Chapter 3

SEQUENCE STRATIGRAPHY

The San Andres Formation within the study area is composed of several lithofacies that were deposited within subtidal, intertidal, restricted intertidal, and supratidal environments associated with a carbonate platform / ramp depositional system. The gross stratigraphic and structural framework of the San Andres was established based on the analysis of six cores (Appendix A), log data from 120 wells, and three-dimensional, multicomponent seismic data. The San Andres in this area represents an overall shallowing-upward interval composed of numerous high-frequency depositional cycles that subdivide the reservoir into alternating zones of high and low reservoir quality. Significant faults, fractures, and features resulting from pervasive diagenesis including dolomitization, karstification, and cementation overprint the primary depositional fabric resulting in additional reservoir complexity (Leary and Vogt, 1990; Adams, 1997). The sequence-stratigraphic analysis discussed here builds upon previous stratigraphic analyses of the San Andres conducted by Capello de Passalacqua (1995), Adams (1997), and Scuta (1997).

The terminology of sequence stratigraphy used in this study is based on a modification of the Exxon-type sequence-stratigraphic terminology as presented by Mitchum and Van Wagoner (1991), and used by Kerans (1995), and Kerans and Tinker (1997). The hierarchy of chronostratigraphic units based on this modification (Figure 3.1) includes, from low- to high-order (frequency), composite sequences, which are made up of high-frequency sequences (HFS), which in turn are made up of high-frequency cycles (cycles). Establishing the hierarchy of cyclicity is essential to construct a representative stratigraphic framework, especially for reservoir modeling. Although it is important to establish the chronostratigraphic framework for a reservoir, relative time relationships among cycles, high-frequency sequences, and composite sequences are more important than absolute time relationships (Kerans, 1995).

Differences in sequence-stratigraphic nomenclature have developed between carbonates and siliciclastics primarily due to the different styles of deposition associated with the two types of sediment. Whereas parasequences are the highest order chronostratigraphic unit of siliciclastic sequence stratigraphy, the high-frequency cycle, or simply "cycle", is a more common term of modern carbonate stratigraphic analysis (Kerans, 1995; Kerans and Tinker, 1997). The term cycle refers to the smallest set of genetically-related lithofacies deposited during a single base-level cycle, comparable to the siliciclastic parasequence (Kerans, 1995; Van Wagoner et al., 1988, 1990), but can contain both deepening and shallowing components (Kerans, 1995; Tinker, 1998). Parasequences are thought to be asymmetrical progradational depositional events recording periods of high-frequency base-level fall or stillstand (Van Wagoner et al., 1990; Kerans, 1995). This is in contrast to carbonate systems in which times of base-level rise may be times of equally great or greater sedimentation rate, and thus a symmetrical record of base-level rise and fall is possible (Kerans, 1995). High-frequency cycles are equivalent in scale to fifth-order cycles (10,000 - 100,000 years in duration) of Goldhammer et al. (1990).

Although not specifically used in this study, a cycle set is a package of cycles that shows a consistent progradational, aggradational, or retrogradational trend (Kerans and Tinker, 1997). A cycle set is analogous to the parasequence set of Van Wagoner et al. (1990).

Mitchum and Van Wagoner (1991) proposed the term composite sequence (Figure 3.2) for those depositional sequences that comprise multiple unconformity-bounded sequences (high-frequency sequences). High-frequency sequences are of intermediate order (fourth-order: 100,000 - 1,000,000 years in duration), are composed of lowstand, transgressive, and highstand systems tracts, and stack into lowstand, transgressive, and highstand sequence sets (Figure 3.2). Similar to depositional sequences, the transgressive systems tract is separated from the highstand systems tract by a maximum flooding surface. Composite sequences are most similar in scale to the seismically-resolvable depositional sequences originally discussed by Vail et al. (1977) and third-order (1,000,000 - 10,000,000 years in duration) cycles (Goldhammer et al, 1990).

The San Andres Formation along the Northwest Shelf can be subdivided into two, third-order composite sequences, herein referred to as the Upper and Lower San Andres (Figures 3.3 and 3.4). The Lower San Andres composite sequence consists of six fourth-order, high-frequency sequences (Figure 3.3; Kerans, 1995). From extensive outcrop work, Kerans (1995) refers to these Lower San Andres high-frequency sequences as L7, L8, and G1 through G4 (i.e. Leonardian 7, Guadalupian 1, etc.; Figure 3.3). The L7 and L8 HFS form a transgressive sequence set and the G1 through G4 comprise an overlying highstand sequence set. Similar to the San Andres in outcrop, these highstand, high-frequency sequences at Vacuum Field record the progradation of subtidal, intertidal, restricted intertidal, and supratidal facies tracts across a Leonardian platform.

