The general depositional history of the Permian Basin is that of carbonate deposition in a broad, shallow marine environment, shallowing upward, terminating with massive evaporites deposited as the basin desiccates.
The regional structural history of the Permian Basin influences the present-day character of the Vacuum Field. Through the Ordovician, Silurian and Devonian Periods, the area was a broad, shallow marine sea bordering the southwestern margin of the North American plate (Hills, 1984; Shumaker, 1992). Known as the Tobosa Basin, cyclic deposition of carbonates, shales and clastic sediments filled the available accommodation space. Transgression and regression are evidenced by unconformities identified in wellbores, surface outcrop, and seismic sections. Tectonic activity in the Permian Basin region was minimal through the Ordovician, Silurian and Devonian Periods (Wright, 1979).
Organic-rich shales accumulated during periods of slow sedimentation and anoxic conditions. Both the Ordovician Simpson Group and the Devonian Woodford Shale are source rock for the hydrocarbon accumulations in the region.
In the Late Mississippian, tectonic activity increased significantly. Compression was directed from the south, associated with the convergence of the South American plate with the southern margin of the North American plate (Kerans and Fitchen, 1995). The late Mississippian and Pennsylvanian tectonics removed the expression of the Tobosa Basin. The present-day structural characteristics of the Permian Basin developed with early expressions of the Delaware and Midland Basins, separated by the elevated Central Basin Platform. Development of these structures may have occurred along pre-existing zones of weakness (Shumaker, 1992). A north-northwest to south-southeast trending fault zone (Figures 2.2 and 2.3) extends into the Vacuum Field project area, forming the boundary between the Delaware Basin and Central Basin Platform. Early work on the structural elements of the Permian Basin described the north-south trending fault zone, which bounds the western edge of the Central Basin Platform and extended northward into the Vacuum Field area, as a strike-slip fault with significant right-lateral movement. Shumaker (1992) alternatively describe the deformation as a system of structural elements and styles consistent with an external rotation of the Permian Basin region. The result is variations in the trends of folds and faults in the Delaware Basin caused by the rotation of individual blocks as depicted in Figure 2.3. This more complex pattern of faulting will likely result in local stress variations in response to a changing regional stress field.
Significant aspects of this tectonic activity produced the structural setting for the Delaware Basin with its associated carbonate deposition through the Permian and the pattern of faulting and fractures that influenced diagenesis and hydrocarbon migration.
Deformation associated with this tectonic event continued throughout the Mississippian and gradually diminished in the early Permian Wolfcamp. Subsidence continued throughout the Permian, providing accommodation space for the deposition of Permian Leonardian through Guadalupian carbonates and siliciclastics.
The Permian Basin region has been largely undeformed since the end of the Paleozoic. Figure 2.4 illustrates a west to east geologic cross-section of present-day structure. The traverse is from the Delaware Basin up onto the Central Basin Platform. Intense folding and faulting during the Pennsylvanian and Early Permian are evident along with the later Permian carbonate and evaporite deposition. Deformation associated with the Laramide Orogeny occurred to the west and north of the region, however, the Permian Basin was uplifted and tilted to the southeast. The present day stress regime shows the regional horizontal maximum stress direction is west-northwest to east-southeast. This azimuth of maximum compressive stress is based on regional studies of stress indicators and borehole breakouts (Zoback and Zoback, 1989), focal mechanisms of seismicity in the Permian Basin (Doser, 1992), observed shear-wave birefringence in the Vacuum Field area (RCP Sponsors Meeting Notes, April 15, 1996), and analysis of borehole breakouts in well WS-26 in the study area (Scuta, 1997; RCP Sponsors Meeting Notes April 10, 1997).
Vacuum Field is located on the northwestern shelf of the Delaware Basin. During the Permian, deep-water basins such as the Midland and Delaware Basins were sites of carbonate deposition along the edges of the basins. During the Permian, the Permian Basin was located within 5 degrees of the Paleoequator (Kerans and Fitchen, 1995). Figure 2.5 shows the stratigraphic units of the Permian Period in the Vacuum Field. The lithology is primarily carbonates, with periodic siliciclastic and evaporite deposition. The general depositional characteristics are cyclic carbonate deposition in a shallowing upward environment with eventually greater amounts of evaporites being formed as the basin desiccates (Wilson, 1975).
There are multiple hydrocarbon-producing intervals in the Vacuum Field ranging from shallow Permian to deep Devonian and Ordovician horizons. In the Central Vacuum Unit, the shallowest producing reservoir is the San Andres - Grayburg at a depth of 4300 feet. This is an important consideration concerning the time-lapse (4-D) aspect of this study. One of the assumptions for processing 4-D, 3-C data is that no bulk property changes occurred above the San Andres Formation due to reservoir processes between the initial and repeat surveys.
The Northwestern Shelf was the site of a carbonate shelf and ramp setting, through much of the Leonardian and Guadalupian Stages of the Permian Period, between the exposed Pedernal Highlands to the northwest and Delaware Basin to the south and southeast. Sedimentation was cyclic in response to relative changes in sea level, tectonism and siliciclastic input (Meissner, 1972).
A carbonate shelf model applicable to the study area is shown in Figure 2.6. This model is based on an extensive outcrop study of the Guadalupian Age San Andres Formation and basin equivalents in the Guadalupe Mountains of New Mexico.
