Figure Captions
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A significant and highly prospective portion of the underexplored
southern Uinta Basin belongs to the Ute Indian Tribe in the Hill Creek
Extension of the Tribe’s Uintah and Ouray Reservation (Figure
1). During 1999, the U.S Department of Energy solicited proposals
for projects designed to encourage energy development on Indian
reservations. Wind River Resources and the Ute Indian Tribe made a
successful grant proposal to the DOE and received cost-share funding for
a fifteen-square-mile 3D seismic survey on tribal surface and mineral
lands in the Hill Creek Extension. The survey area grew to include a
total of twenty-seven square miles, with most of the additional acreage
consisting of split-estate lands (tribal surface overlying either state
or federal minerals) that included the small Flat Rock oil and gas
field, which produced from two of ten prospective geologic formations.
Survey Design
The North Hill Creek 3D Seismic Survey was the first large-scale
exploration-oriented 3D survey conducted in the Uinta Basin. It was
designed to delineate the structure and stratigraphy of a portion of the
Hill Creek anticline. The Hill Creek anticline had been previously
identified in the work of John Osmond, Donald Stone, and others as a
subsidiary structure associated with the Garmesa wrench fault zone,
located a few miles to the north. This fault forms the north edge of the
northwest-plunging Uncompahgre uplift. Although the Hill Creek anticline
was noted in many publications and shown on several maps, its exact
location and nature were not well understood (Figure
2).
The North Hill Creek 3D survey was designed to image a geologic section
ranging from the Wasatch at a depth of 2,000 ft. to the Entrada at more
than 11,500 ft. (Figure 3). When the project
was conceived, it was believed that the most important objectives would
be shallow Wasatch Formation oil and gas and deeper gas potential in the
Dakota, Cedar Mountain, and Morrison formations. Secondary objectives
included stream-channel mapping in the Mesaverde Group, fracture
delineation in the thick Mancos Shale section, and mapping of structural
and stratigraphic traps in the eolian Entrada Sandstone. Drilling based
on the interpretation of the North Hill Creek 3D data has proven each of
these objectives to be gas-productive and identified additional deep
objectives that were not originally considered. The additional deep
objectives were sands in the Kayenta Formation and the eolian Wingate
Sandstone, neither of which had been productive elsewhere.
A mixed-source design was chosen due to extremely rough terrain in the
survey area, a 7,500-ft. elevation mesa bounded and dissected by
1,000-ft. deep canyons. The final survey consisted of 81% Vibroseis and
19% dynamite shot-hole sources. The source lines were oriented
northeast-southwest and spaced at 1,320 ft. with source points spaced at
220 ft. intervals. Receiver lines were oriented east-west and spaced 660
ft. apart with receiver stations every 220 ft. Sources were recorded
into a ten-line swath with each line containing 108 channels. These
parameters provided nominal 45-fold data in 110-ft. bins.
Surveying for the North Hill Creek 3D Seismic Survey began in September,
2000. WesternGeco started acquisition in late October and concluded work
on December 7, 2000. Prior to the start of the survey a zero-offset
vertical seismic profile (VSP) was run in an 11,600-ft.-cased hole
awaiting completion at a location within the survey area.
Initial processing was performed in late 2000 and early 2001 by
WesternGeco’s Denver Processing Center. The data volume was reprocessed
during the summer of 2002 by Precision Interactive Processing in Denver.
The presence of the highly reflective Horse Bench Sandstone member of
the Green River Formation at, or within a few feet of, the surface in
most of the survey area made it impossible to see the data on the field
records. The entire survey was shot trusting that the processors would
be able to make the data usable, and they did.
Initial data interpretation was greatly aided by the VSP data, as
calibration issues were easily overcome. Although a gross structural
interpretation was made very quickly, the original detailed
interpretation effort was concentrated on the shallow Wasatch Formation,
where both structural and stratigraphic traps were identified for
testing. This work was performed for Wind River Resources by Black Coral
LLC and Fall-Line Exploration. The data clearly imaged a
northwest-southeast trending asymmetric anticline with a faulted south
flank and a gently dipping north flank. This feature looks very much
like the one described by Osmond and Stone decades earlier. The fault
associated with the Hill Creek anticline is a deep-seated high-angle
reverse fault that displaces the Precambrian through Dakota formations.
The Wasatch and Mesaverde are also faulted, but by shallow faults
oblique to the deeper faulting. These faults are down-to-the-south
transtensional faults.
Prior to seeing the first 3D data it was generally believed that the
Flat Rock field was located atop the crest of the Hill Creek anticline.
