The increase of oil exploration in carbonate provinces requires a better understanding of stratigraphic entrapment. Photomicrographs of reservoir and trap rock from a Nesson anticline field in North Dakota illustrate this problem. The pore geometry and petrology of rocks with a similar relation would be beneficial to the exploration geologist.

A schematic diagram illustrates the interstices of a reservoir rock. The total porosity is a ratio of the rock’s void space to its bulk volume. Under subsurface reservoir conditions, this porosity is occupied by fluid of two phases. The non-wetting oil phase occupies this porosity according to the size and distribution of the rock’s pore system. The displacement of interstitial water by oil depends on the size of pore throats. What is not effectively displaced by oil remains as irreducible water saturation within the reservoir.

In the laboratory, the mercury injection method of capillary pressure measurement simulates these reservoir properties and pore size and equivalent reservoir characteristics can be calculated. The capillary pressure curves may be investigated by the same statistical methods used on cumulative curves from sieve analysis of unconsolidated sands. The hysteresis of the capillary pressure curve depends on the vuggy makeup of the rock.

Seven distinctive petrophysical characteristics were evident from two hundred samples of Williston basin carbonate rocks studied. These characteristics may be classified by displacement pressure, effective porosity, and pore distribution. Examples of typical rocks show good and intermediate reservoir rock and a specific reservoir-trap rock. A photomicrograph of reservoir rock with low effective porosity gives an insight into the reason for expecting highly water-cut production from carbonate reservoirs. This petrophysics is independent of the porosity and permeability routinely measured on reservoir rock.

Stratigraphic entrapment of oil in a field example is explained by petrophysics of the reservoir and trap rock. These petrophysical distinctions can be observed from sample examination without extensive laboratory measurements.

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