Tectonic activity diminished and carbonate deposition progressed after the structural formation of the Delaware Basin and stable basin margins, including the Northwestern Shelf, due to the tectonic convergence of the North and South American plates. During the late Leonardian, the platform-to-basin transition was characterized as a carbonate ramp system. Through the Guadalupian, the system evolved into a more distinctive ramp, and then shelf crest, separating the inner shelf depositional environment from the basin sediments (Sarg and Lehmann, 1986). Kerans and Fitchen (1995) described the San Andres Formation in terms of 15 high frequency cycles (Figure 2.7) superimposed on 2 longer cycles of relative sea level rise and fall.
The two longer cycles, perhaps equivalent to third-order sequences as described by Sarg and Lehmann (1986), resulted in deposition of the Lower San Andres and Upper San Andres sequences. The resulting thickness of the San Andres is 1400 to 1500 feet in the study area. A significant lowstand period separated the Lower and Upper San Andres, resulting in periods of exposure, karst development and sediment bypass into the deeper Delaware Basin to the south and southeast. In an outcrop study (Kerans and Fitchen, 1995), relative falls in sea level produced erosional surfaces with karsting, brecciation, fracturing and bypassed sedimentation at the top of the lower San Andres, top of the upper San Andres and within both sequences.
Diagenesis of the carbonate section, including the San Andres, occurred with calcium carbonate components of the original limestone being replaced with magnesium to form dolomite. There is evidence for multiple periods of anhydrite deposition. Alteration of the original depositional fabric was both chemical (limestone to dolomite diagenesis) and physical (periods of fracturing and healing with anhydrite). Core descriptions for three wells in the study area were assembled by Capello de Passalacqua (1995), and separately by Scuta (1996). Their descriptions include evidence interpreted as indicating periods of lowstand with erosion and karst development, multiple periods of fracturing and brecciation, emplacement of anhydrite, and periods of sediment bypass into the deeper basin. The original depositional framework, diagenesis and physical alteration resulting in a highly heterogeneous reservoir.
A regional San Andres structure map from well control based on 1292 wells (Purves, 1990) is shown in Figure 2.8. The location of the RCP Phase VI 4-D, 3-C survey is shown along with the Bridges State Lease studied by Purves. The San Andres shelf edge trends east-west in the western portion of the RCP study area, then the strike of the shelf edge changes to southwest-northeast in the eastern portion of the study area. The general east-west orientation of the San Andres shelf is a regional feature coincident with the northern shelf of the Delaware Basin. The local variation in the strike of the San Andres shelf coincides with the projection of the north-northwest to south-southeast trending fault system which bounds the Delaware Basin and Central Basin Platform (Figures 2.2 and 2.3). A detailed structure map of the top of the San Andres in the survey area is shown in Figure 2.9. The circular area of the 4-D, 3-C seismic survey is indicated. The southwest-northeast trending San Andres shelf edge, with the inner and middle shelf facies to the north and west, is evident, as is the deeper basin to the south and southeast.
Well-log signatures for the San Andres and Grayburg from well CVU #345 are shown in Figure 2.10 (Scuta, 1997; RCP Sponsor Meeting Notes, October 10, 1996). Four of the high frequency cycles indicated by shallowing-upward cycles are annotated. The gamma ray response shows low API values for the upper and lower San Andres (dolomites) with higher values for the Lovington and Grayburg sandstones. Both sandstones/siltstones coincide with periods of lowstand and sediment bypass.
There is a strong correlation between neutron porosity, log density and core porosity at Vacuum Field. Log signature indicates greater porosity in the upper part of shallowing upward cycles.
The study at Vacuum Field during RCP Phase VI was undertaken by an interdisciplinary team of students under the direction of Tom Davis and Bob Benson. Max Scuta (Ph.D. candidate, Geology) compiled a three dimensional model of the San Andres and Grayburg formations from well control. Figure 2.11 presents a northwest to southeast structural cross-section from the model. The cross-section shows a distribution of the gamma ray response from well control along with interpreted chronostratigraphic surfaces (RCP Sponsors Meeting Notes, October 10, 1996). A schematic of interpreted stratigraphic sequences is shown with the lower San Andres sequence bounded above by an exposure surface (associated with the Lovington sandstone) and the upper San Andres sequence bounded above by the Grayburg Sandstone. The SS1 sandstone represents possible sediment bypass from a higher frequency cycle and related exposure surface. The Permian carbonate shelf edge was a persistent feature, at which vertical aggradation dominated over progradational and retrogradational carbonate deposition in the Vacuum Field area throughout the Leonardian and Guadalupian. Three-dimensional geologic models, cross sections and examples of seismic data are contained in RCP Sponsors Meeting Notes, October 14, 1996 and RCP Sponsors Meeting Notes, April 10, 1997.
Seismic data show faulting and fracturing associated with the shelf edge (Talley, 1997; RCP Sponsor Meeting Notes, April 10, 1997). The faulting and fracturing involved reactivation of the earlier Paleozoic fault system. An analysis of seismic event discontinuity by Talley (1997) shows extensive faulting, branching upward through the brittle San Andres and Grayburg dolomites. The San Andres is predominately dolomite while the later Permian section (Seven River, Yates, Salado) increases in anhydrite, siliciclastic and evaporite content.
A map of maximum compressive stress orientations for New Mexico and West Texas is shown in Figure 2.12. This map is compiled from Zoback and Zoback (1989) and includes the maximum horizontal present-day stress orientation computed by Scuta, reported in the RCP Sponsors Meeting Notes, April 10, 1997, from a borehole breakout study in the well Warn State 2-26 in the Vacuum Field area. A study of microseismic focal mechanisms in the Permian Basin by Doser (1992) also indicates a general southeast to northwest maximum horizontal compressive stress. Present microseismic activity in the Permian Basin is indicative that subtle movement and reactivation of existing faults is presently ongoing.
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