Reservoir Characteristics of Putnam Zone (Silurian Interlake Formation) Lithofacies, Southwestern Williston Basin

INDEN, RICHARD, LSSI, Denver, CO, CHRIS OGLESBY, Bass Enterprises, Fort Worth, TX, ALAN BYRNES, Geocore, Loveland, CO, and BOB CLUFF, The Discovery Group, Denver, CO

Reservoirs in the Putnam zone (lower Interlake Formation) in the southwestern part of the Williston basin include oolitic-pellet dolomite grainstone, fossil-pellet grainstone, and a wide spectrum of reef-related,

fossil-corral dolomite packstones and coral-stromatoporoid rudstone/boundstones. Each of these potential reservoirs has a unique pore system and, thus, a different set of petrophysical properties which define their reservoir characteristics.

Oolitic grainstones have a homogeneous intercrystalline-microcrystalline pore system, whereas the fossil-pellet dolomite grainstone facies consists of separate mesovugs dispersed in well-interconnected intercrystalline porosity. Measured resistivity parameters m and n values for this rock type are approximately 2.2 and 1.7, respectively, but each has a distinctive porosity/permeability trend. Reef-related reservoir facies can be so porous that the best quality reservoir rocks cannot be sampled because they crumble. Rocks in this group contain not only intercrystalline and mesovuggy porosity but also macrovugs and microfractures; m and n are from 1.66-2 and 1.85, respectively.

Capillary pressure curves indicate that pore-throat heterogeneity is greater, and entry pressures lower, for reefal lithofacies than for pelletal grainstones. These curves also demonstrate why many of the producing fields tend to have high water cuts. In many oolitic-pellet grainstone units, irreducible water saturations of 10% would not be reached until a hydrocarbon column of 700 ft was reached. High water production characteristics are therefore expected because Red River/Interlake structures attain only 50-100 ft of closure.

This, however, does not mean that Putnam is not an economic zone, especially as a secondary objective. Wells in Putnam and Crane fields, for instance, have reserves in excess of 300,000 bbl of oil. The reservoirs here may be dominated by the reef-related facies, which have an extremely high relative permeability to oil.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)