The Upper San Andres composite sequence contains nine fourth-order, high-frequency sequences (Figure 3.3), the lower seven (Guadalupian 5-11 of Kerans, 1995) being basin-restricted, deep-water siliciclastic high-frequency sequences equivalent to the Brushy Canyon Formation (Kerans, 1995; Fitchen, 1997; Gardner and Sonnenfeld, 1996). These basinal sequences form a wedge which onlaps a submarine erosional surface at the basin margin (Figures 3.3 and 3.4). The basin-restricted sequences were deposited during a period of relative base-level fall and are equivalent to a regionally-extensive karst surface (subaerial unconformity) at the top of the Lower San Andres composite sequence and to thin sandstone and siltstone deposits (Lovington Formation) on the platform. The Lovington Formation is a dolomitic siltstone unit that separates the Upper and Lower San Andres composite sequences. The Lovington represents eolian silts and sands that were deposited along the ramp margin and is most likely the shelfal equivalent of the Brushy Canyon. Two high-frequency sequences of the Upper San Andres composite sequence (Guadalupian 12 and 13 of Kerans, 1995) lie directly on top of the Lower San Andres on the platform and are capped by a regionally extensive subaerial unconformity (Figures 3.3 and 3.4). This gross stratigraphic framework was used to construct a three-dimensional geologic model of the reservoir for use in flow simulation (Chapter 6).

Six cores were available for detailed stratigraphic analysis of four San Andres high-frequency sequences that comprised the most productive reservoir interval within the study area. These sequences included the upper two high-frequency sequences of the Lower San Andres composite sequence (approximately equivalent to G3 and G4 of Kerans, 1995; Figures 3.4 and 3.5) and the upper two high-frequency sequences of the Upper San Andres composite sequence (approximately equivalent to G12 and G13 of Kerans, 1995). Individual high-frequency sequences were further divided into numerous higher-order (fifth-order) depositional cycles represented by distinct vertical lithofacies successions (Figure 3.5).

The sequence-stratigraphic interpretation of the San Andres was aided by utilizing key indicator facies that represent interpreted depth / energy positions such as shoreline, fair-weather wave base, and storm wave base (Kerans and Tinker, 1997). Three indicator facies were identified based on lithology, allochems, and sedimentary structures. For the San Andres interval, these included fenestral algal laminites, peloidal ooid dolograinstones / dolopackstones, and fusulinid dolowackestones / dolopackstones. Vertical lithofacies successions in conjunction with key indicator facies and exposure surfaces were used to define the finer-scale stratigraphic framework for the San Andres Formation within wells that have core.

Figure 3.5 is a dip-oriented cross section through three cored wells that illustrates vertical lithofacies successions and the high-frequency sequence and cycle framework for the San Andres. Within each high-frequency sequence (HFS 1-4), individual cycles generally are thickest at the base and become thinner toward the top. Upward cycle thinning combined with an increase in the ratio of peritidal lithofacies to subtidal lithofacies in successive cycles reflects decreased accommodation (shallowing upward trend) associated with carbonate deposition and progradation.

Karst breccias, terrigenous siltstones, and increases in the proportion of supratidal lithofacies were used to identify significant high-frequency sequence boundaries that, in some cases, also coincided with composite sequence boundaries. The base of the Lovington Siltstone (top of Lower San Andres composite sequence) and top of the Upper San Andres are examples of both composite sequence and high-frequency sequence boundaries. Karst breccias filled with anhydrite cement and fractures filled with siliciclastic sediment are common features at these boundaries (Figure 3.6).

Based on vertical lithofacies successions observed in core, there is a pronounced seaward facies tract offset across the boundary between the upper two high-frequency sequences within the Lower San Andres (Figure 3.5; top of HFS 4 or approximately G3 of Kerans, 1995). In CVU-60, this abrupt transition is marked by a vertical facies succession of shallow subtidal lithofacies overlain by supratidal, fenestral, algal-laminated mudstones. In addition, this boundary is characterized by laterally-extensive exposure features that include karst breccias, silt-filled fractures, and evaporites. Karst features also extend across the platform at the top of the Lower San Andres composite sequence. A major seaward facies tract offset is recorded at this boundary defined by the sharp transition from shallow subtidal and intertidal facies to eolian siltstone (Lovington Siltstone; e.g. CVU-100). This sequence boundary is considered to be a major eolian siliciclastic bypass surface, equivalent to greater than 1500 feet of deep-water sandstone within the Delaware Basin (Kerans, 1995; Fitchen, 1997; Gardner and Sonnenfeld, 1996).

The Lovington Siltstone and Grayburg Sandstone are both interpreted as lowstand systems tract deposits that were transported by eolian and shallow-water marine coastal processes. During periods of relative base-level fall, the shelf was subaerially exposed, eroded, incised by wadi deposits (arid area streams), and partially covered by eolian-derived silt and sand (Figure 3.7). The Lovington Siltstone thins toward the shelf margin and is only present across the northwestern portion of the study area (Figure 3.8). The observed distribution of the Lovington Siltstone may be due to sediment bypass at the shelf margin resulting in siliciclastic transport to the basin via suspension or sediment-gravity processes. An isopach map of the Grayburg Sandstone (Figure 3.9) also supports the interpretation of eolian and fluvial-type deposits that cut across the shelf margin during the major period of relative base-level fall that followed San Andres deposition.

