of 3-D seismic data with several types of seismic attributes can reveal
geologic factors that control the location of productive algal mound reservoirs
in the Paradox Basin.
Routine seismic mapping of the producing interval did not reveal the presence
of a regional strike-slip fault that is clearly shown using attribute
analysis. This previously unknown strike-slip fault controls the local
stratigraphy and extends for over 30 miles.
Although each attribute when used alone has some level of ambiguity, it
is important to note that when a number of attributes with different mathematical
algorithms yield similar results, the reliability of the geologic interpretation
Several commercially available seismic attributes, along with attributes
generally incorporated within most workstations, collectively have had
a significant impact on the understanding of how and where the algal mound
reservoirs form, and indicate that their stratigraphic development is
often not a random act. The purpose of this article is to demonstrate
that the small cost of time and money required to perform attribute analysis
is far outweighed by the increased understanding of the geologic dynamics
of an area.
The ability to map more geologic detail will ultimately result in reduced
risk of drilling dry holes.
Miller Energy, of Kalamazoo, Mich., and its partners completed the Miller
Horse Canyon 1-10 in 1998 for an initial potential of 960 barrels of oil
per day and 940 MCFG/D from a depth of 5,850 feet from an algal mound
reservoir. The well was featured in the 1999 EXPLORER.
Figure 1 shows the location
of the Paradox Basin in the southwestern United States, where shelf carbonate
buildups within the Pennsylvanian basin have produced oil and gas since
Generally these carbonate buildups are the result of algal debris from
Ivanovia (these Ivanovia algal skeletons are much the same size as cereal
corn flakes). In the 1980s it became generally known that there could
be a seismic expression resulting from these carbonate buildups, but the
2-D seismic data that existed then added little to the understanding of
the genesis of these features.
Figure 2, a geologic cross
section through the discovery well, is a good reference for the various
geologic strata that will be mentioned throughout this article. The well
was completed in an Upper Ismay carbonate buildup; however, the test also
encountered a deeper Desert Creek carbonate buildup that is further revealed
with attribute analysis.
A seismic tuning effect owing to thickness changes in the Hovenweep shale
creates a mappable high amplitude that indicates where the shale is extra
thick, as shown in
Figure 3. This thick resulted
in a positive “island” that existed during the time of deposition of the
Upper Ismay. The island is outlined in black and will be shown on subsequent
Note in Figure 2 how the
Hovenweep thick is located to the left of the discovery well.
The isochron thickness map of the Upper Ismay (from the top of the Hovenweep
to the top of the Upper Ismay) shown in Figure
4 demonstrates that there is an atoll shape of increased thickness
of Upper Ismay surrounding the Hovenweep island.
A series of different seismic attribute displays are shown in Figures
5 through 10 (5, 6,
which reveals a linear fault zone that affected the formation of the Hovenweep
thick and thus the locations for mound development. The attribute analyses
include several types of methods, including multivariate analysis, edge
detection, spectral decomposition, wavelet cross-correlation and wavelet
amplitude map of the Desert Creek horizon demonstrates a distinctive NW-SE
trend (Figure 5). Originally
it was believed that this NW-SE trending amplitude anomaly was related
only to the Desert Creek carbonate buildups.
An attribute process that could be characterized as “wavelet classification”
was run on the shallower Upper Ismay seismic horizon - and again, the
NW-SE trend was observed. This wavelet classification technique divides
the horizon wiggle into 12 characteristic wavelets, and then makes a color
map indicating areas of similar wavelets. Small changes in the wavelet
character often reveal significant geologic features like porosity and
thickness changes (Figure 6).
Field analyses from other fields on trend with the fault demonstrate that
the “atoll model” is not unique to this discovery.
Another attribute, which can be classified as multivariate attribute analysis,
combines the properties of autocorrelation, amplitude and phase of seismic
traces. The output from the multivariate attribute analysis of the Upper
Ismay is shown in Figure 7,
and also demonstrates this same strong NW-SE trend.
It is important to note that both the wavelet classification map and the
multivariate attribute analysis have entirely different mathematical algorithms,
and yet both show the same NW-SE lineament. One therefore has to give
more credibility to the lineament being the result of a geologic phenomena
rather than some “black box” artifact.
The multivariate attribute analysis creates a cube output that allows
one to evaluate a lineament (fault?) spatially throughout a seismic data
Edge detection technology has been used in the seismic industry for several
years, and is designed to compare each trace of the data with its neighbors
in order to map dissimilarities. Figure
8 (previous page) is the edge detection horizon slice of the Upper
Ismay horizon. A fault was interpreted through the linear discontinuity
on the horizon slice. This package also has the advantage that it creates
a cube of data output that can be loaded on the workstation to be interpreted
at different time or horizon slices.
Figure 9 is another coherency
slice that is approximately 1,500 feet below the Upper Ismay, and again
this same lineament can be mapped. Two-D seismic has been shot in the
area for decades, and this very small-offset fault (or perhaps zone of
weakness) has not been observed. In fact it was not even observed on the
3-D data until output was displayed and interpreted using the various
Figure 10 is an amplitude
map of the top of the Akah salt, and it is believed that the dim shown
in green and red is the result of salt dissolution.
Note how the amplitude dim conforms with the overlying Hovenweep thick.
We postulate that the fault may have been a conduit to cause dissolution
of the Akah salt. A horizon slice slightly above the Akah salt has a very
distinctive linear pattern that also could be the result of differential
As another attribute comparison, spectral decomposition is a very precise
frequency measurement tool that can also be viewed as a wavelet measurement
tool. Figure 11 is the result
of spectral decomposition over the Upper Ismay horizon, and again one
can see the fault, Hovenweep “island,” and the flanking atolls.
Many workstations have add-on packages that allow some trace processing.
A horizon windowed seismic wiggle from the area of the producing well
was cross-correlated with the entire 3-D volume, and those areas that
had similar correlation values were mapped on the workstation to produce
Note the similarity between Figures
12 and 6 . Both the
cross-correlation and wiggle classification mapping techniques indicate
strong potential for new drill site locations on the south side of the
The enhanced geologic understanding gained from the various attribute
analyses far outweighs the small costs and interpretation time required
The similar patterns that emerge by running many different techniques
reinforces the interpreter’s confidence when mapping subtle geologic variations.
This improved geologic understanding leads to reduced risk in both development
and exploration projects.
Often seismic interpreters are discouraged from mapping anything but the
primary producing zone. Our experience on this stratigraphic play shows
that attribute analysis of the section surrounding the producing reservoir
leads to a better understanding of the porosity development in the productive
zones, as well as showing how to project the extension of the trend for