Selecting Seismic Attributes and Enhancing Their Mapability Through Proper
Selection of Display Parameters, A Study Conducted on Sooner Field, CO
ABSTRACT
DOE and Diversified Operating Corporation acquired 3-dimensional seismic data over
Sooner Field, a combination structural and stratagraphic pool in the D-sand of Weld County, Colorado.
Evaluation of these public domain data through the means of an isochron map of the D-sand, mapped at
resolution estimated at one order of magnitude below the seismic sample rate, as supported by positive
correlation with a known isopach of the valley fill sand, and a mapped instantaneous frequency response of
the D-sand, demonstrate that the thicker portion of the sand body can be mapped indirectly through the
isochron and instantaneous frequency seismic attributes. The selection of proper display attributes in order
to view and present the data to maximum effect is also discussed.
PURPOSE
Having decided to re-enter the geophysical profession after a significant hiatus, this interpreter undertook a
self re-education program to become familiar with the new tools for interpreting 3-dimensional seismic
data. Consultation with friends still active and research on the web brought me to Seismic MicroTechnology, Inc. and their Kingdom Suite™ of 3-D interpretation software. Upon contacting the company,
they graciously provided access to Kingdom Suite v. 7.6 and Kingdom Suite v. 8.0 (Beta) for 30 days of
evaluation, testing and education.
INTRODUCTION
The geology of the Sooner Unit has been characterized as “The Sooner Unit area encloses approximately
1440 acres (Fig. 1) and produces 40° API oil from the lower-most Upper Cretaceous D Sandstone. The
sandstone reservoir was deposited in a fluvial and estuarine setting with the majority of clastic sediments
being deposited in an erosional valley as sea level rose.”, and “The first productive D Sandstone oil well
within the confines of the current Sooner Unit boundary (NWSE section 28, T.8N., R58W.) was completed
in December 1985.” (Cannon, 1998)
The Vibroseis™ seismic data were acquired in 1993 by Diversified Operating Corporation and the
Department of Energy (DOE), and were provided with the evaluation copy of Seismic Micro-Technology’s
Kingdom Suite™ software.
As these data are assumed to be in the public domain and presumably have been analyzed by innumerable
geoscientists, this interpreter does not expect to present any “new” findings. Precisely because this was a
learning exercise, this interpreter chose to learn by discovery and not by mimicking what has already been
done. Therefore, no search of the literature was made until most of this work was completed.
RESOLUTION
Much has been published regarding the topic of “resolution.” It is well known that the seismic tool is
limited in vertical resolution by the bandwidth of the seismic wavelet, requiring both high and low
frequencies. The “velocity of the rocks” from which the reflections come directly affects the wave length
and therefore the resolution. Furthermore, the layering of the reflecting horizons introduces constructive
and destructive interference, which can produce “tuning” and effect resolution.
It is also well known that there is lateral resolution of the seismic tool, described as the Fresnel zone, the
diameter of which varies in proportion to wavelength. Because longer wavelengths reflect from a broader
surface area at one instant of time than do shorter wavelengths, there is a resulting in loss of lateral
resolution at lower frequencies.
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William C. Overman M.S. – Geophysical Interpreter
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Figure 1: Sooner Field, a D-Sand oil pool of the D-J Basin, located in Weld County, Colorado.
Presented are
the locations of wells with Gamma Ray logs available to the author. Wells are identified by their API number, which
can be completed with the prefix “05-123-”. The 3-D seismic grid crosslines are oriented E-W. Also presented are the
Sooner Field unit boundary, the outline of the D-sand production and the D-Sand isopach, contoured from well control
and which clearly suggests the shape of the interpreted valley fill sand. The major isopach contours are 22.5, 35.0 and
47.5 feet, thickening toward the central maximum, while the minor C.I. is 2.50 ft.
A third arena of resolution is not related to rocks or sound waves, but to our ability to perceive and
integrate the presented data. In addition to examining various seismic attributes, much of this presentation
will touch on the selection of display attributes such that the user can easily perceive what the analysis has
discovered.
