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GCCrooks
Gap Field, Wyoming: Limitations of 3-D
Seismic
Interpretation![Next Hit](/images/arrow_right.gif)
By
Robert E. Wellborn1
Search and Discovery Article #40065 (2002)
*Adapted for online presentation from the Geophysical Corner
column in AAPG Explorer November, 2000, entitled “Finding Limits of 3-D
Seismic
,” and prepared by the author. Appreciation is expressed to the
author, 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.
1Chief geologist, Wold Oil Properties, Casper, Wyoming
Some of you may recall the early 1950s, when the reflection seismograph was making great advances. Some advisors said conventional petroleum geology exploration was being replaced and it might be wise to change college majors to some other field, perhaps geophysics. At the same time, I recall graduate school at Berkeley, with professor C.M. Gilbert musing that “geology is an art, not a science,” referring to the importance of an educated guess. The talk about being replaced was flat-out wrong, but Gilbert's idea still remains true.
Admittedly
the giant strides in the science of seismology, including 3-D and 4-D seismic
,
coherency cubes, complex processing parameters and the like, are mind boggling
to many of us. It might be easy for some individuals to succumb to the final
seismic
product as being “gospel” -- and perhaps being able to replace
old-fashioned geology. However, there are circumstances where the
seismic
can
begin to fail us, particularly in steep-dip areas where reliable reflection
migration becomes more difficult. And even worse than recording “no-
data
” in
these steep-dip areas, we sometimes see erroneous events “sneaking in,” which
can lead to an incorrect structural
interpretation
.
Various
geophysicists have told me they have encountered a similar problem, as
illustrated by the 3-D seismic
program over Crooks Gap Anticline in Fremont
County, Wyoming.
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Click here for sequence of Figure 3a and Figure 3b.
Click here for sequence of Figure 4a and Figure 4b.
Click here for sequence of Figure 5a and Figure 5b.
Crooks Gap Field is in the northeast corner of the Great Divide
Basin (Figure
1), in a structurally complex area thought to be suited to 3-D
As a result of this survey (Figure 2), three different operators with their own geophysical departments drilled three unsuccessful Crooks Gap option wells. All drilled close to prognosis to the Mowry Shale, which then became anomalously thick due to a fault repeated and overturned section caused by multiple bedding plane detachments. Also, all of the beds on the southwest flank rapidly roll over into very steep to vertical dips. Figure 3a is a geologic cross section through some earlier wells across Crooks Gap Field, and defines an anticlinal axial plane dipping at 18 degrees from vertical. Significantly, the #12 well has dipmeter dips changing from the northeast to the southwest within the Frontier Formation.
The first indications of problems with the 3-D
Of particular interest is the strong southwest continuation of the
deep Phosphoria-Tensleep (Pt)
Although the farmees were aware of the geologic discrepancies, they
decided the Undaunted by the previous failure, another company took a farmout to drill a Nugget test just 800 feet southeast of the #25 dry hole. With velocity problems seemingly resolved, the #22 location was chosen on what was thought to be an antithetic forelimb detachment thrust (Figure 5b). Both the Dakota and Nugget formations were interpreted to be unusually high (dashed orange and yellow lines). Unfortunately, this attempt was even farther off-structure, and drilled near-vertical Mowry Shale for nearly 1,000 feet (Figure 5a).
With a prior structural analysis from existing well
A third operator drilled yet another dry hole at the north end of
the anticline based on similar migrated
Now one might ask, "what happened?" I was advised by one
geophysicist that the problem lies in the complex velocities associated
with the thrust faults and in the migration of extreme dips beyond the
ability of the software to handle the
At this point a structural geologist should have been involved,
using perhaps as much art as science -- and an occasional “educated
guess.” Detailed structural studies, utilizing all possible
Most importantly, when an exploration company is gathering |