Bob A. Hardage1
Search and Discovery Article #40044 (2002)
*Adapted for online presentation from two articles by the same author in AAPG Explorer (October and November, 1998), respectively entitled “Collapse Effect Viewed” and “Seismic Supports Collapse Concept.” Appreciation is expressed to the author and to M. Ray Thomasson, former Chairman of the AAPG Geophysical Integration Committee, and Larry Nation, AAPG Communications Director, for their support of this online version.
1Bureau of Economic Geology, The University of Texas at Austin ( ). Coauthors: David Carr, consulting geologist, Austin, Texas; D. Lancaster, S.A. Holditch and Associates, College Station, Texas; J. Simmons, Jr., Bureau of Economic Geology, The University of Texas at Austin; Virginia Pendleton, Integrity Geophysics, Tulsa, Oklahoma; Robert Elphick, Schlumberger, Denver, Colorado.
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.
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 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.)
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.
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.
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.
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.
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.
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:
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.Return to top.