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William C. Overman M.S. – Geophysical Interpreter
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When establishing display parameters, one objective is to select the upper and lower display values such
that they are limited to the data actually being presented. In this way, the dynamic range available from the
display medium is not squandered mapping to values which will never be presented, thus preserving the
best visual contrast available. An extension of this is to further restrict the values presented such that only
the features of interest are displayed and all other values, even though present on the map or seismic
section, are visually “discarded” having been mapped to some constant default background color. A third
tool is to rotate the color bar in such a way that two highly contrasting colors are presented adjacent to one
another for the purpose of highlighting a specific feature of interest with that contrast.
ATTRIBUTES
The reflection amplitude from a horizon is a well known seismic attribute. “Bright Spots,” “Dim Spots,”
and AVO are all attempts to relate the strength of returned energy to the reflecting lithology and pore
fluids. Figure 2 maps the amplitude of the reflection from the top of the D-Sand. Clearly there is
correlation of strongly reflected energy with the isopach of the known sand body in sections 21 and 28.
However, the broad distribution of strongly reflected energy does not discriminate between thick sand
accumulation and the regional sand. So the Amplitude attribute does not present itself as a useful, stand
alone measure by which to map the thicker Sooner Field sand body.
Another attribute of seismic reflection amplitude is the Peak to Trough Ratio. This is a measure of the
amplitude ratio between consecutive peaks and troughs. Assuming that either the peak or trough adjacent to
the horizon of interest has a consistent reflection strength, and can thus become a “standard of
comparison,” this attribute would highlight lateral variations in the other.
The Peak to Trough Ratio, as calculated at the top of the D-sand, is presented in Figure 3. This interpreter
would suggest that this attribute (green/red/yellow) generally correlates with the known valley fill sand in
parts of sections 21 and 28. However, it does not correlate to the known valley fill in the north half of
section 21 and also presents many questionable indications outside of the valley fill sand.
FIGURE 2: Seismic Amplitude Attribute
from the top of the D-Sand, with the D-Sand Isopach
superimposed. The color bar has been rotated to emphasis
correlation (green/red/yellow) with the known sand body.
Figure 3: D-sand Peak to Trough Seismic
Attribute. D-Sand Isopach contour map, as calculated
by SMT and provided with the training data set, is
superimposed. The color bar has been rotated to
emphasize correlation (green/red/yellow) with the known
sand body.
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Furthermore, this interpreter would anticipate that a seismic attribute mapping a sand body would reflect
the geometry of that sand body. By that criteria, neither of the geometries presented by these seismic
attributes (Fig. 2 and Fig. 3) is very satisfying.
This Instantaneous Frequency Spectrum (Fig. 4) is of Crossline 82, an E-W line across the heart of the
Sooner Field sandbar (Fig. 1). The color bar is set such that any frequency 35 Hertz or lower will display
in cyan and any frequency 85 Hertz or greater will display in white. The section is flattened to 1.460
seconds, on the D-sand top, as identified in the SMT training set, and as selected by the auto-picker. The
black wiggle traces are the seismic amplitude and show that the D-sand top event is a peak, identified by
the horizontal black line. The D-sand base is also identified as the thicker red line between 1.467 and 1.469
seconds. Comparing the separation between the black and red picks, a subtle isochron thickening of the Dsand interval is suggested, between inlines 82 and 114.
Figure 4: Instantaneous Frequency Spectrum of Crossline 82, between 35 and 85 Hertz, from 1.409
to 1.500 sec., with the seismic amplitude super imposed as wiggle traces. Using this color bar, any value less than or
equal to 35 Hertz, displayed in cyan, and any value greater than or equal to 85 Hertz, displayed in white.
While the isochron separation of the two picked horizons is subtle to the point of being dismissed, the
downward deflection of the yellow/orange/red colored frequency band of the D-Sand base, and it’s
separation from the green/cyan colored frequency band of the D-Sand top is unmistakable. Taner and
Sheriff observed, “Lateral changes in the color pattern [of the instantaneous frequency] indicate that
something has changed about the reflection. Such changes focus the interpreter’s attention on the places
where the change occurs even though the nature of the change might not be indicated.” (Taner and Sheriff,
1977). This exaggerated separation between the colored bands is attributed to constructive and destructive
interference, or tuning effects, associated with the very small variation in isochron thickness. The observed
change might be caused by any combination of the isopach thickening of the D-sand, the localized scouring
of the J-silt to form the trough of the D-sand deposition, variations in the oil or gas content of the pore
spaces, change in the lithology laterally across the D-sand body and more. To explain why this change has
occurred is beyond the scope of this investigation. Let it be sufficient to observe that an isochron thickening
and a change in the instantaneous frequency (Fig. 4) appear to coincide with a mapped isopach thickening
of the D-sand (Fig. 1).
