Facies and Fracture Architecture of the Tensleep Sandstone, Bighorn Basin, Wyoming: Preiiminary Result of an Outcrop and Subsurface Study

Arya Aviantara

The Middle Pennsylvanian to Lower Permian Tensleep Sandstone has been the most prolific producer in the state of Wyoming. Now in its advanced stages of production, it is critical to accurately characterize the Tensleep facies and fracture architecture. Two outcrop locations have been selected: Aikali Creek has cross-strata dip parallel exposures at close spacings for eolian facies architecture; Ziesman Dome is an NW-SE trending asymmetric anticlinal closure with the SE axis having a marked dextral dislocation, and is selected for analysis of fracture architecture. Byron Field is also an anticlinal closure with NW-SE trend which is selected for location of subsurface study. Methods of study for outcrop includes high- precision electronic surveying, photomosaics, geologic m pping, and fracture spacing and orientation measurements. The subsurface study includes conventional wireline logs, cores, and porosity and permeability data. In addition, a microresistivity log was evaluated from the Lindsay#3H horizontal well.

Tensleep facies architecture is defined by a hierarchy of elements. Parasequences are made up of marine to marginal marine dolomitic sandstones and sandy dolostones (<10.^-2 to 10² md) in the lower 20-50%, and eolian dune, interdune and large-scale deformed sandstones in the upper part. Eolian dune sandstones, the primary reservoir (2 to 3 x 10²md), are characterized by very large-scale (up to 40 ft thick) tabular-planar cross strata with lower permeability wind ripple strata (1 to 2 x 10⁹ md, strongly anisotropic) concentrated above 1st- and 2nd-order bounding surfaces. Medium-scale trough cross stratified intrasets are common; however, their spatial dimensions and distribution pattern are uncertain at this time.

Fracture architecture is to include the geographic and stratigraphic distribution, orientation, density and laterallvertical continuity. Although data are limited at present, some trends have been identified. For example, at Ziesman Dome the dominant laacture set trends 76° azimuth on the northwest flank and 59° on the south flank. The south flank also contains high density fractures grouped in clusters at 500 ft spacing at the highest rate of fold axis plunge and dislocation. At the subsurface location, fracture sets identified include 312° azimuth spaced 4.3 ft and 59° azimuth spaced at 11.6 ft.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California