracture permeability can be related mathematically to bed thickness and structural curvature (the second derivative of structure). It is found that fracture porosity varies directly as the product of bed thickness times curvature and that fracture permeability varies as the third power of this product. A map of structural curvature in the Sanish pool shows good 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 overlying, very petroliferous Bakken Shale is the immediate, as well as the ultimate, source of this oil. The role of the Sanish fracture system is primarily that of a gathering system for many increments of production from 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 be present anywhere in the Williston basin where a permeable bed, of limited areal extent, is in direct contact with either of the two Bakken shale beds.