The same crossline is presented in Figure 5, however, the frequency range of the color bar has been tightly
limited to 62 - 64 Hertz. Notice that all of the data are still present on the section, so there is no alteration of
the seismic signal as would occur if a filter had been applied, but limiting the values of the color bar has the
effect of drastically increasing the visual contrast, making certain features more observable.
Again, the isochron separation of the horizon picks is subtle, but the downward deflection in the, now
white, frequency band is obvious. Also note that the D-Sand top (black) is picked through a zone of rapid
frequency transition, meaning that the time pick frequently cycles through many different colors (Fig. 4) or
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William C. Overman M.S. – Geophysical Interpreter
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many transitions from cyan to white (Fig. 5). Similar variations are apparent on the D-Sand base pick (red).
In a map representation, these rapid variations are observed as “noise” or “chatter,” possibly obscuring
trends or disrupting the spatial geometry of the underlying geology. This could manifest as a visual
resolution issue.
Figure 5: Instantaneous Frequency Spectrum of Crossline 82, between 62 and 64 Hertz, from 1.409
to 1.500 sec., with the seismic amplitude super imposed as wiggle traces. Using this color bar, any value less than or
equal to 62 Hertz, displayed in cyan, and any value greater than or equal to 64 Hertz, displayed in white.
Because the actual isochron separation of the D top and base is almost indiscernible, the question is forced:
“Is this a meaningful measurement or is this interpreter overly fond of adult beverages?”.
The D-sand isochron interval is presented below (Fig. 6). The geometry of the isochron map presents an
isochron thickening (based upon time picks not color bands) consistent with the valley fill sand mapped
from well control. In sections 28, 21, and the NE ¼ Sec. 20, where well control is good, the agreement
between isopach and isochron is very satisfying. Because the SW ¼ Sec. 17 has limited well control, the
anomalous isochron “thick,” which is on trend with the known valley fill sand, suggests an inviting
prospect, but, according to the Colorado Oil and Gas Conservation Commission (COGCC) web site, as of
this writing, it remains untested and the suggested sand build up remains unconfirmed.
Observing that the isochron increment on the color bar is finely divided to 5 decimal places (0.00001
seconds), that the data were acquired and processed using a 2 millisecond sample rate (0.002 seconds) and
that the entire variation on the isochron map is only 2 milliseconds (0.007 to 0.009 sec.), or 1 seismic
sample, one should reasonably question the validity of the isochron time increment. SMT’s Kingdom Suite
users’ manual states that the auto-picker fits a parabola to the three points nearest the peak or trough being
picked, so as to interpolate the most accurate time and amplitude. (The manual is an unpublished
document.)
This interpreter attempted to correlate isochron times, selected very precisely at the well bore locations,
against the D-sand isopach from the respective well logs. The result was very noisy and no correlation or
relationship was established. This suggests that the error in any single map location, be it noise in the
seismic trace or lithologic or pore fluid content variation at the well bore, makes any individual trace an
unreliable measure of the immediate geology. Yet, the lateral coherence and geologically appealing spatial
geometry of the isochron suggest that, on average, the interpolated estimates are accurate. Examination of
the isochron data file shows data values stored to 0.0001 sec. By empirical observation, this interpreter
accepts the isochron time interpolation as reasonable significantly below the sample rate.
Therefore, this interpreter would assert that, while the isochron variation is extremely subtle (Fig. 6), far
below the seismic sample rate, it is very meaningful to these specific data, in this geologic environment,
and that it might be similarly meaningful in additional D-J fields or prospects.
