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GCRocky Mountain Subthrust Exploration Using Pre-Stack Depth Migration*
By
Mary Sue Purcell1, Steven G. Siguaw2, and Steven G. Natali3
Search and Discovery Article #40118 (2004)
*Adapted from the Geophysical Corner column in AAPG Explorer, September, 2003, entitled “Where Should We Put the Drill Bit?,” and prepared by the authors. Appreciation is expressed to them, to R. Randy Ray, Chairman of the AAPG Geophysical Integration Committee, and to Larry Nation, AAPG Communications Director, for their support of this online version.
1Geophysicist, Samson Resources, Denver; CO ([email protected])
2Geophysicist, Cabot Oil & Gas, Denver; CO ([email protected])
3Exploration director, Williams Production RMT, Denver, CO ([email protected])
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., 2000).
uGeneral StatementuFigure captionsuExploration, Owl Creek thrustuSeismicuExploratory welluConclusionuReference
uGeneral StatementuFigure captionsuExploration, Owl Creek thrustuSeismicuExploratory welluConclusionuReference
uGeneral StatementuFigure captionsuExploration, Owl Creek thrustuSeismicuExploratory welluConclusionuReference
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Exploration along Owl Creek Thrust
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
Survey parameters are:
The
preliminary
Figure 2 shows a north-south traverse from
the time-processed
To further
evaluate the prospective structure, velocity analysis and depth
Figure 3 shows the pre-stack depth migration
of the same
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 (Figure 4).
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
The #31-2 Wyatt Draw wildcat was not a commercial discovery, despite finding a closed subthrust structure. Log analysis indicated marginal oil saturations and generally reduced porosities in both the Tensleep and Madison reservoirs.
Three-D seismic data, in combination with modern pre-stack depth migration techniques, can be used satisfactorily to 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.
Reference
Natali, S.G., R. Roux, P. Dea,
and F. Barrett, 2000, Cave Gulch
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