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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 3-D survey (Figure 1). 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:
The preliminary 3-D interpretations, based on conventional time-processed data, indicated the presence of numerous anticlinal folds in subthrust 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, Precambrian 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 subthrust Chugwater velocities can be as low as 12,000 feet per
second. This significant lateral
To further
evaluate the prospective structure,
Figure 3 shows the pre-stack depth
Based on
the results of the pre-stack depth 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 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
ReferenceNatali, S.G., R. Roux, P. Dea, and F. Barrett, 2000, Cave Gulch 3-D survey, Wind River basin, Wyoming, in R.R. Ray and M.W. Longman, eds., 3-D seismic success stories in the Rocky Mountain region: The Mountain Geologist, v. 37, p. 3-13.
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