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Figure 6: D-Sand isochron map, demonstrating lateral coherence and spatial geometry in good agreement with
the superimposed D-sand isopach contours. The isochron time range is 0.007 to 0.009 seconds, mapping only the 0.002
sec. variation in the interval. The color bar has been “rotated,” such that the sharp cyan to yellow color contrast at
0.00756 seconds maximizes the visual contrast between the Sooner Field isochron thickening (green/red/yellow), the
inferred valley fill sand response, and that attributed to the peripheral sands.i
However, notice the minor N-S and E-W “streaks” in the map’s color (Fig. 6). The most prominent is the
yellow “streak” on inline 90 (N-S) between crosslines 112 and 122 (E-W). It is impossible to discern which
of these streaks are real and which are artifacts of the “noise” or “chatter” mentioned above (Fig. 4), but
those which parallel the inlines or crosslines are very suspect, especially if the magnitude is small.
While the general spatial geometry of the D-sand isochron (Fig. 6) matches well with the isopach of the
D-sand at Sooner Field, it is still not aesthetically pleasing, being more suggestive of the Planariaii flat
worms this interpreter encountered in 10th grade biology, than a valley fill sand. However, because the
outline is reasonably well constrained to the mapped isopach, or extending its known trend, it represents an
improvement over the Amplitude attribute (Fig. 2), which showed little constraint to the valley fill sand,
and the Peak to Trough attribute (Fig. 3), which was not present over the entire valley fill sand, much less
presenting prospects beyond the established production.
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The instantaneous frequency presented in cross section demonstrated a significant separation of the color
bands associated with the top and base of the D-sand (Fig. 4), suggesting instantaneous frequency as a
mappable seismic indicator of D-sand thickness.
In Figure 7 this interpreter raised the low frequency display limit and in Figure 8, lowered the high
frequency display limit to discover which limits take full advantage of the dynamic range.
Figure 7a: Frequency Range 20 to 80 Hertz
Figure 7c: Frequency Range 40 to 80 Hertz.
Figure 7b: Frequency Range 30 to 80 Hertz.
Figure 7d: Frequency Range 50 to 80 Hertz.
Figure 7: Instantaneous Frequency of the top of the D-Sand, with D-sand isopach contours super
imposed. All maps are identical, except that the lower limit of displayed frequencies was progressively raised. The
color bar is such that all data values falling outside of the display range will paint as white.
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William C. Overman M.S. – Geophysical Interpreter
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Figure 7 presents Instantaneous Frequency in four versions (Figs. 7a – 7d), identical except for raising the
lower frequency limit of the display. The color bar has been rotated down one position so that any value
outside of the display range will present in white. All four maps suggest positive spatial correlation
between the Instantaneous Frequency attribute and the D-sand isopach. However, none of the maps
presents a reasonable valley fill sand geometry or a tightly focused drilling target. The “blotchy”
appearance of each map also suggests a manifestation of “noise” associated with the picks on the top of the
D-sand (Fig. 4 and Fig. 5).
Figure 8a: Frequency Range 35 to 80 Hertz.
Figure 8b: Frequency Range 35 to 75 Hertz.
Figure 8c: Frequency Range 35 to 65 Hertz.
Figure 8d: Frequency Range 35 to 55 Hertz.
Figure 8: Instantaneous Frequency of the top of the D-Sand, with D-sand isopach contours super
imposed. All maps are identical, except that the upper limit of displayed frequencies was progressively lowered.
Following the low frequency indication from Fig. 7, the lower frequency limit is fixed at 35 Hertz. The color bar has
been rotated down 1 position, such that all data values falling outside of the display range will paint as white.
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Other than changing the color distribution of the display, the maps remained stable up to 30 Hertz (Fig. 7a
& 7b). There was significant display loss in the core of the anomaly when the lower limit was raised to 50
Hertz (Fig 7d) but only a little display loss when the lower limit was raised to 40 Hertz, which “clipped” a
bit of data at crossline 120, inline 60 (Fig 7c). Based on these observations, a lower limit of 35 Hertz was
selected for the displays in Figure 8.
Lowering the high frequency limit (Fig. 8) introduces a more profound, yet useful effect. At 55 Hertz (Fig.
8d) much of the high frequency “noise” peripheral to the valley fill sand and outside of the area of interest
has been set to the default white color, thus effectively focusing attention on the valley fill sand and other
areas deemed prospective because of similar frequency response. Such an area is to the northwest, in Sec.
