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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 thrustuSeismic
uExploratory
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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 velocity variation severely distorts the time-migrated image below the thrust fault.
To further
evaluate the prospective structure, velocity 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 subthrust nose lacking north dip, the Wyatt Draw prospect now as mapped has four-way closure. This closure also was evident on the post-stack depth migration, although the image was of lower quality than the pre-stack depth volume.
Based on
the results of the pre-stack depth migration, a wildcat
The
A dipmeter
obtained in the
The #31-2
Wyatt Draw wildcat was not a commercial discovery, despite finding a
closed subthrust structure.
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.
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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