Abstract The Horseshoe atoll is an arcuate chain of reef mounds, composed of mixed types of bioclastic debris, that accumulated in the interior part of the developing intracratonic Midland basin during late Paleozoic time. The atoll is 175 mi (282 km) long and locally is almost 3,000 ft (914 m) thick. The reef environment was established early in basin history and retained because of the limited amount of terrigenous clastic material transported to the basin interior. About 1,800 ft (549 m) of limestone accumulated during the Pennsylvanian, and primary dips commonly as great as 8° developed along the margins of the atoll. During earliest Permian time the reef was restricted to the southwest side of the complex, where more than 1,100 ft (335 m) of additional limestone accumulated before death of the reef. Reef mounds were buried by prograding Early Permian terrigenous clastic material which progressively covered the atoll from northeast to southwest. Westward tilting of the reef complex after burial elevated Pennsylvanian mounds along the east side of the atoll 1,400 ft (428 m) higher than Permian mounds along the southwest side. The updip migration of hydrocarbons was uninhibited in the lower part of the reef, and most mounds along the eastern half of the atoll are full to the spill point. Some mounds along the trend are not productive because of Wolfcamp sandstone contacts with the upper surface of the mounds. Fifteen individual fields, containing 2.54 billion bbl of recoverable oil, are present along the crest of the atoll. The Scurry field is the giant of the trend. It includes approximately 73,000 productive acres (295 km 2 ), has a maximum oil column of 765 ft (233 m), and ultimately will yield 1.72 billion bbl. This field has no active wafer drive, so pressure mainte- ance was initiated early to achieve maximum efficient recovery. Scurry field has produced 521 million of the 857 million bbl of oil produced from reef rocks along the crest of the atoll. Scurry was discovered in 1948 with reflection-seismic methods, but only a small part of the field was mapped before the drilling of the discovery well.

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 The Sacroc Unit of the Kelly-Snyder field, located on the eastern portion of the Pennsylvanian Horseshoe Atoll, northern Midland Basin, has produced over a billion barrels of hydrocarbons since 1948. Cross sections based on core descriptions and supplemented by electric log correlations allow reconstruction of the ancient facies tract across the middle of the field. Updip dolomitic fenestral lime mudstone is interpreted as a tidal-flat deposit. Pellets, small intraclasts, and forams were concentrated by currents to form well-sorted grainstone bodies in tidal creeks. A wide phylloid algal zone interfingered with other proximal-shelf deposits just downdip of these tidal flat deposits. Phylloid algal mounds contained abundant Eugonophyllum , palaeotextulariid forams, Apterrinella, Bradyina, Globivalvulina, Tetraxis, Tubertina and unidentified tubular forams. Local binding and encrustation by blue-green algae and small forams was common in the phylloid algal mounds. Luxuriant algal mounds grew behind discontinuous, wave-fronting sponge-algal-bryozoan mounds. These latter mounds grew into waters as much as sixty feet deep along the mid-section of the Kelly-Snyder field. Finger-sized calcareous sponges were characteristic of the sponge-algal-bryozoan mounds but did not directly bind or stabilize the substrate. Binding by blue-green algae, fenestrate and massive bryozoans, and Tubiphytes was identified in thin sections by cathodoluminescence. Oolite shoals flanked and possibly overlapped shoaling portions of sponge-algal-bryozoan mounds in the northeastern part of the field. Sponge-algal-bryozoan mound-derived debris and ciasts accumulated as submarine debris flows basinward of the buildups and were interbedded with, basin-margin shaly lime muds. Subaerial exposure and concomitant meteoric diagenesis left excellent secondary porosity. Oomoldic porosity locally exceeds 20%. Leaching of algal thaiIi and skeletal grains provided permeabilities of 10 to 25 md or more.

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.