17. Without elaboration, the “prospective” areas to the east of the field (Fig. 8d) result from poor data at the
edge of the survey. Thus, by narrowing the frequencies displayed, much of the “noise” that arose from the
time picks (Fig. 4 and Fig. 5) has been removed from the display and brought the interpreter’s focus to the
valley fill sand.
A guitar string changes pitch from lower to higher notes as it’s length is shortened by the guitarist’s fingers.
So too, we expect the dominant instantaneous frequency to change (within limits not addressed here) from
low to high frequency (pitch) as the sand body thins (shortens). Likewise, as the sand body thins, we
would expect a corresponding decrease in isochron times.
Figure 9a: D-Sand Isochron
(Fig. 6 reproduced)
Figure 9b: Frequency Range 35 to 55 Hertz.
(Fig. 8d reproduced)
Figure 9: Comparison of the D-Sand isochron and the Instantaneous Frequency at the top
of the D-sand.
Comparing Fig. 9a to Fig. 9b, the excellent spatial correlation between the isochron and instantaneous
frequency maps is expected. Furthermore, there is also good visual correlation of the maximum isochron
times (green/red/yellow of Fig. 9a) to the lower frequencies (green/red/yellow of Fig 9b.) and the minimal
isochron times (cyan of Fig 9a.) to the higher frequencies (white of Fig 9b.).
The cross plot of the Instantaneous Frequency map (Fig 7a.) and Isochron map (Fig 9a) in Figure 10
confirms a linear relationship between the D-sand isochron time and the instantaneous frequency from the
top of the D-sand, displaying the expected relationship of longer isochron times with lower frequencies and
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shorter isochron times with higher frequencies. This correlation further supports accepting the presentation
interval of the isochron map (Fig. 6 and Fig. 9a) at finer than seismic sample rate.
Figure 10: Cross Plot of Instantaneous Frequency over
Isochron Times, with a Numerical Regression RMA trend
line fitted.
Figure 11: Cross Plot of Isochron Time over Isopach
Thickness, with a fitted 2nd degree trend.
While the cross plot between isochron time and isopach thickness (Fig. 11) suggests a relationship, it is
certainly not conclusive. The fitted trend line suggests that sands thicker than 30 feet can be recognized and
the thickness estimated. However, because the isochron data is “noisy” (Fig. 4) and the isopach data is
sparse, there is too much random scatter in the cross plot to allow a reliable relationship to be established.
Still, it is interesting to note that the apparent lower limit of 0.0075 seconds to distinguish sands 30 feet and
thicker from the surrounding thinner sands, is very close to the value of 0.00756 seconds visually selected
as the abrupt cyan to white transition used to visually separate the interpreted (green/red/yellow) valley fill
sand from the thinner surrounding (cyan/green) sheet sands (Fig. 6 and Fig 9a.)
This interpreter observed that the geometry of the Amplitude (Fig. 2) and Peak to Trough (Fig. 3) seismic
attributes were not satisfying because they do not resemble the expected geometry of the known sand body.
Although improved, the same observation can be applied to the detailed geometry of the isochron and all of
the instantaneous frequency maps so far presented.
While stepping down through time slices of the instantaneous frequency attribute, flattened on the D-Sand
Top, it was observed that the “noise” in the data surrounding the D-sand body reduced significantly. The
clearest correlation between any tested seismic attribute and the interpretation of the known D-sand body
was empirically observed on the instantaneous frequency time slice 4 mils. below the flattened pick.
Refer again to Figure 4. The position of the time slice is indicated by the fine red line at 1.464 seconds on
the flattened seismic section. Notice that by selecting a time within the D-sand isochron, that is between the
black and red picks, that the time slice maps a portion of the Instantaneous frequency attribute which is far
more stable than when the attribute is mapped directly on the D-sand top. Simplistically, the time slice does
not pass through nearly as many colors. The same observations apply upon examination of Figure 5. The
effect of this is to reduce the previously mentioned “noise” or “chatter” on the map display.
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Figure 12a: Frequency Range 35 to 70 Hertz.
Figure 12b: Frequency Range 62 to 68 Hertz.
Figure 12c: Frequency Range 62 to 66 Hertz.
Figure 12d: Frequency Range 45 to 65 Hertz.
