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The AAPG/Datapages Combined Publications Database

Williston Basin Symposium

Abstract

SKGS-AAPG

Eighth International Williston Basin Symposium: Core Workshop Volume, October 21, 1998 (SP13A)

Pages 67 - 88

Facies and reservoir characteristics of Silurian Interlake of the central and southwestern Williston Basin

Richard Inden, LSSI, Denver, Col..
Alan Byrnes, Kansas Geologic Survey, Lawrence, Ks.
Bob Cluff, The Discovery Group, Denver, Col.

ABSTRACT

Vertical variations in Interlake lithofacies and reservoir characteristics are dramatic and offer a significant challenge to assess for their hydrocarbon potential. The Interlake is one of the most enigmatic productive zones in the Williston Basin because of several factors: 1) lack of any distinctive log character that might distinguish productive from non-productive intervals, 2) its apparent homogeneous appearance in well cuttings ("white, finely crystalline dolomite"), 3) the characteristic apparent high water saturations in oil-productive intervals and 4) limited understanding of reservoir types, the distribution of salt-plugged porosity, and fracturing. As such, many feel it is a prime candidate for by-passed pay potential, especially off of the Nesson Anticline where it is only rarely productive now.

The Silurian Upper Interlake Subgroup (Porter and Fuller, 1959; LoBue, 1983; Magathan, 1987) of the central Williston Basin is made up of a monotonous sequence of thin shallowing upward cycles. On the Nesson Anticline and in surrounding areas (Fig. 1) the interval has a characteristic and ubiquitous "spikey" nature on neutron, density, sonic, and resistivity log curves over most of its thickness (Fig. 2). This character is due to the stacking of light colored, microcrystalline dolomite mudstones of subtidal origin which grade upward into light to dark gray fenestral algal boundstones and paleosols of tidal flat origin. These fenestral algal boundstones retain excellent reservoir qualities and contain an abundance of horizontal, solution enhanced desiccation fractures, that, along with the solution-enlarged irregular fenestrae (meso - macrovugs and channels), produce an effective pore system. Dolomite mudstones, on the other hand, are ubiquitously saturated with water because they are composed of very finely crystalline dolomite with extremely small pore throats and resultant high pore-entry capillary pressures.

Porosity in reservoir rocks ranges from 5 to 24 %. This range overlaps the observed porosities for non-reservoir lithologies. Reservoir rocks, therefore, cannot be clearly distinguished from non-reservoir lithologies on the basis of porosity alone. Because the Upper Interlake has these characteristics, it is difficult to make reliable completion or recompletion decisions without more definitive rock, petrophysical, and engineering data sets.

Reservoirs in the Lower Interlake and Putnam Zone in the southwest portion of the Williston Basin include oolitic-pellet dolomite grainstone, fossil-pellet grainstone, and a wide spectrum of reef-related, fossil-coral dolomite packstones, and coral-stromatoporoid rudstone/boundstone. Each of these potential reservoirs has a unique pore system and thus, a different set of petrophysical properties that define their reservoir characteristics. As in the Upper Interlake, this sequence is contains an abundance of dolomite lithologies composed of very finely crystalline dolomite with small pore throats.

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