Abstract Dolomite and dolomite reservoirs are common in Ordovician and middle to upper Permian carbonates in the ‘Permian Basin’ area of the SW United States. Scattered, but significant, dolomite reservoirs also occur in the Silurian and Devonian. In the middle to upper Permian, platform and shelf-top carbonates were dolomitized, while limestones in slope and basinal environments were not. Most dolomitization of Ordovician and Permian carbonates occurred in evaporated seawater shortly after deposition (reflux dolomitization). Most dolomitizing brines apparently formed in tidal flat and restricted lagoonal environments in the platform and shelf interiors, and on depositional highs near shelf and platform margins. More than 26 billion barrels of oil have been produced from the Permian Basin, with most of that oil coming from Palaeozoic (mainly Permian) dolomites. Permian Basin dolomites have very heterogeneous porosity and permeability on a wide range of scales. Field-scale (km-scale) variations in porosity are commonly related to position in the original dolomitizing system. Porosity generally increases away from the apparent source of the dolomitizing brines because a greater volume of dolomite was precipitated in proximal parts of the dolomitizing system than in the distal parts; hence, porosity is greater in dolomites in the basinward parts of fields. Most Permian Basin dolomite reservoirs are structural traps with stratigraphic enhancement of closure by loss of porosity and permeability towards the shelf or platform interior. Many traps were formed by compactional drape over the same features that created highs during deposition. Hence, the structurally highest parts of many fields have the poorest porosity and permeability because they coincide with proximal parts of the original dolomitizing system. The most porous, permeable and productive dolomites are on the basinward flanks of structures, and often near the oil-water contact. Dolomite reservoirs in the Permian Basin are quite variable. Ultimate oil recoveries from these fields range from <1000 barrels to 2 billion barrels, with the largest fields in shallow middle Permian (San Andres/Grayburg) reservoirs. Reservoir depths range from 1500 to more than 14 000 ft (500–4300 m). Average porosities for fields are 1–21%, with porosities generally decreasing with depth. Average permeabilities are 1–1000 mD. Many deeper reservoirs have high permeability related to fractures in karstified Ordovician reservoirs. Recovery efficiencies are 10–65% of the original oil in place, with higher recovery efficiency associated with larger pores and higher permeabilities.

Abstract Reinecke field is an upper Pennsylvanian to lowest Permian carbonate buildup in the southern part of the Horseshoe Atoll, west Texas, United States. The field and surrounding areas have been imaged with three 3-D seismic surveys and penetrated by many wells. Although Reinecke is commonly referred to as a reefal reservoir, deposition occurred in stratified sequences, 50–100 ft (15–30 m) thick, dominated by wackestones, packstones, and grainstones. Boundstones (mainly rich in phylloid algae) constitute only 16% of the buildup. Seismic reflectors within the buildup parallel sequence boundaries and are truncated at the margins of the buildup. Three-dimensional seismic surveys show that the top of the Reinecke buildup is highly irregular with more than 470 ft (143 m) of relief. Deep-marine shales overlie the reservoir and act as a seal for this stratigraphic trap. Reinecke's irregular, mounded morphology is the result of localized carbonate growth and erosional truncation. Much of the erosional truncation probably occurred in a deep-marine environment. Reinecke's south dome acts a single continuous reservoir dominated by limestone (70%) with 25% dolomite. Limestone porosity is generally 5–18% (average of 11.2%) and permeability is 1–1000md(average of 166 md). Dolomite porosity is lower (average of 8.3%), but permeability is higher (average of 894 md). Discontinuous low-permeability layers parallel to stratification serve as low-permeability baffles; however, patchy replacive dolomites cut through stratification and act as high-permeability vertical conduits. Good reservoir continuity, low-permeability baffles, and artificially enhanced bottomwater drive helped to recover more than 50% of the original oil in place. Excellent vertical reservoir continuity has allowed implementation of a crestal CO 2 flood at Reinecke field. CO 2 is being injected into the top of the structure, displacing residual and bypassed mobile oil downward for recovery in lower parts of the reservoir.

