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,
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
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.
have told me they have encountered a similar problem, as illustrated by
the 3-D seismic program over Crooks Gap Anticline in Fremont County, Wyo.
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 seismic
In 1994, a 27-square-mile
seismic to drilling option survey was conducted to further define the
Crooks Gap structure for possible additional drillsite locations and to
check for other structural leads.
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 seismic interpretation
show up with the seismic cross section (Figure
3b), which follows this same line of wells. It became obvious that
erroneous seismic events were continuing updip beyond the known crest
of the structure for Dakota and deeper formations. Also, the structural
axes for all of the seismic horizons were stacked up vertically, oblivious
to the inclined axial plane.
Of particular interest
is the strong southwest continuation of the deep Phosphoria-Tensleep (Pt)
seismic events. In two wells, which penetrated the Paleozoics at Crooks
Gap, E-log correlations indicate overturned beds in the Phosphoria, contrary
to the flat seismic reflections.
Although the farmees were aware of the geologic discrepancies, they decided
the seismic data were too strong to the southwest to be ignored. They
programmed the #25 well to penetrate both the Dakota and Nugget reservoirs
in a structural position high to the #6 well (Figure
4b, dashed orange and yellow lines).
Formations came in as expected to the top of the Mowry, followed by a
"wadded up" Mowry Shale, and the hole ended up on the steep southwest
flank of the structure (Figure 4a), missing
the crest at the Dakota level.
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
With a prior structural
analysis from existing well data, this failure might have been avoided.
The postulated antithetic thrust was large enough that it should have
been recognized in nearby wells, particularly the #25 hole; however, evidence
of a fault is lacking.
On the seismic section,
this fault appears to die out too rapidly upward and does not displace
or even fold the Frontier seismic events.
A third operator
drilled yet another dry hole at the north end of the anticline based on
similar migrated seismic data, believing they could get higher than the
previous wells. Instead, the hole ended up structurally low, in 60 degree
southwest dips, much like the other examples.
A review of previous
hole deviations in this area could have ruled out this third attempt,
since wellbore deviation plots indicate these earlier wells were on the
structural axis at Dakota level.
Now one might ask,
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 data properly. Prestack depth migration,
the next logical step, was not conducted at the time because of the difficulty
associated with building an accurate velocity model.
At this point a structural
geologist should have been involved, using perhaps as much art as science
-- and an occasional "educated guess."
studies, utilizing all possible data such as field mapping, air photos
and well control, should be done prior to the seismic survey. Structure
cross sections should be constructed to compare to the seismic sections
(using the same horizontal scale) to determine where and why they might
differ. One should honor the dipmeters as they are usually more accurate
in the immediate area than the seismic.
data, one can use directional surveys or even non-directional hole deviation
surveys, in locating a structure's crest.
when an exploration company is gathering seismic in a complex structural
area, their geophysicist should also be an experienced structural geologist,
or the structural geologist should be a competent geophysicist.
Since most of us
lack dual expertise, it is essential the geophysicist and structural geologist
work together to avoid incorrect interpretation and costly mistakes.