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Figure Captions
 Figure 1. Fort Worth Basin study area.
 Figure 2. Well control within the study area.
Click here to view sequence of Figures 2, 3,
4, and 5.
 Figure 3. Interpreted time structure map for
the top of Vineyard. Features 1, 2 and 3 are only three of numerous
depressions across this chronostratigraphic surface.
Click here to view sequence of Figures 2, 3,
4, and 5.
 Figure 4. Interpreted time structure for the
top of Caddo. Features 1, 2 and 3 are circular depressions on this
surface. Note that these depressions follow a NW-SE linear trend, an
alignment along a deeper basement fault.
Click here to view sequence of Figures 2, 3,
4, and 5.
 Figure 5.  Seismic reflection amplitude
response across the Vineyard surface, a sequence near the base of the
Bend Conglomerate section.
Click here to view sequence of Figures 2, 3,
4, and 5.
 Figure 6. Seismic profile along line ABC,
which traverses three of the disrupted zones (white areas) on the
Vineyard reflection amplitude surface. Each disrupted zone extends
vertically from the Ellenburger up to the Caddo level (more than 2,000
feet). These collapsed zones are assumed to be genetically related to
post-Ellenburger karsting.
 Figure 7a. The Great McKelligon Sag, east
face, southern Franklin Mountains.
Click here to view sequence of uninterpreted
and interpreted
photograph (Figure 7b).
 Figure 7b. Same area interpreted, showing
distribution of collapse breccia and the collapse of the Ordovician
Montoya Group into the Ranger Peak Formation. (B=breccia, C=blocks of
Cindy Formation, M=blocks of Montoya Group.)
Click here to view sequence of uninterpreted
(Figure 7a) and interpreted photograph.
 Figure 7c. Outcrop mapping and diagram of the
El Paso caverns showing collapse of the Ordovician Montoya, development
of breccia pipes and development of caverns in the Fusselman Formation.
 Figure
7d. Uninterpreted photograph of breccia
pipe exposed in an unnamed Ellenburger outcrop in the Franklin Mountains
(courtesy of F.J. Lucia). The Ranger Peak through Cutter section is
Ordovician age (Ellenburger equivalent); the Fusselman is a Silurian
unit.
Click here to view sequence of uninterpreted
and interpreted photograph (Figure 7e).
 Figure 7e. Interpreted photograph of the breccia pipe.
Click here to view sequence of uninterpreted
(Figure 7d) and interpreted photograph.
 Figure 8. Initial pressure measured in the
Sealy C-2 Caddo completion is comparable to other wells there: Little
drainage from surrounding production.
 Figure 9. Time structure map of the Caddo in
the vicinity of the Sealy C-2 well, which is positioned on a structural
high that was created when surrounding strata sank into a ring of karst
collapsed zones.
 Figure 10a. Profile along Line A of
Figure 9,
showing how the Sealy C-2 Caddo reservoir is structurally
compartmentalized by surrounding Ellenburger-related karst collapse
zones. MFS90 means “maximum flooding surface 90,” and is the position of
the sequence boundary for the top of the Caddo. MFS55 is the top of the
Bridgeport; MFS20 is the top of the Vineyard. Step changes show where
zones were perforated.
 Figure 10b. Profile along Line B, showing how
the reservoir is structurally compartmentalized by Ellenburger-related
karst collapsed zones. Location shown in Figure
9.
 Figure 10c. Profile along Line C. Location
shown in Figure 9.
 Figure 10d. Profile along Line D. Location
shown in Figure 9.
 Figure 11. History match of production data
from the Sealy C-2 well provides estimates of reservoir properties such
as permeability and drainage area.
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General Statement
At Boonsville Field in the Fort Worth Basin,
gas production occurs throughout the Bend Conglomerate interval, a
Middle Pennsylvanian clastic section having a thickness of 900 to 1,300
feet (275 to 400 meters) in the project study area (Figure
1). The base
of the interval is approximately 6,000 feet (1,830 meters) deep.