The seismic data show that the field is actually located on the north
flank of the anticline.
Del-Rio Resources’ technical staff and consultants undertook the initial
deep interpretation effort, concentrating on their acreage in the
northern portion of the survey area. Subsequently, the Black Coral and
Fall-Line team performed a detailed deep interpretation of the entire
reprocessed data volume. Both groups of geophysicists saw deep features
that could best be described as sand dunes.
The first wells drilled to test seismic anomalies were Wasatch wells in
the 4,000 ft. depth range. One well was completed naturally for 1,600
thousand cubic feet of gas per day (Mcfgpd) from an obvious stream
channel deposit. The second well targeted a series of sands on an
apparent closed dome structure. Initially this well produced yellow oil,
but it was recompleted to a shallower gas interval which became wet
following hydraulic fracture stimulation.
The first two deep wells drilled on the basis of the North Hill Creek 3D
were Entrada wells drilled by Del-Rio Resources in the Flat Rock field.
These wells located more than two miles north of the crest of the
anticline confirmed that the sand-dune interpretation was correct.
Significantly, they produced pipeline-quality gas. Each well was
completed at approximately 2,500 Mcfgpd.
During the fall of 2002 Wind River Resources and a partner embarked on a
ten-well deep drilling program to explore the area along the crest of
the Hill Creek anticline. All of these wells penetrated the eolian
Wingate Sandstone, and Wingate completions were attempted in all but
two. The best Wingate well was completed at 7,100 Mcfgpd.
To date sixteen wells have been drilled and completed on the basis of
the interpreted 3D data, establishing production in eleven formations
ranging from the Wasatch at approximately 3,500 ft. to the Wingate at
more than 12,000 ft. Some wells have certainly been better than others,
but for the past three years several of these wells have consistently
been among the top gas producers in Utah.
Each formation imaged by the North Hill Creek 3D involves an interesting
story. This presentation is focused on the gas-bearing dune sands of the
Entrada and Wingate sandstones. These formations underlie a large area
in the southern Uinta Basin and were previously unrecognized as
important gas reservoirs.
Mapping of the Jurassic eolian Entrada Formation, which ranges in
thickness from 200 ft. to 250 ft. in this area, has revealed multiple
porous, gas-filled sand dunes separated by low-porosity and
low-permeability interdune deposits. The dunes appear to be stacked as a
result of dune migration in an area with a fluctuating water table. This
tended to preserve the dune bases of all but the topmost layer of dunes,
which appear to be fully formed. Barchan dunes, linear dunes, and large
star dunes are clearly visible and appear to be separated by barren
interdune mud flats.
A regression analysis of the seismic amplitudes correlated with
porosity-feet in the upper 70 ft. of the Entrada resulted in a
correlation coefficient of 0.8831, implying that seismic amplitudes can
be reliably used to map reservoir-quality sands. Production comes from
several intervals in the upper two-thirds of the Entrada section. The
best wells exhibit a well developed Curtis Moab Tongue immediately above
the Entrada. Although the Entrada is well known as a carbon dioxide and
low Btu gas producer in the San Arroyo area, at North Hill Creek and
Flat Rock it produces methane with a Btu value greater than 1,000.
It is likely that the Entrada is prospective over a large area in the
southern Uinta Basin. Gas quality will presumably become an issue
somewhere between North Hill Creek and San Arroyo, which is
approximately 25 miles to the east.
The Triassic Wingate Formation was deposited as apparent linear ridge
dunes. A 50-ft. thick sand near the top of the Wingate appears to be a
clean gas-charged reservoir in all of the wells producing from this
formation. Productive Wingate wells are located in relatively
high-amplitude areas. A regression analysis similar to that performed
for the Entrada resulted in a correlation coefficient of 0.9, again
implying that reservoir-quality sand limits and thickness can be mapped
using this tool. The best Wingate producer was drilled into a relatively
high-amplitude anomaly and established an initial production rate of
7,100 Mcfgpd.
The North Hill Creek 3D Seismic Survey established the usefulness of
this sort of survey as an exploration tool in the Uinta Basin. It was
successful in delineating the planned targets and initiating a new play
by illuminating new ones. To date more than 140 square miles of 3D
seismic data have been acquired by six companies, and more than 100
square miles of additional survey are planned.
Reference
Stone, D. S.,
1977, Tectonic history of the Uncompaghre Uplift, in H. K. Veal,
ed., Exploration frontiers of the central and southern Rockies: Rocky
Mountain Association of Geologists, p. 23-30.
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