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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 69 (1985)

Issue: 11. (November)

First Page: 2045

Last Page: 2045

Title: Practical Characterization of Eolian Reservoirs for Development: Nugget Sandstone, Utah-Wyoming Thrust Belt: ABSTRACT

Author(s): Sandra J. Lindquist

Article Type: Meeting abstract

Abstract:

The Jurassic-Triassic Nugget Sandstone of the Utah-Wyoming thrust belt is a texturally heterogeneous eolian reservoir with anisotropic directional properties that have been inherited primarily from the depositional environment, but also somewhat modified by diagenesis and overprinted by tectonism. Porosity ranges from several percent to the mid-twenties, and maximum horizontal permeability covers five orders of magnitude from a darcy to hundredths of millidarcys. Where productive, the formation ranges from approximately 800 to 1,050 ft (244 to 320 m) thick, at depths of about 7,500 to 15,000 ft (2,286 to 4,572 m) below the surface. Because some reservoirs are fully charged with hydrocarbons, an understanding of their internal complexity is important to monitor production erformance and maximize hydrocarbon recovery.

Depositional processes determine the three-dimensional distribution of facies-related bedding types, which can have unique grain size and sorting textures, primary and diagenetic mineral compositions, and tectonic deformational fabrics, resulting in characteristic porosity, permeability, permeability directionality, and pore geometry attributes. Such characteristics can be quantified or calculated from core analysis, capillary pressure, nuclear magnetic resonance, conventional log, dipmeter, and production data to generate a practical geologic model on whatever scale of detail is necessary.

Nugget dune deposits (good reservoir facies) consist of cross-bedded grain-flow and grain-fall strata and generally lower angle, wind-ripple-generated laminae. Interdune, sand-sheet, and other nondune deposits (poor reservoir facies) are characterized by wind ripples and more irregular bedding types, some of which are commonly associated with the presence of damp or wet conditions (e.g., bioturbation, wavy bedding, and soft sediment deformation). Dune grain-flow laminae exhibit the best reservoir quality and the least heterogeneity in bedding texture, followed respectively by grain-fall and wind-ripple strata. Anisotropy in permeability directionality resulting from bedding can range over several orders of magnitude. Irregular bedding types, especially those associated with wetter con itions, commonly contain an abundance of the finest grained material available in the depositional environment and generally have the poorest reservoir properties.

Postdepositional modification to the reservoir include diagenesis and tectonism. Although compaction, cementation, clay mineralization, and dissolution events have occurred, diagenesis has not ultimately altered the relative hierarchy of bedding-controlled reservoir quality determined by depositional texture. Tectonic deformation has primarily created networks of low-permeability gouge-filled fractures and microfaults, although intermittently open fractures with somewhat more effective permeability also exist locally.

Depositional models incorporating dune morphologies, facies distribution, permeability directionality, and theoretical concepts regarding dune migration through time are useful in defining the correlative intervals that are most likely to have continuity and potential communication of reservoir properties. Well-devised stratigraphic models can be adapted for engineering reservoir simulation studies and can also be used in solving structural resolution problems if a correlatable vertical sequence or a relatively consistent cross-bed orientation are known.

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Copyright 1997 American Association of Petroleum Geologists