Previous studies have established that the Bend Conglomerate was
deposited in a fluvio-deltaic environment. These productive Bend
Conglomerate clastics are underlain by extensive Paleozoic carbonates,
the oldest and deepest of these being the Ellenburger Group of
Ordovician age.
Evidence of karst processes is frequently
observed in Ellenburger rocks. The data shown here illustrate that some
of these Ellenburger-related karsts create collapsed zones that extend
to considerable heights, sometimes 2,000 feet (600 meters) or more. The
resulting structural sags affect the overlying stratigraphy and can
compartmentalize younger, siliciclastic reservoir systems. Karst-generated
collapses created a reservoir compartment in a sandstone facies
approximately 2,000 feet (600 meters) above a karst origin. For a more
detailed discussion of this study, please refer to Hardage (1996). for
those who wish additional information about the karst collapse phenomena
that are presented.
3-D
Seismic Program
A 3-D seismic grid at Boonsville Field
covering approximately 26 miles2 (67 kilometers2)
is the major part of of this study. This 3-D survey, beginning at the
west shore of Lake Bridgeport, extends westward across Wise County and
into Jack County. The area covered by the 3-D survey is outlined in the
accompanying map (Figure 2), which also shows the extensive well control
that exists in this active gas field.
In addition, this map shows the locations of
wells where vertical seismic profile (VSP) and checkshot data were
recorded to permit log-defined depths of key sequence boundaries to be
converted to accurate two-way seismic traveltime coordinates.
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Data Interpretation
One unique aspect of Fort Worth Basin
stratigraphy revealed by the 3-D seismic images created in this study is
the manner in which Atokan-age (Middle Pennsylvanian) sedimentation has
been influenced by solution collapse that originated in deep,
Ordovician-age Paleozoic carbonate rocks. Time structure maps produced
during the course of interpreting the Boonsville 3-D seismic data (Figures 3 and
4) show, respectively, the topography near the base of
the Bend Conglomerate (the Vineyard surface) and the topography at the
top of the Bend Conglomerate (the Caddo surface).
Inspection of the deeper Vineyard structure
map (Figure 3) shows that several depressions occur in a seemingly
random pattern across the Vineyard chronostratigraphic surface. These
depressions tend to have circular to oval shapes, with diameters ranging
from about 500 feet (150 meters) to about 3,000 feet (915 meters).
Groups of collapse features sometimes occur along linear
northwest-southeast trends, suggesting a genetic relationship between
these stratigraphic disruptions and basement faults.
The seismic-interpreted Caddo surface
developed in this study (Figure 4) shows that depressions similar to
those at the Vineyard level also occur across this shallower Caddo
surface. An important observation is that these Caddo depressions,
particularly the three prominent ones labeled 1, 2, and 3, are
positioned directly above equivalent depressions in the Vineyard
surface, approximately 1,000 feet (300 meters) deeper, implying that
there is a genetic relationship between the Caddo depressions and the
older Vineyard depressions.
The seismic reflection response inside each of
these structural depressions differs from the reflection response in
unaffected areas. This variation in seismic reflection behavior can be
documented by displaying the seismic reflection response across any
interpreted chronostratigraphic surface within the Bend conglomerate interval. One example of the seismic reflection sensitivity
to these surface depressions is shown in Figure
5, which is a display of
the reflection amplitude on the Vineyard time-structure surface.
Profile ABC (Figure
5) traverses three of the
seismic reflection anomalies on the Vineyard surface: one rather large
anomalous area between A and B and two smaller, circular anomalies
between B and C. A section view of the seismic behavior along this
profile is provided in Figure 6, and in this view the consistently
near-vertical attitude and the extreme height of these stratigraphic
disruptions are striking. Each structural disruption begins not far
below 1.2 s, which is the position of the Ellenburger Group (Ordovician
age), and extends vertically into--or completely through--the Bend
Conglomerate clastics (Pennsylvanian Atokan age), causing the vertical
extent of these disrupted zones to be as much as 2,000 to 2,500 feet
(600 to 750 meters) throughout the Boonsville 3-D seismic grid.
