Abstract Ooid grainstones in Upper Jurassic carbonates in East Texas have been hydrocarbon exploration targets for some thirty years. Shoaling-upward sequences through mudstones/wackestones and packstones culminate in well-sorted ooid grainstones in many cores. A succession of several shoaling-upward sequences, each averaging 12-18 m (40-60 ft) thick, is commonly developed in upper Smackover carbonates around the basin margin where it produces large cumulative thicknesses of grainstones. In contrast, basinal carbonates contain fewer and thinner grainstone sequences. Porosity and permeability within the grainstones are controlled by their diagenetic history. Along the northern flank of the East Texas Salt Basin it is possible to trace a lateral continuation of the northern, transitional and southern diagenetic zones recognized in Louisiana and Arkansas. Oomoldic porosity is well developed in the northern zone and patchily present in the transitional zone. Early complete dolomitization is prevalent at the top of the Upper Smackover in the northern and transitional zones; late dolomitization is minimal. Little or no dolomitization occurs in the southern zone. On the western flank of the basin early dolomitization of upper Smackover carbonates is less common but some percentage of later, post-compaction dolomite is present in both updip and downdip carbonates. In updip areas fracturing is common, with fracture fill calcite cement precipitation. Little or no fracturing is observed in the downdip carbonates. Diagenesis of the more basinward carbonates is comparable to that of the southern diagenetic zone of Louisiana-Arkansas, with the exception of the presence of some replacive baroque dolomite. In addition, there is no evidence of late, solution-enhanced porosity development in East Texas. Porosities and permeabilities are highest in the dolomitized updip Smackover grainstones; oomoldic limestones have good porosity but often low permeability. In general, downdip and basinward grainstones possess lower porosities and permeabilities.

ABSTRACT Production from the Bryan’s Mill, Frost, and Carbondale Fields in the Bryan’s Mill area is from Jurassic Upper Smackover dolomitized ooid grainstones. Within the Upper Smackover, three coarsening-upwards sequences culminating in ooid grainstones extend across the area. Shortly after deposition, leaching by meteoric fluids resulted in development of oomoldic porosity in many of the ooid grainstones; other carbonate facies were little affected. During early diagenesis, dolomitization enhanced permeability by preserving existing porosities and by generating an effective intercrystalline porosity. After dolomitization, brittle compaction during burial further increased permeability by interconnecting oomolds. The center of the Bryan’s Mill area was a positive feature during late Smackover deposition. Faulting during Buckner deposition resulted in a series of southeasterly tilted fault blocks, perhaps due to regeneration of basement structures. Later, post-Hosston faulting had an east-west trend and was possibly associated with doming of the sediments over two highs. There is little evidence of Jurassic or Early Cretaceous movement on the east-west trending faults. Hydrocarbon production in the area depends on a combination of structural, stratigraphic, and diagenetic factors. Although the grainstones are continuous across the northernmost structural high, production is limited to the dolomitized areas. Thus, diagenesis, and in particular dolomitization, is the ultimate control on production in the area.

Abstract The hydrocarbon potential of the Smackover Formation in South Texas is virtually untested. Much of the section penetrated is impermeable; however, reservoirs as thick as 33 feet (10 m), with porosity ranging from 4 to 26 percent and permeabilities ranging from 0.1 to 6.5 millidarcys, have been cored at depths below 18,000 feet (5,500m). Nearly complete dolomitization has resulted in the development of intercrystalline porosity in innershelf wackestones and shoal-complex grainstones. In addition, some grainstones have subsurface-derived oomoldic porosity. In the grainstone facies, four general stages of diagenesis affected porosity: Stage 1 (marine-phreatic environment)-precipitation of an isopachous carbonate cement and extensive grain micritization; Stage 2 (shallow-meteoric environment)-precipitation of very coarse-crystalline syntaxial calcite and fine-crystalline equant calcite, dissolution of aragonitic skeletal grains, and incipient solution-compaction; Stage 3 (regional fluid-mixing environment)-intrapore precipitation of and grain/matrix replacement by fine- to medium-crystalline rhombic dolomite; and Stage 4 (subsurface environment associated with basinal fluid expulsion)-dissolution of ooids and dolomite, microstylolitization by solution compaction resulting from decarboxylation of kerogen, and precipitation of coarse-crystalline calcite and baroque dolomite. The magnitude of each general diagenetic stage varies regionally. Reprinted with permission from Transactions of the Gulf Coast Association of Geological Societies, v. 31, p. 339-346, 1981 . Porosity development in dolomitized wackestones is sporadic but within grainstones porosity is in part facies related. Hydraulically equivalent oolitic grainstone and oolitic quartzarenite facies are commonly dolomitized and have associated porosity development. In addition, dolomitization of oolitic facies is more extensive in the updip half of the shoal-complex trend. These facies/diagenetic relationships can be used as an exploration tool in the Smackover Formation in South Texas.

