Recent
work in Utah's Ferron coalbed methane (CBM) trend has demonstrated that
seismic can be useful in evaluating CBM plays.
Objective horizons
in CBM trends are typically shallow and acoustic impedance contrasts between
the coals and interbedded sandstones are high, making many coalbed methane
plays attractive candidates for seismic study. Although keeping costs
low in these plays is important, 2-D seismic is now relatively inexpensive
to acquire and can help greatly to:
- Establish areal extents
of coal deposits.
- Identify structures that
can enhance permeability.
- Guide stratigraphic and
structural interpretation.
- High-grade areas for pilot
tests and further development.
- Despite these potential
benefits, seismic is used infrequently in CBM evaluations — but successful
use of seismic in the Ferron play suggests that its value in CBM trends
is underestimated.
Ferron Coal Play
One of the most
significant domestic CBM plays has been the Ferron trend along the western
flank of the San Rafael Swell, near the town of Price, Utah (figure
1). In this area, intertonguing coals, sands and shales were deposited
in a fluvial-deltaic environment on the western side of the early Cretaceaous
Interior seaway (figure 2).
Development began
on the play's north end, at Drunkards Wash Field in 1992. Since then,
the producing part of the trend has been extended both south and north
and now spans an area about 65 miles long and six to 10 miles wide.
Despite the lateral
extent of known Ferron production, large areas between producing fields
remain undeveloped. Limited well control across these open areas has made
characterizing the extent, thickness, quality and production characteristics
of the coals uncertain, and development has proceeded cautiously.
Coalbed gas contents
also decrease southward in the play and project economics become progressively
leaner. In these areas there is an additional need to focus development
by locating production sweet-spots, or "fairways," quickly.
Because of these
factors, Texaco used seismic to help guide its most recent development
activity in the play.
A Requisite
First Step
In two stages
during late 1999, we uniformly reprocessed and interpreted about 140 miles
of 1980's vintage 2-D seismic over the Ferron trend. The first stage was
conducted over the Drunkards Wash Field, where dense well control was
used to determine if seismic could identify the causes of observed production
variations. The second stage was intended to expand what was learned at
Drunkards Wash into the trend's southern parts, where development is much
less complete. Seismic lines from each stage are presented in this article.
The emphasis
in reprocessing was on attention to detail, particularly for imaging shallow
horizons. Changes observed on seismic had to represent true changes in
the Ferron interval, and even subtle features could not be ignored since
they might prove to be important in understanding the stratigraphy.
To achieve reliable
results, only lines with similar acquisition parameters were chosen. Reprocessing
efforts included applying refraction statics, assuring zero-phase data,
detailed velocity analyses, correction of CDP geometry errors, careful
selection of shallow mutes and using improved deconvolution routines.
All steps proved
to be very important in increasing the data's signal to noise ratio, and
in optimizing the quality of shallow reflectors.
The Key Link
The Ferron interval
in the coal-bearing areas is characterized by an upper sandstone, a middle
interval with coal and interbedded siltstones, and a prominent sandstone
at the base known as the Lower Ferron Sandstone. Synthetic seismograms
from this sequence typically show a distinctive pattern (figure
3). A weak reflection peak is generated from the upper Ferron sandstone
due to a low impedance contrast with the overlying siltstones in the Bluegate
Shale member. This is followed by a strong trough (or trough doublet)
produced from relatively thick sections of high-contrast, negative reflection
coefficient coals immediately below the upper sandstone.
At the interval
base, the tight sandstone of the lower Ferron once again contrasts sharply
with coals, and another high amplitude peak is produced.
Sonic logs were
available from a few wells close to seismic lines in the Drunkards Wash
area, and synthetic seismograms from these matched the newly-reprocessed
seismic quite well. The tool of choice in coal plays is the density log,
however, and nearly all the wells in the field have one of these.
Where wells had
both sonic and density logs, psuedosonic curves created from the density
log produced synthetic seismograms that were nearly identical to those
produced from the sonic log.
Drunkards Wash
is fairly densely developed at 160-acre well spacing. Since the density
logs from these wells could be used reliably to generate synthetics, we
were able to tie many nearby wells into the seismic at closely sampled
points along each line. Stratigraphic and lithologic changes observed
in the wells could then be related directly to changes seen on seismic
with high confidence and little interpolation.
Stratigraphic Observations,
Interpretations
Figure
4 shows a series of coincident lines that both demonstrate our results
and show how lithologic and stratigraphic changes seen on well logs in
the Ferron interval translate into changes observed on seismic:
- Figure
4a is a Gamma Ray/Density-log stratigraphic cross-section constructed
from wells located on the seismic line.
- Figure
4b is a seismic model constructed from the synthetics generated
from those wells.
- Figure
4c is the actual seismic line along the same wells.
The displays
have been hung on a flattened Lower Ferron sandstone and scaled similarly
so they can be compared with ease and accuracy.
On the west (or
left) side of figure 4, well logs show a thicker
Ferron interval. Individual coal seams occur in groups, forming relatively
thick upper and lower packages. These are resolved as dual high-amplitude
troughs (with maximum negative amplitudes shaded yellow) and are separated
by a peak generated by the intervening siltstones and shales. The last
high-amplitude peak at the interval base is caused by tight Lower Ferron
sandstone.
Moving east along
the line, the coal sections merge and thin, and the intervening siltstone
is lost. Similarly, on the seismic the intervening peak disappears and
coal troughs merge to form a doublet. Amplitudes are observed to diminish
as the section thins and tuning effects cause destructive interference
in the seismic signal.