In a few instances, a disruption continues
into the Strawn section above the Bend conglomerate. These structural collapse zones occur at a rather high
spatial density, with adjacent collapses often separated by only one
mile (1,600 meters) or less (Figure 5). As noted, each zone extends
completely through the Pennsylvanian-age Bend Conglomerate, or at least
through a significant part of the Bend
conglomerate interval. Because of the stratigraphic disruption that
these collapses cause within the Pennsylvanian section, some of these
Ordovician-related structural sags were a significant influence on
Pennsylvanian and Mississippian sedimentation, and thus these phenomena
need to be considered when evaluating prospects in basins underlain by
karst-prone carbonates.
These extensive vertical collapse zones are
interpreted to be the result of post-Ellenburger carbonate solution,
which occurred during periods of subaerial exposure. This karst model is
adopted because karst-generated vertical collapse zones can be observed
in Ellenburger outcrops in the Franklin Mountains at El Paso, Texas, and
because Ellenburger karst plays are pursued by operators across the
Permian Basin of West Texas.
In the Franklin Mountains outcrops, the
measured lateral dimensions of the collapsed features correspond to the
diameters of several of the disrupted zones observed in the 3-D seismic
image at Bonnsville (Figure 7a, 7b,
7c, 7d,
and 7e). The outcrop features also have extensive
vertical dimensions, as do the seismically imaged collapses at
Bonnsville, with some of these outcrop collapses extending vertically
for at least 1,200 feet (365 meters) in the larger outcrop exposures. It
is important to note that the Ellenburger karst collapse zones observed
in outcrops in the Franklin Mountains and the Ellenburger-related
collapse zones observed in these Boonsville 3-D data in the Fort Worth
Basin document that this Paleozoic karsting phenomenon spans a distance
of at least 500 miles (800 kilometers).
The influence of this deep karst collapse on
younger sedimentation needs to be studied at several sites between these
two widely separated control points (El Paso and Wichita Falls) to
understand better how karsting phenomenon affects hydrocarbon production
and exploration strategy throughout the Permian and Delaware basins of
West Texas.
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Compartmentalization of Siliciclastic Caddo Reservoir Due to Karst
Collapse Processes
One example of
deep-seated Ellenburger karst collapse that created reservoir
compartmentalization at the Caddo level some 2,500 feet (760 meters)
above the Ellenburger and in much younger, Atokan-age clastic rocks, is
the situation associated with the Sealy C-2 well, located in the
northeast quadrant of the 26-mile2 (67-kilometer2)
3-D seismic study area. The Sealy C-2 well drilled through the basal
Bend Conglomerate to a total depth of 5,830 feet (1,777 meters). The
pressures measured in the Upper Caddo were higher than expected and
suggested only partial pressure depletion had occurred in this interval.
The Upper Caddo was perforated from 4,886 to
4,902 feet (1,489 to 1,494 meters) and treated with 2,000 gallons of 15
percent HCl. Following cleanup of the acid treatment, a pressure buildup
test was conducted, and an average reservoir pressure of 1,300 psi was
estimated for the Upper Caddo.
Following the shut-in period, the Sealy C-2
produced at a rate of 1.04 MMscf/d during a 24-hour flow test. Figure 8
is a plot of initial pressures in the Upper Caddo measured from wells in
the project area over time. The value of initial pressure for the Sealy
C-2 well is similar to those reported in wells drilled and completed in
the 1950s. Note that in each case the pressures reported are the best
estimates that could be obtained for particular wells using available
data sources (both operator and public domain records). The estimated
initial reservoir pressure for the Sealy C-2 of 1,300 psi represents a
pressure gradient of about 0.3 psi/ft (1 psi/m). This pressure suggests
that the Sealy C-2 location has been drained partially by surrounding
production, although the pressure is higher than would be expected,
given the extent of the offsetting production from the Upper Caddo. This
Caddo reservoir is in an underpressured sequence and the original
pressure gradient is of the order of only 0.35 to 0.4 psi/ft.