Abstract A sequence stratigraphic framework has been established for the Middle Pennsylvanian (Desmoinesian) section in southeastern Utah using: 1) surface exposures at Honaker Trail, Raplee Anticline, and Eight Foot Rapids located 25 to 40 miles (40-64 km) west of the Aneth field, 2) core and well logs in SE Utah, S W Colorado, NW New Mexico, and NE Arizona, and 3) regional seismic. Bounding discontinuities (sequence boundaries and maximum flooding surfaces) have been correlated over several thousand square miles in the Four Corners region. Systems tracts of 3rd-order composite sequences (0.5-5.0 m.y.) are comprised of 4th-order sequences (0.1-0.5 m.y.) and 5th-order depositional cycles or parasequences (0.01-0.1 m.y.). Nineteen discrete and mappable high-frequency depositional cycles are recognized within three fourth-order depositional sequences of the Desert Creek and lower Ismay section (Middle Desmoinesian) at the McElmo Creek Unit of the Aneth field, southeastern Utah. These simple sequences stack into parts of two third-order sequence sets. Facies analysis of 12,000 feet (3,660 m) of core was tied into the chronostratigraphic framework to constrain correlation of high-frequency depositional cycles (parasequences). Facies mapping within parasequences permitted 1) prediction of porous and permeable facies and 2) characterization of variability in reservoir pore systems. Syndepositional dolomitization and dissolution in peritidal facies, at shoal crests of parasequences and at sequence boundaries caused modification of reservoir character. Since discovery of the Aneth field (1956), 370 million barrels of oil (∼ 1.3 billion barrels original oil in place) have been produced from Middle Pennsylvanian carbonates of the Ismay and Desert Creek intervals. Stratified reservoirs occur within lowstand, transgressive, and highstand systems tracts. Siltstone, dolostone, and evaporites form lowstand wedges that were deposited 150 feet below the crest of the Aneth carbonate platform. Porous dolomudstone and dolowackestone are productive where they onlap and pinch out against the Aneth carbonate platform and are isolated from reservoirs on the platform. Within transgressive systems tracts, lagoonal/tidal flat dolomudstone/wackestone compose parasequences and display intercrystalline and solution-enhanced secondary porosity. Core analysis and production/performance data indicate that significant fluid pathways are developed in dolomudstone deposited on the carbonate platform on paleodepositional highs. In the lower Desert Creek, initial parasequences of the highstand systems tract represent a time of mound building and platform development as a result of coalescing biologic communities of phylloid algae. Interparticle and shelter porosity dominate. Subsequent parasequences within the lower Desert Creek highstand systems tract are composed of skeletal and nonskeletal wackestone to grainstone. Porosity is developed on paleodepositional highs at the top of parasequences where shoal water facies have preserved primary pore systems that are secondarily enhanced by leaching of less stable carbonate minerals by meteoric water. Reservoirs dominated by primary pore systems provide the best long term production and account for the majority of oil produced in McElmo Creek. In the upper Desert Creek highstand systems tract, ooid/peloid grainstones aggrade and prograde to fill available depositional space. Hydrocarbons are produced on the platform and along the platform to basin margin from carbonate sand sheets and allochthonous debris aprons. Grainstone debris aprons may also be deposited during early lowstand conditions of the lower Ismay sequence. On the platform meteoric diagenesis resulted in the formation of oomoldic porosity in ooid grainstone deposits beneath the upper Desert Creek sequence boundary. Within moldic pore systems storage capacity is favorable, but permeability is low, generally less than 1 md. Facies composed dominantly of moldic porosity in the absence of significant primary porosity are poor reservoirs.