Abstract Internal architecture, three scales of reservoir heterogeneity, and porosity-permeability compartments can be reconstructed for Middle Mississippian Salem Limestone reservoirs of the Illinois Basin by integrating bedform architecture, depositional fabrics, and trends of porosity development. In sawed quarry walls of south-central Indiana, excellent exposures of the Salem Limestone allow detailed mapping of a wide range of bedforms representative of an extremely dynamic hydrographic regime similar to that of the shoreface zones of siliciclastic shelves. Echinoderm- and bryozoa-rich grainstones and fossiliferous oolitic grainstones are the predominant depositional facies and were deposited as subtidal shoals in an upper shore-face setting. Vertical and spatial relationships of facies and bedforms reveal hierarchies of depositional units and bounding surfaces that have been used to reconstruct four architectural packages (AP’s) of the scale of mesoforms. AP-I is a channelized mesoform that defines a NNE-SSW intrashoal channel. AP-II is a large storm-deposited sandwave complex that filled the channel. AP-III is a dark-gray grainstone/packstone that grades laterally into packstones and was deposited in a relatively low energy setting. AP-IV consists of oscillatory trough cross-stratified megaripples of oolitic grainstones. Numerous stylolites occur and add to the mesoscale heterogeneity. Together, microfacies, bedforms, and bounding surfaces compartmentalize porosity and permeability and lead to micro- and mesoscale heterogeneities within the mesoform scale architectural packages. Lessons learned from such outcrop models can be applied to the producing Salem reservoirs in the subsurface. Here, in the distal parts of the inner ramp, the shoal complex is even more compartmentalized because each AP is represented by a “complete” suite of upward-shallowing facies: a basal outer shelf sponge-spicular wackestone, a main shoal grainstone, and an overlying restricted marine lagoonal wackestone. When integrated with mercury-injection capillary-pressure and production data, these packages and respective scales of reservoir heterogeneity help delineate flow units for reservoir characterization and simulation purposes and determine how the reservoir facies will drain during primary and secondary recovery. The results can also be used to reduce uncertainty (geologic risk) during the pre-drill economic assessment, field appraisal, and development phases.

Abstract Reinecke field is a carbonate buildup in the southern part of the Horseshoe Atoll. Since discovery in 1950, it has produced more than 82 million barrels of oil. The south dome of Reinecke field has been characterized with core, wireline logs, 3D seismic, crosswell tomography, and 3D cellular models of porosity, permeability, and fluid saturations. Four main depositional sequences, approximately 60-80 ft (18-24 m) thick, were identified in the Upper Pennsylvanian reservoir interval. Nine depositional facies were recognized, including mudstones, wackestones, packstones, grainstones, and boundstones. The reservoir is approximately 70 percent limestone and 30 percent dolomite. Porosity is widespread in both lithologies. Most depositional facies have average porosities of 9-13 percent where still limestone. Important pore types in limestones include intercrystalline microporosity, molds, intergranular pores, fractures, and vugs. Limestones dominated by microporosity have low permeability, commonly 1-30 mD. Limestones with fractures and vuggy pores commonly have permeability greater than 100 md. Lime mudstones are rare but have distinctly lower porosity (average of 1.4 percent) and permeability (average of <1 mD). Average limestone porosity is 11.2 percent, and average limestone permeability is 165 mD. In contrast, dolomite has generally lower porosity (average of 8.3 percent) but much higher permeability (average horizontal of 894 mD). Discontinuous shales compose less than 1 percent of the gross reservoir. Discontinuous lime mudstones and shales in the lower part of sequences form low-permeability baffles. Therefore, the south dome of Reinecke is characterized by relatively continuous vertical and horizontal porosity and permeability with high-permeability streaks and discontinuous low-permeability baffles. Excellent reservoir continuity and water injection into the underlying aquifer have allowed a good bottom water-drive and excellent primary and secondary recovery (55 percent of the original oil in place). Pore systems that are well connected throughout the reservoir have allowed a crestal CO 2 flood to be designed for Reinecke field. CO 2 is being injected into the top of the structure to mobilize residual oil and push an oil bank down through the reservoir to recover residual and bypassed mobile oil.

Abstract The Guadalupe and Delaware mountains of west Texas and southeastern New Mexico contain superb outcrops of the Capitan depositional system (Fig. 1). The Capitan depositional system as used here includes the reef/forereef of the Capitan Formation, shelfal equivalents (Seven Rivers, Yates and Tansill formations), and the basinal equivalent (Bell Canyon Formation) (King, 1948; Newell et al., 1953; Hayes, 1964) (Fig. 2). Exposures in the Guadalupe Mountains are immense (1000 m vertical; 5-10 km in dip direction; 70 km along strike), and have suffered minimal structural deformation. The Capitan depositional system of the Guadalupe Mountains has been visited and studied by many of the world’s premier geologists. In addition, it serves as one of the primary training grounds for earth scientists from the petroleum industry and academia. As a result, the Capitan depositional system has been the focus of numerous stratigraphic, facies, paleontologic, and diagenetic studies, many of which have become classic works in their respective fields. Many facets of the Capitan system have been and continue to be controversial, and much excellent geological research is still being done on the Capitan reef and related units. An informal meeting of stratigraphers, sedimentologists, and paleontologists was held in Carlsbad, New Mexico, and the Guadalupe Mountains in October of 1996. This international conference was cosponsored by the University of Texas at Austin and Cambridge University in England. The purpose of the conference was to let Capitan researchers meet each other and to share

