3-D Reservoir Characterization of the House Creek Oil Field, Powder River Basin, Wyoming, V1.00

The Upper Cretaceous Sussex "B" sandstone was deposited as a probable
transgressive-marine sand-ridge complex in a mid-shelf position. The "B"
sandstone is bounded by upper and basal disconformities and encased in
mudstones and low-porosity and low-permeability sandstones of the Cody Shale.
Reservoir characteristics are controlled primarily by depositional and
diagenetic heterogeneity at megascopic (field), macroscopic (well), and
microscopic (rock sample) levels. To simplify, this means production of oil is
controlled by stacking and interbedding of sandstone and mudstone beds and by
geochemical changes through time that affect flow of fluids through the rock.

More than 24.8 million barrels of oil (MMBO) have been produced from the Sussex
"B" sandstone in the House Creek field, Powder River Basin, Wyoming. Greatest
oil production, porosity, and permeability, the thickest reservoir sandstone
intervals, and best lateral continuity of the primary reservoir facies are all
located parallel and proximal to field axes. Decrease in reservoir quality west
of the axes is due to greater heterogeneity from interbedding of low- and
moderate-depositional-energy facies, with associated drop in porosity and
permeability. Decrease in production east of the axes results primarily from a
combination of seaward thinning of the primary reservoir facies and
non-deposition of sand ridges.

The House Creek field has two axis orientations; these are related to
depositional patterns of the four sand ridges. Deposition of the "B" sandstone
began in the southeastern corner of the field with sand ridge 1; axis
orientation is about north 20 degrees west. Later-deposited sand ridges 2
through 4 are located west and north of sand ridge 1; their axis orientations
are approximately north 32 degrees west. Progressive northward deposition of
later sand ridges is probably concurrent with uplift of the northeast-trending
Belle Fourche arch. Movement along the arch and of lineaments may have caused
topographic highs that localized Sussex and Shannon deposition proximal to the
arch.

[Summary provided by the USGS.]