Figure 12: Instantaneous Frequency of the time slice 4 mil. below the top of the flattened
D-sand horizon. Note: The color bar is in “neutral“ position. Frequencies greater than the maximum will “wrap
around” to the color of the lowest value (cyan), while frequencies less than the lowest value will “wrap around” to the
color of the highest value (white).
Figure 12 presents the instantaneous frequency attribute 4 mils. into the valley fill sand, below the flattened
D-sand top horizon. The difference in the maps is again the upper and lower limits of the frequencies
displayed. Because the instantaneous frequency changes rapidly with time, the frequency spectrum of the
maps here (Fig. 12a - 12d) are not directly comparable to the maps of the D-sand top (Fig. 7a – 7d, Fig. 8a
– 8d). However, it was observed that the same relative insensitivity to changes in the lower frequency limit
observed previously (Fig. 7) also applies to this time slice. Because the previously observed sensitivity to
changes in the high frequency limit (Fig. 8) was also observed, Figures 12a to 12d are presented in order of
smoothly decreasing high frequency, but quite variable low frequency limits.
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Progressing from Figure 12a to Figure 12d, as the high frequency limit was slowly reduced by only 5
Hertz, from 70 to 65, the amount of spurious “noise,” that is non-cyan color points peripheral to the valley
fill sand, was dramatically reduced. In contrast, re-introducing significant low frequencies in Figure 12d as
compared to Figure 12c, introduced no observable effect beyond broadening the color spectrum. As
suggested by the correlation between isochron and frequency (Fig. 10) the greens and reds of Figure 12d
suggest the thickest sands, but that is only inference as no calibration has been established.
Figure 13a: Instantaneous Frequency
Figure 13b: Instantaneous Frequency
with isopach contours superimposed.
contoured between 56 (core) and 66 (exterior) Hertz.
Figure 13a & b: Instantaneous Frequency of the time slice 4 mil. below the top of the
flattened D-sand horizon, tightly restricted to 62 to 64 Hertz. The color bar is rotated up 1 position so that
frequencies above the upper display limit are cyan.
The process of narrowing the frequency limits was carried to an observable extreme in Figure 13, where the
displayed limits are 62 to 64 Hertz. Figure 13a demonstrated excellent correlation between this
representation of the Instantaneous Frequency response and the isopach of the valley fill sand. Figure 13b
shows contours of the Instantaneous Frequencies on this map. As the full frequency spectrum was available
for contouring, contours of frequencies higher then 66 Hertz were manually removed, so as to present a
clear outline of the inferred sand body. This contour presentation is subsequently used as a surrogate for the
valley fill isopach.
Thus, by selecting the Instantaneous Frequency as the key seismic attribute, because of the exaggerated
separation observed in the cross section (Fig. 4), limiting the visual “noise” in the seismic time picks by
limiting the frequencies displayed (Fig. 5), and mapping the time slice in the center of the D-sand isochron
to avoid the “noise” of the reflection interface, this map clearly presents an attribute that corresponds very
well to the size, shape and position of the known valley fill sand. Furthermore, it looks like a valley fill
sand! But what does it really map?
Figure 13 is a map of the relative time separation of the instantaneous frequency band associated with the
base of the D-Sand from the top of the D-sand (Fig 5). It is presented as an indirect indicator of isochron
and isopach thickening of the D-sand over the Sooner valley fill sand, resulting from “tuning effects.” The
Frequency range of 62 to 64 Hertz was selected, through trial and observation, as the limits which cause the
“best” response by two measures,
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1) the geometry of the response best matching the distribution of the known valley fill sand with
2) the fewest peripheral white points or “noise.”
This selection was interpretive to model the known geology and not based upon just the numbers.
This interpreter would assert that this (Fig. 13a) is the best representation of the geometry of the isopach
“sweet spots” within the Sooner Field valley fill sand. The interpreter would also assert that, other than
geometry, it represents very little about the geology of the field. It is not, for example, an isopach of the
valley fill sand, only an estimate of where the sands are “thicker” relative to the surroundings.
FIGURE 14: Isochron color map
Figure 15: Peak to Trough color map
with instantaneous frequency contours superimposed.
with instantaneous frequency contours superimposed.