Abstract The spectacular mixed siliciclastic/carbonate exposures of the Guadalupe Mountains include 30 high-frequency sequences (HFS) that stack together to form six composite sequences (CS), the CS9 through CS14. These sequences include carbonate ramps and reef-rimmed platforms as well as basin-restricted lowstand sequences. The Capitan Formation represents the shelf-margin and slope facies tracts of the upper 12 HFS. The Capitan is examined in the context of this late Leonardian-Guadalupian ramp-to-rimmed-shelf system by focusing on extrinsic controls on platform development. Eustatic changes initiate and punctuate larger scale changes in platform evolution. Rapid shifts of large magnitude, such as the latest Leonardian (L7-L8 HFS) eustatic rise, are a first-order control on platform architecture and reef formation. The model for the late Permian eustatic curve based on the present stratigraphic framework suggests that by the time the Capitan was established, eustatic amplitudes were in the range of 20 m or less. This amplitude variation does not cause major shifts in the shelf-margin location but is sufficient to affect critical accommodation factors that influence reef depth and faunal composition. Antecedent topography, whether of tectonic, depositional, erosional, or compactional origin, is the critical parameter in controlling the timing and development of the Capitan and other buildups in the Leonardian-Guadalupian sequences, as well as the primary control driving the ramp-to- rimmed-shelf transition. The shelf-slope break, whether a ramp or rimmed shelf, is only one of numerous geometric parameters that can be used to describe the dynamic evolution of carbonate platforms. Changing styles of carbonate-platform progradation and aggradation, which are responses to changes in platform and basin accommodation and sediment-supply, can be captured using P/A (ratio of progradation to aggradation for a given chronostratigraphic unit) and SMP/A (shelf-margin progradation/aggradation ratio) ratios. P/A values > 25 are characteristic of ramps and lowstand wedges, whereas P/A values < 25 are indicative of either transgressive-dominated ramps or reef-rimmed margins. SMP/A values within the Capitan- equivalent sequences can be used to document the complex but systematic and predictable progradational-aggradational-progradational response of the shelf margin to changing base level. Within the high-frequency sequence framework, other analytical tools, including facies tract substitution and facies proportions, can be used to better constrain interpretations of the dynamic water-depth setting of the Capitan margin and factors controlling its position on the profile. This holistic approach, which draws on relationships from outer-shelf and shelf-crest facies tracts in the interpretation of the Capitan margin, demonstrates the power of a stratigraphic framework for sedimentologic analysis.

Abstract The Capitan depositional system was studied in the subsurface using seismic and well data from the northeastern Delaware basin. Seismic data of the Capitan depositional system show characteristics that include (1) a massive prograding reef/slope, (2) back-reef/shelf reflectors that dip and diverge basinward before disappearing into the massive reef, and (3) layered bottomset beds that thicken basinward by the addition of younger reflectors. A wireline log cross-section of nearby wells illustrates the stratigraphy in more detail than the seismic line. Basinward-dipping shelf strata are interbedded sandstones and carbonates that diverge and pass basinward into massive carbonate of the reef. Correlative markers within the massive reef are difficult to find. Slope carbonate beds thin and basinal siliciclastics thicken toward the basin. Bottomset beds in the basin consist of interbedded sandstones/siltstones and low-porosity carbonates. This subsurface stratigraphy is very similar to outcrop stratigraphy described in the Guadalupe Mountains. Lithologic differences between outcrops and their subsurface equivalents are due largely to variations in dolomitization and evaporite dissolution on outcrops. Distribution of porosity in the Capitan depositional system is closely related to depositional facies. Back-reef sandstones and some shelf carbonates adjacent to the reef have good porosity and moderate permeability, but porosity and permeability in those strata generally decrease landward. The subsurface Capitan reef has moderate porosity and high permeability and is a regional aquifer. Carbonate beds in the basin are generally not porous, but some basinal sandstones filling elongate channels have good porosity and moderate permeability. Hydrocarbons are not present in the Capitan reef because it does not occur in a setting that allows structural or stratigraphic closure and/or isolation from active meteoric aquifers. Many oil fields (10-400 million barrels recoverable) occur in back-reef equivalents of the Capitan reef, primarily the Seven Rivers and Yates formations, on the Northwestern Shelf and western edge of the Central Basin Platform. Those reservoirs are generally in stratigraphic or combination stratigraphic-structural traps, where porous and permeable sandstones pass up-dip into impermeable sandstones/siltstones, carbonates, and/or evaporites. Oil also occurs in channelized basinal sandstones equivalent to the reef, but the basinal fields have <5-30 million barrels of oil recoverable, and hence are generally smaller than those of the back-reef.