The northeast quadrant of the Caddo time
structure map (Figure 4) is enlarged in Figure
9, and the locations of
the Sealy C-2 and several neighboring wells are identified. This map
shows that the Sealy C-2 well was drilled on what appears to be a
structural high. However, when the structural and stratigraphic details
associated with the Sealy C-2 well are viewed in seismic section views
along line A, B, C or D (Figure 9), it is apparent that the well is not
positioned on a structural high created by tectonic action, but rather
it is on a portion of the Caddo surface where the terrain surrounding
the well collapsed because of underlying Ellenburger-related karsting.
Seismic lines A, B, C and D from the 3-D
seismic survey are presented as Figure 10a,
10b, 10c,
and 10d to support this karst-generated
compartmentalization model. All profiles show that vertical, seismically
disrupted, collapse zones extend from the Ellenburger (approximately 1.2
s) up to the Caddo, and that these collapse zones completely surround
the Sealy C-2 well. These vertical seismic sections indicate that
numerous low-displacement faults, or structural collapses, often with
throws of only 20 to 30 feet (six to nine meters), separate the Sealy
C-2 well from the surrounding terrain. This is the same order of
structural collapse observed in Ellenburger outcrop studies by Jerry
Lucia, Bureau of Economic Geology (personal communication).
The estimated Upper Caddo reservoir pressure
of 1,300 psi encountered in the C-2 well and the subsequent production
history (Figure 11) suggest that these low-displacement faults can be
partial barriers to fluid flow at the Caddo level. The area inside the
circumference of this seismic-defined ring of karst collapse is
approximately 130 acres. Thus, if it is assumed that the karst collapse
zones are partial flow barriers, then the Sealy C-2 well is producing
from a Caddo reservoir compartment spanning about 130 acres.
Figure 11 shows the actual production from the
Sealy C-2 well. This is a log-log plot of gas flow rate versus time. The
well came on line in November 1992 and produced 800 to 900 Mscf/d for
the first couple of months. After that, the gas flow rate gradually
declined to about 200 Mscf/d after just over 2 years of production. The
production data, when plotted this way, show an influence of reservoir
boundaries, as evidenced by the concave downward shape of the later-time
data.
The production data were history-matched with
an analytical reservoir model to estimate reservoir properties and gas
in place. As Figure 11 shows, the analytical model provides a good match
of the actual production data. From this analysis, a permeability of 2.2
md, a skin factor of -2 (indicating slight stimulation following the
acid treatment), and a drainage area of 128 acres were determined. This
production-based estimate of a reservoir area of 128 acres is, for all
practical purposes, identical to the 130-acre reservoir size identified
from the seismic interpretation.
Reservoir performance thus supports the
seismic interpretation concept of an Upper Caddo reservoir compartment
created by Ellenburger karst-collapse zones that surround the Sealy C-2
well. However, none of the reservoir pressures measured in the Bend
Conglomerate interval (Caddo through Vineyard) in this well are
considered initial reservoir pressures, because all measurements
indicate varying degrees of pressure depletion. Therefore, the
low-displacement faults associated with the karst collapse features at
this particular location seem to act as partial, not total, barriers to
gas flow. The degree of reservoir isolation caused by this low-magnitude
faulting appears to vary from sequence to sequence through the Bend
Conglomerate interval.
Several fundamental research questions remain
to be answered, with the following issues being some of the more
obvious:
-
Should wells be positioned inside or outside karst collapse zones?
- How
does a karst process extend through an extensive clastic section such
as the Bend Conglomerate? Does the collapse occur as episodic events
or as a single, catastrophic event?
- What
is the genetic relationship between karsts and faults, and what causes
the collapse features observed in this study to be almost perfectly
vertical?
Currently, we have only speculative answers to
these questions. Both the drill bit and the coring bit will continue to
provide valuable information about these intriguing karst phenomena.
Three-D seismic data also will be critical in any such future
investigations.
Hardage, B.A., D.L. Carr, D.E. Lancaster, J.L. Simmons, Jr., R.Y.
Elphick, V.M. Pendleton, and R.A. Johns, 1996, 3-D seismic evidence of
the effects of carbonate karst collapse on overlying clastic
stratigraphy and reservoir compartmentalization: Geophysics, v. 61, p.
1336-1350.
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