In Figure 14, the good correlation expected between instantaneous frequency contours (Fig. 13b) and the
isochron color map (Fig. 6) is demonstrated. Figure 15 presents a surprisingly good correlation between the
instantaneous frequency contours (Fig. 13b) and the Peak to Trough attribute (Fig. 2). As previously
observed, this attribute offers limited predictive indications beyond the established production in the S½
Sec. 21 and N½, Sec. 28, but had a wildcat been drilled based upon the attribute as mapped, it almost
certainly would have been the discovery well and therefore cannot be dismissed.
The purpose of Figure 13 is to focus attention on the “thicker” sand as the intended target. When the nontechnical client or management asks “Where, on this rainbow of colored maps, do we drill?” the summary
response could be “Drill the white, avoid the cyan.” But there is the question: “Would such a simplistic
recommendation be sufficient?”
Figure 16 was prepared to evaluate the assertions of this analysis against the real world results. Initial
Production Potentials (IPP) for oil only was chosen as a convenient surrogate of profitability and was
gathered from the scout tickets on the COGCC web site and then plotted iii. The IPPs ranged from a low of 5
BOPD to a high of 960 BOPD, very significant for this basin, with the best IPPs from the closure in the
W½ NE¼ of SEC. 21.
Using the Instantaneous Frequency contours (Fig. 13b) as a surrogate for the valley fill sand and overlaying
those contours on the IPP colored base map (Fig. 16), on Figure 17 we observe … a problem!
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FIGURE 16: Initial Production Potential (IPP)
Figure 17: Instantaneous Frequency
for oil only of the wells at Sooner Field. C.I. = 50 BOPD.
(Fig. 13b), over IPP base map colors (Fig. 14).
Drilling on the Instantaneous Frequency map alone, that is “Drill the white, avoid the cyan” would have:
 only found the production in the N ½, Sec. 28 and the S ½, Sec. 21,
 drilled a number of dry holes in Sec. 20 and Sec. 21 (which were drilled anyway), and
 missed the very best IPPs in the field, in the W½, NE¼, Sec. 21, 1860 BOPD!
Using a constant velocityiv of 4375 ft./sec. to convert the D-sand Top 2-way time structure map into a depth
map referenced to the seismic datum (Fig. 18), the source of the “failure” is quickly discovered! The top of
the valley fill sand crosses an approximately 40 foot structural low in the central part of Sec. 21. Even
though the total relief on the D-sand Top in this study area is less then 100 feet total, it is apparent that
Sooner Field has a significant structural component.
Figure 18: Structure on the D-sand Top
over IPP base map colors (Fig. 16).
Page 14 of 16
William C. Overman M.S. – Geophysical Interpreter
(720) 251-4117
(A U.S. phone number whch, via internet, follows me globally )
Bill@WCO-Exploration.com
The isochron (Fig. 9a), isopach (Fig. 9b) and Instantaneous Frequency (Fig. 9b) maps all suggest that the N
½ Sec. 28 would have the best combination of thick sands and favorable structure (Fig 16), yet the best
IPPs were from the comparatively thin sheet sands east of the valley fill sand, sands which were not
resolved by this analysis. Clearly there is also a stratagraphic component, making the Sooner Field pool a
combination structure and stratagraphic trap which could have been discovered using the seismic tool, but
not fully developed without geologic insight, well logs and the drill bit.
OTHER WORKS
As indicated above, this interpreter chose to do the work before reference to the literature. Subsequently, a
search of the web disclosed an announced presentation to the Geophysical Society of Houston, 09/18/2003,
by Mr. Paul Jones, co-authored with Tom Wittick, on their analysis of these data (GSH, 2003). The
announcement states “…A high resolution 3D seismic survey [Sooner 3D] is tied with digital well logs
from 27 key wells using synthetic seismograms and a multiple linear regression scheme identifying seismic
attributes most sensitive to net sand variations in four horizons.
The four attributes that rank highest are instantaneous frequency, amplitude, isochron of peak to trough,
and a depth-converted horizon of the Huntsman Sand. These attributes are combined using the multiple
linear regression technique to predict net sand thickness.”
Their work addressed four horizons, this only the D-Sand. This interpreter is gratified to have re-discovered
the importance of the instantaneous frequency and isochron attributes as well as the need for structural
control. This analysis of the D-sand did not find the amplitude attribute to be useful but did observe that the
peak to trough attribute could have located a successful discovery well.