Abstract: Many carbonate reservoirs are located on top, or down the flanks, of extant structural highs or syndepositional palaeo-highs. This study examines diagenesis in Pennsylvanian oolitic reservoirs close to the crest and down the flank of a long-lived anticline. It illustrates that the position of the best reservoir quality shifted back and forth during successive diagenetic events. Cement stratigraphy shows that early diagenesis did not enhance reservoir character significantly. Most oomoldic porosity formed penecontemporaneously with compaction. Fluid-inclusion and stable isotope data indicate that late cements precipitated during burial conditions by refluxing brines and later hydrothermal fluids. After initial burial, greater permeability existed downdip, where smaller amounts of early meteoric cement allowed for compaction. Subsequent reflux cementation initially degraded downdip reservoirs preferentially and then progressed updip, resulting in relatively uniform reservoir porosity. Later hydrothermal events are most important in affecting the distribution of the highest quality present-day reservoir. Highest porosity is preserved in wells down the flanks of the structure, where hydrothermal cements are not as prevalent. Understanding the effect of diagenesis on location of the best reservoir in relation to palaeotopographical and structural highs allows for the prediction of reservoir quality using seismic and mapping data typically available in the subsurface.

Abstract A generalized stratigraphic framework for the upper Jurassic is suggested, in which the name Haynesville Formation is utilized, and the name Lower Cotton Valley Lime is suppressed in favor of the Gilmer, where appropriate. Eustatic sea-level fluctuations have resulted in patterns of sedimentation common to upper Jurassic sequences across the entire northern Gulf of Mexico region and hence indicate the wide applicability of this generalized stratigraphic framework. The presently accepted model of the Smackover-Haynesville sedimentation must be modified to take into account sea-level fluctuations, subsidence, and sediment availability. The model developed here is simply one of lower Smackover basin fill during a rapid transgressive phase and upper Smackover regional shoaling during a sea-level stillstand in which sedimentation was in equilibrium with subsidence. The lower and upper Smackover are not necessarily time equivalent, but represent two separate sedimentologic sea-level regimens. The Haynesville Formation is thought to be a separate sedimentologic package that was deposited during the next sea-level rise; the Gilmer Limestone formed a shelf-margin barrier behind which the lagoonal Buckner evaporites were deposited. The evaporites graded landward into quartzose clastics. Predictable regional porosity patterns have developed in the Smackover-Haynesville, in response to early diagenetic overprints, controlled largely by eustatic sea-level subsidence interactions. These patterns include: updip oomoldic porosity in a regional meteoric-water system developed during the upper Smackover sea-level stillstand; downdip porosity preservation under marine conditions along the shelf margin; regional dolomitization associated with reflux of evaporitive waters from the Buckner lagoon behind the Gilmer shelf-margin barrier. Structural hydrocarbon traps associated with salt movement are the most common type of Late Jurassic trap. Buckner evaporites or Haynesville shales usually form the seals in the Late Jurassic reservoirs. Jurassic source rocks are probably lower Smackover limestones and Norphlet shales. Sourcing is generally local with migration into updip areas, particularly where regional dolomitization has occurred. The time of migration, which is a key factor in a viable Smackover exploration strategy, varies across the Gulf in response to the subsidence history of each individual basin. Future Jurassic exploration will center on south Texas, the Gilmer shelf margin, and the updip Smackover along the bounding graben fault systems. Most production will be gas, but some oil should occur along the updip faulted fairways.

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.