Ooid Grainstone Research Articles

CHARACTERISATION OF EPEIRIC “LAYER‐CAKE” CARBONATE RESERVOIRS: UPPER MUSCHELKALK (MIDDLE TRIASSIC), THE NETHERLANDS

Upper Muschelkalk (Middle Triassic) carbonates produce natural gas at Coevorden field in the NE Netherlands. This is currently the only field which produces gas from this succession although several other prospects have been identified nearby. In order to help develop these hydrocarbons, this study proposes a facies and reservoir model of the Upper Muschelkalk in the NE Netherlands together with a regional framework intended to assist in further evaluation.Distribution of facies and reservoir properties of the Upper Muschelkalk carbonates in the NE Netherlands indicate deposition on a storm‐dominated epeiric ramp with a very low gradient. The predominantly muddy and marly lithofacies types in proximal and distal parts of the ramp gradually interfinger with a shoreline‐detached “shoal”‐like ooidal grainstone complex. The best reservoir quality (permeability up to 60 mD) is recognised within dolomitised peloid ooid grainstones. These are interpreted as high‐energy backshoal deposits. Reservoir quality decreases in the limestone‐dominated “shoal” facies and the muddier foreshoal facies. A four‐fold hierarchy of depositional cycles describes the systematic and thus predictable vertical variation in reservoir quality (permeability) and quantity (net‐to‐gross). High‐resolution correlation suggests that medium‐scale cycles (5 to 15 metres thick) can be traced for hundreds of kilometres. Small‐scale cycles (1 to 3 metres thick) are persistent for several tens of kilometres and have sheet‐like geometries. Individual reservoir units (several decimetres thick) appear to be laterally continuous over a maximum of a few kilometres although internal flow barriers might be expected. Mapping of Upper Muschelkalk thickness and facies has clearly defined backshoal, ”shoal and foreshoal facies belts with distinctly different reservoir characteristics. Typically, reservoir quality and quantity decrease with increasing thickness of the Upper Muschelkalk and the underlying Middle Muschelkalk halite. The systematic variations in thickness are apparently controlled by a combination of palaeogeography and palaeotectonics. The best reservoir quality and highest quantity is found on a palaeohigh characterised by a relatively thin Upper Muschelkalk succession and the absence of underlying halite. These features can also be recognised in seismic data.The results of this case study can also be applied in the integrated characterisation of similar epeiric carbonates constituting highly productive reservoirs in the Middle East, including the Khuff and Arab Formations.

Improving recovery from mature oil fields producing from carbonate reservoirs: Upper Jurassic Smackover Formation, Womack Hill field (eastern Gulf Coast, U.S.A.)

Reservoir characterization, modeling, and simulation were undertaken to improve production from Womack Hill field (eastern Gulf Coast, United States). This field produces oil from Upper Jurassic Smackover carbonate shoal reservoirs. These reservoirs occur in vertically stacked, heterogeneous depositional and porosity cycles. The cycles consist of lime mudstone and wackestone at the base and ooid grainstone at the top. Porosity has been enhanced through dissolution and dolomitization. Porosity is chiefly interparticle, solution-enlarged interparticle, grain moldic, intercrystalline dolomite, and vuggy pores. Dolostone pore systems and flow units have the highest reservoir potential. Petroleum-trapping mechanisms include a fault trap (footwall uplift with closure to the south against a major west-southeast–trending normal fault) in the western area, a footwall uplift trap associated with a possible southwest-northeast–trending normal fault in the south-central area, and a salt-cored anticline with four-way dip closure in the eastern area. Potential barriers to flow are present as a result of petrophysical differences among and within the cycles, as well as the presence of normal faulting. Reservoir performance analysis and simulation indicate that the unitized western area has less than 1 MMSTB of oil remaining to be recovered, and that the eastern area has 2–3 MMSTB of oil to be recovered. A field-scale reservoir management strategy that includes the drilling of infill wells in the eastern area of the field and perforating existing wells in stratigraphically higher porosity zones in the unitized western area is recommended for sustaining production from the Womack Hill field.

Dual-lateral horizontal wells successfully target bypassed pay in the San Andres Formation, Vacuum field, New Mexico

This case study of the San Andres Formation in the mature Vacuum field, New Mexico, shows how seismic data can be used to target bypassed pay with horizontal wells. These dual-lateral wells were the first attempt at horizontal development in the Vacuum San Andres field and in the San Andres Formation in New Mexico. The primary reservoir facies consist of ramp crest and outer ramp dolomitized peloidal packstones, skeletal and ooid grainstones, and fusulinid packstones. Vertical facies successions form numerous high-frequency carbonate depositional cycles and cycle sets that create distinct reservoir zones. Structural blocks created by small-scale faults (25 ft [8 m] vertical displacement) and bypassed pay located in thin depositional cycles were identified with three-dimensional compressional-wave seismic amplitude and coherency volumes and well data and targeted using medium-radius horizontal wells. Horizontal wells penetrated fault blocks and depositional cycles that were not adequately drained by existing vertical wells.Production curves show a significant increase in production from the horizontal wells and no interference with production from offset vertical wells. This suggests that the faults are partially sealing.

