Nugget Formation Reservoir Characteristics Affecting Production in the Overthrust Belt of Southwestern Wyoming
Authors: Lindquist Sandra J. 
Published: December 2007

Lindquist, Sandra J., Amoco Production Co.

The Jurassic/Triassic Age Nugget sandstone of the south western Wyoming over thrust belt is a texturally heterogeneous reservoir with anisotropic properties that have been inherited primarily from the depositional environment but also have been modified by diagenesis and overprinted by tectonism. Predominantly eolian processes deposited cross bedded, low-angle to horizontally bedded and rippled, very- fine- to coarse-grained sand in dunes, interdune areas, and associated environments. Original reservoir quality has been somewhat modified by compaction, cementation, dissolution, clay mineralization, and the precipitation or emplacement of hydrocarbon asphaltenes or residues. Low-permeability gouge- and carbonate-filled fractures potentially restrict hydrocarbon distribution and negatively affect producibility whereas discontinuous open fractures enhance permeability in some intervals. Contrast in air permeability between dune and inter dune deposits ranges over four to five orders of magnitude. Dune and inter dune intervals are correlatable locally with the aid of core log, conventional log, and stratigraphic dipmeter data. Stratigraphic correlations then can be utilized to model the lateral and vertical extent of directional properties in the reservoir.

Introduction

A large proportion of the sizable hydrocarbon reserves in the Utah/Wyoming over thrust belt are within the Jurassic/Triassic Age Nugget formation. Upper Nugget data were obtained from three fields in Uinta County, southwest Wyoming-Clear Creek, East Painter, and Glass cock Hollow-all of which are closed anticlinal structures on the upper plate of the Absaroka thrust (Fig. 1). Nugget pay fluids are probable retrograde gas condensates and volatile oils. In the producing trend, the Nugget sandstone is primarily a clastic eolian deposit ranging (north to primarily a clastic eolian deposit ranging (north to south) from 800 to greater than 1,000 ft (250 to greater than 300 m) in thickness. It probably is conformable with the underlying Triassic Ankareh formation and certainly unconformable with the overlying Jurassic Twin Creek formation. Lithology is predominantly sandstone, with some siltstone. Siltier lithologies are more abundant toward the base of the formation, concurrent with probable increased influence of moisture, water tables, and/or water processes in the depositional environment. Many eolian sandstone reservoirs mistakenly have been considered homogeneous from a reservoir performance standpoint because the importance of subtle, facies-related performance standpoint because the importance of subtle, facies-related controls on reservoir properties within these thick sandstone sequences has been overlooked. Textural heterogeneity results from bedding variations in grain size and sorting, which are directly related to the geometry of eolian facies. Within the Nugget formation, these primary depositional heterogeneities are amplified by diagenesis and further modified by tectonic stresses that folded and faulted the region.

JPT

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