The analysis of the San Andres sequence stratigraphy from core and three-dimensional seismic suggests that the San Andres first developed in an overall aggradational (vertically stacked) pattern followed by a decrease in accommodation and basinward progradation (seaward-stepping facies tracts). This interpretation is in agreement with several other interpretations of the San Andres Formation from outcrops within the Guadalupe Mountains (Sarg and Lehmann, 1986; Sonnenfeld and Cross, 1993; Kerans, 1995).

Four major carbonate depositional environments characterize the San Andres Formation within this portion of Vacuum Field (Figure 3.10). In an overall regressive sequence, these include subtidal, intertidal, restricted intertidal, and supratidal environments. Previous stratigraphic studies of the San Andres at Vacuum Field and other fields within the Permian Basin, and of San Andres outcrops within southeastern New Mexico document similar depositional environments and diagenetic histories (Sarg and Lehmann, 1986; Friedman et al., 1990; Leary and Vogt, 1990; Purves, 1990; Ruppel, 1990; Sonnenfeld and Cross, 1993; Capello de Passalacqua, 1995; Kerans, 1995; Kerans and Fitchen, 1995; Adams, 1997; Scuta, 1997).

Three lithofacies that are representative of the subtidal environment include fusulinid crinoidal dolopackstones, fusulinid dolowackestones / dolopackstones, and peloidal dolopackstones / dolograinstones (Figure 3.11).

Fusulinid dolowackestones and dolopackstones exhibit varying degrees of fusulinid preservation and are primarily present within the basal portion of the reservoir interval. In many cases, fusulinids have been completely removed through dissolution resulting in biomoldic porosity (Figure 3.11). Although porosity values within fusulinid dolopackstones may exceed 10 percent, effective porosity is generally low within fusulinid-bearing lithofacies since anhydrite cement occludes much of the pore space (and pore throats) or the biomoldic pores are not connected. Anhydrite-filled molds of various mollusks are also common. The presence of fusulinids, bryozoans and echinoderms suggests that this facies developed in normal-marine conditions within 30 feet of water depth and greater (Sonnenfeld and Cross, 1993; Kerans et al., 1994; Kerans and Fitchen, 1995).

Peloidal dolopackstones / dolograinstones are more abundant than fusulinid dolowackestones. This lithofacies consists of peloids and skeletal debris including mollusks, fusulinids, and echinoderms. Extensive dolomitization has resulted in both fabric-selective and non-fabric selective replacement and recrystallization. As a result, pore types within this lithofacies are primarily intercrystalline and interparticle. Evidence of primary depositional fabrics and sedimentary structures has been reduced by dolomitization and bioturbation. Open and healed fractures are common. In general, this lithofacies exhibits relatively high porosity and permeability associated with intercrystalline porosity. However, reservoir quality is often altered by anhydrite cement, gypsum, or fractures (Figure 3.12).

Intertidal deposits consist of skeletal and ooid dolograinstones that were deposited within water depths of 0 to 15 feet (Kerans and Fitchen, 1995) and represent relatively higher-energy deposits (Figure 3.13). Skeletal dolograinstones are composed of dasycladacean green algae (Adams, 1997), mollusk fragments, and occasional fusulinids and are commonly cross-bedded or exhibit planar laminations (Figure 3.13). Skeletal grainstones and associated packstones have been interpreted to represent deposition in a migrating complex of skeletal sand shoals (Ruppel, 1990). Intercrystalline and interparticle pore types are most common among these lithofacies with lesser amounts of fenestral porosity. These deposits are generally very porous and permeable, however reservoir quality can be reduced by anhydrite-filled fractures and gypsum cement. In general, skeletal and ooid dolograinstones do not comprise a significant proportion of the reservoir within this area of Vacuum Field.

The restricted intertidal environment is represented by laminated to nonlaminated dolomudstones and dolowackestones (Figure 3.14). The restricted fauna, abundant carbonate mud, and presence of terrigenous silt and clay layers suggest that these facies were deposited within a low-energy, shallow-water intertidal or subtidal setting. Sparse skeletal fragments include dasycladacean algae, mollusks, echinoderms, and fusulinids (Adams, 1997). Anhydrite cementation is highly variable. Burrows, root casts, faint laminations, and stylolites are common sedimentary features (Figure 3.14). Reservoir quality associated with these lithofacies is generally very low.

Lithofacies representing supratidal deposits include fenestral and algal-laminated dolomudstones and dolomitic quartz siltstones (Figure 3.15). Tidal flat lithofacies are used as a shoreline monitor and generally provide the cap to shallowing-upward cycles (Kerans and Tinker, 1997). Various combinations of mudcracks, stromatolites, fenestrae, intraclastic breccias, tepees, and cryptalgal laminations are present within these lithofacies and suggest subaerial exposure with alternating wet and dry conditions (Shinn et al., 1965; Lucia, 1968). In addition, karst-related features are commonly associated with the supratidal deposits due to prolonged exposure. Karst breccias and solution-enhanced fractures are very common and are typically filled with anhydrite cement (Figure 3.16). Reservoir quality associated with supratidal deposits is generally very low and, given their lateral continuity, these zones may reduce or eliminate vertical flow between higher-quality reservoir intervals.

If present, fractures may allow vertical communication across supratidal lithofacies.