In the Rockies we are all looking for a
hidden structure that will discover a new big gas field. In 1994,
Barrett Resources (now Williams Production Company) made a significant
Rocky Mountain gas discovery at Cave Gulch Field, with reserves
in excess of 600 BCF. The field, located in the eastern half of
Wyoming’s Wind River Basin, produces from multiple stacked sandstone
plays.
Following the initial discovery well, Barrett acquired
a 3-D seismic survey that proved essential in determining the proper
structural placement of subsequent development wells. Due to strong
lateral velocity variations associated with the Owl Creek Thrust,
the use of post-stack depth migration was mandatory to resolve the
correct subsurface structure. (Natali et. al., January 2000, "The
Mountain Geologist").
Building on its success along the Owl Creek Thrust,
Williams partnered with Cabot Oil and Gas and caused them to acquire
in 2001 the 118.5-square-mile Boone Dome 3-D survey. The center
of the survey is located approximately 15 miles southeast from the
Cave Gulch 3-D survey along the Owl Creek Thrust trend.
Survey parameters are:
- Fold: 36 nominal.
- CDP Bins: 110 feet x 110 feet.
- Source: 8 Vibroseis trucks.
- Recording patch size: 9,240 feet x 35,420
feet.
The preliminary 3-D interpretations, based on conventional
time-processed data, indicated the presence of numerous anticlinal
folds in sub-thrust positions throughout the survey. One such area
was the Wyatt Draw prospect, a three-way time closure positioned
along the Clark Ranch thrust, east of the main Owl Creek fault.
Figure 2 shows a north-south
traverse from the time processed 3-D survey. Along the thrust, Pre-Cambrian
granite is juxtaposed against shales of the Triassic Chugwater formation.
Granite interval velocities in this area are as high as 19,000 feet
per second, while sub-thrust Chugwater velocities can be as low
as 12,000 feet per second.
This significant lateral velocity variation severely
distorts the time-migrated image below the thrust fault.
To further evaluate the prospective structure, velocity
analysis and depth processing of the seismic data was needed. The
decision was made to utilize both pre-stack and post-stack depth
migration techniques. Once the velocity field is properly accounted
for, an accurate depth image of the subsurface can be achieved.
Figure 3 shows the pre-stack
depth migration of the same 3-D traverse as in figure
2.
There is a notable difference between the two sections,
especially along the footwall portion of the Clark Ranch Thrust.
By properly accounting for the high velocities associated with the
granite and the lower Paleozoic rocks in the hanging wall, the structure
is now correctly imaged. Instead of having a sub-thrust nose lacking
north dip, the Wyatt Draw prospect now has mapped four-way closure.
This closure also was evident on the post-stack depth
migration, although the image was of lesser quality than the pre-stack
depth volume.
Based on the results of the pre-stack depth migration,
a wildcat well was drilled by Cabot and Williams during the summer
of 2002. The Williams #31-2 Wyatt Draw well, Section 2, T35N, R85W,
Natrona Country, Wyo., was located to take advantage of the footwall
structural crest for both the Tensleep sandstone and Madison carbonate
formations. Both are prolific producers in other parts of the Wind
River Basin, as well as other basins in the Rockies.
The well was deviated due to unfavorable surface
conditions above the preferred target at depth, and drilling plans
involved minimizing the amount of granite that needed to be drilled
in the hanging wall.
While drilling, formation tops came in very close
to prognosis for both the hanging wall and footwall; these predictions
were based on the pre-stack depth seismic volume combined with area
well control.
A dipmeter obtained in the well indicated the presence
of critical north dip below the main thrust, and also that the bottom
hole location was near the top of the structure. A velocity check
shot survey was obtained and used to create a final pre-stack depth
migrated image. The crestal position of the structure moved very
little and the final depth calibration was changed by only 15 feet,
confirming the quality of the original depth processing.
The #31-2 Wyatt Draw wildcat was not a commercial
discovery, despite finding a closed sub-thrust structure. Log analysis
indicated marginal oil saturations and generally reduced porosities
in both the Tensleep and Madison reservoirs.
In conclusion, 3-D seismic data, in combination with
modern pre-stack depth migration techniques, can be used to satisfactorily
image complexly thrusted structures, and can place a drill bit in
a favorable structural location with a relatively high degree of
certainty.
Three-D seismic depth imaging opens the door for
future exploration along basin bounding faults in other Rockies
basins.