CONCLUSIONS
The selection and evaluation of all of the seismic attributes were driven to identify the known valley fill
sand. The valley fill sand was successfully identified by isochron variations mapped at less than the seismic
sample rate, confirmed by a measure of the instantaneous frequency and supported by the peak to trough
attribute. Although the thickest sands were identified by seismic attributes, the actual structure of the field
proved to dominate the trap and pool geometry. The attributes are too subtle to be identified by analysis of
seismic alone, and any interpretation must be directed by geologic forethought and attention paid to the
basic geology and structure.
Likewise, the production from the sheet sands east of the valley fill would not have been found by any
attribute analyzed. That production had to be discovered by a geologist with well logs and insights into the
depositional environment.
Despite those limitations, this analysis did locate the “sweet spots” in the sand body and did locate the
structural low in Sec. 21. In retrospect, application of the seismic tool could have lead to more efficient
wildcat and development drilling, thereby conserving drilling capitol and increasing overall profitability.
The Instantaneous Frequency (Fig. 13) suggests two more “sweet spots” in Sec. 17. These are supported by
encouraging correlative isochron “thicks” (Fig. 6), on a structural “plateau” at the same elevation as the
production in Sec 21 and 28 (Fig. 18). So this interpreter suggests that there still might be production
established in a north west extension of the field.
Finally, the proper selection of display parameters is critical. Maximizing the dynamic range of the
displayed information or even masking non-critical noise by restricting the values displayed to only the
range of interest, has a dramatic effect upon what data is presented and how it is perceived.
Page 15 of 16
William C. Overman M.S. – Geophysical Interpreter
(720) 251-4117
(A U.S. phone number whch, via internet, follows me globally )
Bill@WCO-Exploration.com
ACKNOWLEDGMENTS
While the author takes full responsibility for any errors or omissions in this text, he would like to thank
William C. Cook III, Senior Geophysicist, for his constructive comments and suggestions regarding this
article.
Likewise, Coerte Voorhies, Sr. Geologist and Account Manager, Seismic Micro Technology, Inc. for his
patience with “newbee” questions and his supporting comments regarding this work.
The author would also like to thank Seismic Micro-Technology, Inc. for providing access to their Kingdom
Suite™ of 2D and 3D seismic interpretation software.
Finally and of chief importance, I would like to thank my wife, Barb, who endured being made a “3D
seismic widow” during this project.
REFERENCES CITED
Cannon, Terry J., 1998. Polymer Treatments for D Sand Water Injection Wells Sooner D Sand Unit
Weld County, Colorado. Prepared for the National Petroleum Technology Office, U. S. Department
of Energy, Tulsa, OK, Under Subcontract G4S60232 and Prime Contract DE-AC22-94PC91008
COGCC web site http://oil-gas.state.co.us/
GSH, 2003 (Geophysical Society of Houston) web reference http://gsh.seg.org/Sep03.html
Tanner, M. T. and Sheriff, R. E., Application of amplitude, frequency, and other attributes to
stratagraphic and hydrocarbon determination, in Seismic Straigraphy –applications to hydrocarbon
exploration, AAPG Memoir 26
Two unsmoothed “grids” were prepared, one on the D-Sand Top (black line of Fig. 4 ) and the other on
the D-Sand Base (red line of Fig. 4). Using the “Math on 2 Maps” calculator, the isochron time difference
was calculated.
i
ii
To refresh the memory…this image of a Planaria is from: www.ebiomedia.com/prod/BOanimals.html
SMT’s Kingdom Suite™ offers a tool called XYZ Grid. Basically, the user clicks on the base map to
obtain the X-Y coordinates and then enters the Z value into a data table. That tool made creating this ad hoc
IPP map a snap, as all I had to do was click on the well symbol and type in the IPP from the scout ticket.
iii
A “proper” velocity map showed a number of “busts” in an otherwise smoothly changing velocity field.
Even though the constant velocity assumption would introduce structural errors, they were clearly regional
in nature (i.e. incorrect regional dip) and applying a constant velocity to the time structure map was judged
adequate for this illustration.
iv
Page 16 of 16
William C. Overman M.S. – Geophysical Interpreter
(720) 251-4117
(A U.S. phone number whch, via internet, follows me globally )
Bill@WCO-Exploration.com