Abstract The apparent prevalence of meter-scale lithologic cyclicity in shallow-marine carbonate and mixed carbonate-siliciclastic platform strata has led to the widely accepted paradigm that such meter-scale cycles are the fundamental building blocks of platforms. This view commonly is challenged, however, on the grounds that lithologic cyclicity or “ideal lithologic cycles” are difficult to demonstrate from statistical evaluations (for example, Markov chain analysis) of facies successions. This issue is of major importance in determining the accuracy of high-resolution chrono- stratigraphic correlations using present sequence stratigraphic techniques. In this study we have used traditional and modified Markov chain analysis to evaluate the presence of cyclicity in backreef strata, the Seven Rivers Formation, of the Capitan reef. Traditional Markov analysis, similar to that used in previous studies, fails to verify the presence of cyclicity. Through the addition of stratigraphic data on the distribution of significant surfaces (subaerial exposure and hiatal) and the two-dimensional distribution of facies bodies, however, Markov analysis verifies the presence of cyclicity and even allows the statistical identification of potential “ideal cycles” in the system. Thus, meter-scale cyclicity does exist in this system, with the implication that previous studies may have failed to incorporate stratigraphic information necessary for an adequate evaluation of the existence of cyclicity.

Abstract The Guadalupian Series, with its type section in the Guadalupe Mountains, west Texas and New Mexico, has generally become the accepted worldwide standard for the Middle Permian. The Cisuralian of Russia is the accepted standard for the Early Permian; and the Lopingian of China has been accepted as the Late Permian standard. Most recently, the task of securing this threefold worldwide standard has been pursued by a host of workers in many specialties in order that no major gaps or overlaps are left for chronostratigraphic correlation. Following established international procedures for defining a chronostratigraphic boundary, the base of the Guadalupian Series is recognized at the stratigraphicaUy lowest occurrence of the conodont Jinogondolella nankingensis in a section recording continuous deposition in the middle of the El Centro Member of the Cutoff Formation, Roadian Stage, Guadalupe Mountains. This designation is in basic agreement with the ranges of important species of three other major faunal groups: the ammonoids, brachiopods, and fusulinaceans. Designation of a formal top of the Guadalupian Series has remained more elusive inasmuch as the final establishment of the base of the overlying Lopingian in China has remained in a rather unsettled state until recently. Enough is now known to insure that the Lopingian base will fall chronostratigraphicaily somewhere between the upper Lamar Limestone, Bell Canyon Formation, and the base of the Castile Formation of the Delaware Basin. This interval includes the so-called “post-Lamar beds” of the upper Bell Canyon Formation. As a means of equating the lithostratigraphy to the chronostratigraphy, we describe herein the Reef Trail Member of the upper Bell Canyon Formation by establishing its type section at McKittrick Canyon, Guadalupe Mountains, as a formal replacement name for the informal “post-Lamar beds.” We also identify the fusulinacean and conodont faunas from composited sections of Lamar Limestone and Reef Trail members, and extend their correlations biostratigraphically from the Guadalupes to the Apache and Glass mountains. The McKittrick Canyon Limestone is a new name given to the “Middle” Limestone (Brown, 1996), following his recognition that this limestone had for too long been misidentified as the McCombs Limestone. Finally, correlation of faunal zones established in the Delaware Basin from the three mountainous areas are extended globally in an attempt to reconcile chronostratigraphic problems.

Abstract The outer shelf sediments of the Capitan Reef Complex (Permian, Guadalupian) dip towards the basin at variable angles of up to 10°. High-resolution correlation of field logs from several canyons in the Guadalupe Mountains, New Mexico, has revealed a geometry that is most likely to have resulted from differential subsidence with the greatest rate of subsidence occurring at the shelf margin. During deposition of the upper Seven Rivers, Yates, and lower Tansill Formations, the reef accreted in several alternating phases of progradation and aggradation in response to fluctuating sea level. As the reef aggraded, loading of the underlying fore-reef and basin deposits gave rise to episodic compaction-induced differential subsidence, which tilted the outer shelf strata in a basinward direction. This process was controlled by the variable compactability of the forereef and basin lithologies as well as by the loading of the aggrading reef. Episodic differential compaction also influenced deposition of outer shelf sediments and induced small-scale relative sea-level changes near the shelf margin that were not experienced by the inner shelf. Mechanical failure of lithified outer shelf sediments occurred as the reef prograded over the compacting fore-reef and basin deposits. Water and liquefied sand from compacting basin sediments appear to have been forcefully injected upwards through mechanically initiated fractures, and the effects of dissolution are apparent along some fracture margins. The importance of differential compaction as a control on platform development must be recognized if the effects of sea level are to be properly understood.