On the line's
eastern end, there is a facies transition to stacked Ferron shoreline
sandstones and the coals are absent by non-deposition. The seismic interval
thins and amplitudes dim further as the coal-sandstone impedance contrast
is lost.
Structural influences
are also recognizable. Disruption of the reflectors due to faulting is
evident on the east-central part of the line (despite horizon flattening).
Once seismic
stratigraphic and lithologic relationships such as these were established
in areas of better well control, the seismic could be used to interpret
significant changes in the Ferron interval elsewhere. Similar seismic
"facies" observed on other lines were used to map significant features
of the Ferron interval in undeveloped parts of the play. For example:
- The thickness of the Ferron
interval was mapped and numerical estimates of stacked coal seam "groups"
were made.
- The approximate lateral
limit of coal was plotted. (The coal seams thin below the resolution
of this seismic data set before they pinch out. However, seam terminations
occur fairly rapidly in the area and the limits could still be estimated
with reasonable accuracy).
- Structure maps were made
and structural features that potentially influence production were charted.
- Maps of amplitude were
constructed.
Attempts were
also made to correlate observed amplitude changes with variations in production.
Although no simple association was recognized, interval thickness variation
occurring close to the tuning thickness may have caused amplitude fluctuations
that obscured meaningful relationships.
Structural Observations,
Interpretations
Where structural
features such as faults and folds enhance coal productivity, seismic can
be particularly helpful in focusing play development — something clearly
demonstrated in our geophysical work at Buzzard Bench field (figure
1).
Initial high
water production from CBM wells can be an indicator of permeability and
is often a good sign for future productivity in newer coalbed methane
plays. However, as Texaco began developing the Buzzard Bench area, many
wells there turned out to be low-volume water producers.
Several wells
on the field's east side did have dramatically higher water (and gas)
production rates (see figure 5). This was
encouraging, but on logs, there was no apparent reason for this enhanced
production. The wells were suspiciously aligned north to south, however,
and production changes in one well seemed to affect the others.
Although this
suggested the presence of a fracture or fault system forming a connected
pathway of enhanced permeability, well control alone could not confirm
it.
The seismic here
aided greatly our understanding of the geology behind the production anomalies.
Seismic line 4 traverses both the area of poor production to the west
(not shown on the line) and the trend of enhanced production to the east.
Where the line
traverses the area of poor production, nothing of significance in the
Ferron interval was observed. As it crosses the area of better production,
however, the line intersects one of the high-volume water and gas-producing
wells, the UP & L 14-55, and clearly shows a large fault at the Ferron
level. It also shows that the well is in an area of pronounced downward
folding (reverse drag-folding) on the fault's downthrown side. Enhanced
production in the 14-55 is thus the result of improved permeability from
fracturing along the main fault and from small-scale antithetic faulting
(below seismic resolution) in this zone of reverse drag folding.
This fault is
also found on other lines in the area and can be traced in a north-south
direction where other more-prolific producing wells also lie along its
downthrown side. With the aid of the seismic, we were able to map a narrow
"fairway" of enhanced production parallel to the fault's downthrown side
and focus further development along this trend.
Recognition of
the fault on seismic also suggested structural causes for other curious
complications in this part of the play — one of which was continued high
water production from the high permeability wells beyond the expected
volume for normal coal dewatering.
The faulted and
folded zone on seismic line 4 at Buzzard Bench resembles a half-graben,
a structure typically associated with extensional tectonics. This zone
and its master fault can be traced from its termination several miles
south of line 4 through the subsurface due north almost to a large fault
of the same strike mapped on the surface geologic map.
As it continues
northward, the surface fault gets larger and becomes part of the Scofield
Graben system in the Wasatch Plateau (figure 1).
The connection to that graben system likely explains the half-graben morphology
of the Buzzard Bench fault zone on seismic.
With its exposure
on the wetter high plateau, the fault system forms a conduit for fresh
water recharge (seemingly confirmed by maps of formation water chlorides
in this area). This makes dewatering of the coals along the fault down
at Buzzard Bench difficult and it may take longer than expected.
Additional wells
would help expedite dewatering of coals in the enhanced permeability fairway.
Therefore, this seismically-supported geologic interpretation provided
another reason to target the area for increased drilling.
Although further
structural discussion is beyond the scope of this article, it is important
to note that the seismic also helped us understand the formation of other
significant structures observed in the Ferron trend, such as the Huntington
Anticline, and the timing of their development.
Additional Opportunities
There are additional
benefits conceivable in the Ferron area using seismic. Most notably, a
definitive link between seismic attributes and production might be possible.
Acquisition of new, higher frequency data and perhaps the use of seismic
inversion techniques could achieve this objective.
Shear wave attenuation
and Vp/Vs ratios from multi-component seismic might also be employed to
predict areas of increased cleating, related higher permeabilities and
the associated better production.
Conclusions
In the Ferron
CBM play, seismic was used in a cost-effective way to:
- Relate coal facies changes
observed on well logs to changes seen on seismic.
- More completely map the
lateral extent of Ferron coals in the play.
- Guide stratigraphic and
structural interpretation in the Ferron CBM play in areas where well
control is lacking.
- Reveal faults and folds
that influence coal permeability and predict "sweetspot" fairways for
targeted development.
- Develop a much greater
understanding of the regional geology of the play.
Seismic is not
used often in CBM plays, perhaps because of cost concerns or because of
a paradigm that little additional information will be gained from the
effort. We learned from experience in the Ferron CBM play, however, that
seismic adds valuable geologic information which helps to guide interpretation
and focus development.