Anhydrite distribution within a shelf-margin carbonate reservoir:San Andres Formation, Vacuum Field, New Mexico, USA

Anhydrite cement causes significant heterogeneity within the San Andres reservoir of the Vacuum Field where it is associated with faults, fractures, karst zones and highly cemented dolomite intervals. The primary reservoir rocks within the Central Vacuum Unit of the Vacuum Field are dolomitized peloidal packstones, skeletal and ooid grainstones and fusulinid packstones. These rocks alternate with lower reservoir-quality dolomite intervals with variable amounts of anhydrite cement. Nodular and pore-filling fabric-selective anhydrite cements are common within the reservoir interval. Cross-plots of apparent matrix grain-density versus apparent matrix volumetric cross-section ( ρ maa – U maa cross-plots), combined with V p / V s seismic attributes and core data, provide insight into the vertical and lateral distribution of anhydrite within the San Andres reservoir. Anhydrite is generally concentrated in thin depositional cycles or intervals that are separated by relatively anhydrite-free cycles that exhibit relatively higher porosity and permeability. Lower V p / V s values correspond to higher percentages of anhydrite and are useful formapping anhydrite distribution. This evaluation of anhydrite distribution providesan estimate of the significant cementation-related heterogeneities within the reservoir that is useful for development-well planning and to target areas of bypassed pay.

Meteoric versus burial control on porosity evolution of the Smackover Formation

Whole-rock 13C, 18O, and Sr compositions of the uppermost ooid grainstones of the Smackover Formation at Black Creek field in Mississippi reveal characteristics of a meteoric system in this area. The trend in 13C signatures indicates that meteoric water acquired its low carbon values at a soil zone and was buffered completely with the host rock before entering the meteoric lens. Identical 18O data suggest that allochems stabilized at the same temperatures in waters of similar 18O compositions during early meteoric diagenesis; that is, calcitization continued to completion before the onset of burial. Sr concentrations reveal differential transformation mechanisms. Low Sr values in the phreatic zones are attributed to slow stabilization by exchange with the bulk solution. High Sr compositions in the vadose zone resulted from rapid, incremental transformation.Meteoric cementation was sparse; therefore, grainstones entered the burial realm with all of their porosity intact. Modification of porosity was a burial process and was directly related to the intensity of chemical compaction, which itself was controlled by three factors: (1) the degree of early cementation, (2) grain type, and (3) grain size. Early-cemented intervals experienced low to moderate compaction regardless of grain size or grain type. In the absence of early cementation, grain type and grain size were the dominant factors of porosity control. Coarse-grained intervals that were composed of micritic grains experienced less compaction than fine-grained, oolitic intervals.Average volumetric percentages of cements as determined by point counting of 63 samples are as follows: (1) marine and meteoric cements = 1.5%, (2) prebitumen calcite = 6.5%, (3) saddle dolomite = 1%, (4) postbitumen calcite = 4.3%, (5) pyrobitumen = 1%. Total cement (sum of 1–5), total burial cement (sum of 2–5), and total burial carbonate cement volumes (sum of 2–4) average 14, 13, and 12%, respectively. Out of an original porosity of 40%, 13 porosity units were lost by cementation, and 27 porosity units were reduced by compaction (mechanical and chemical).Present-day intergranular porosity of these grainstones is zero, whether they belonged to the vadose zone or the phreatic interval. Burial cementation and compaction destroyed all intergranular porosity, regardless of early diagenetic overprint or textural variables. The reservoir became compartmentalized during burial. Some intervals were active producers of calcium carbonate; others were passive recipients.

Geology of the Beltana Willemite Deposit, Flinders Ranges, South Australia

Beltana is a high-grade hypogene willemite deposit hosted in Lower Cambrian carbonate rocks in the Arrowie basin, northern Flinders Ranges, South Australia. It is situated adjacent to a major growth fault on the basin margin. Ooid grainstone units of the Woodendinna Dolomite and units of Archaeocyathid -rich Wilkawillina Limestone are the main host lithologies. Lead minerals in subeconomic quantities are also present in karstic collapse breccias surrounding the willemite orebodies. Mineralization is structurally controlled and associated with brecciation and extensive hematite-rich hydrothermal zincian dolomitization. Ore minerals include willemite and coronadite with lesser mimetite, hedyphane, and smithsonite. Late-stage gangue minerals include manganocalcite, dolomite, and minor quartz. The texture of willemite is heterogeneous, resulting from various depositional mechanisms such as partial to massive replacement of the carbonate host rock, internal sedimentation, fracture fill, brecciation, and vein fill. On the periphery of the deposit, smithsonite formed by weathering of willemite. Beltana is centered on a karstic collapse breccia that extends at least 100 m vertically, formed in part through corrosion by acidic ore solutions. The geochemical signature of the orebody includes high levels of Zn, Pb, Cd, As, and Mn. Notably, silver is absent from the deposit and sulfur concentrations are low (<20 ppm). Fluid inclusion studies yield a low minimum temperature range of ore deposition between 50° and 170°C. K-Ar dating of coronadite associated with the willemite orebody indicates an age of formation of ~ 435 ± 5 Ma. Premining resources of willemite ore were 850,000 t at 36 percent Zn, and an associated body of subeconomic lead contained more than 800,000 t at 8.9 percent Pb, 3.9 percent Zn and 1 percent As. The deposit has some similarities with Mississippi Valley-type deposits but differs in ore and alteration mineral assemblages.

Sequence Stratigraphy and Depositional Facies of Lower Ordovician Cyclic Carbonate Rocks, Southern Missouri, U.S.A.

ABSTRACT Lower Ordovician cyclic carbonate strata of southern Missouri were deposited in a warm, shallow, epeiric sea on a fully aggraded carbonate platform. Sedimentological characteristics distinguish the Jefferson City and Cotter dolomites from the underlying Gasconade and Roubidoux formations. Mixed carbonate-siliciclastic sedimentation characterizes the Roubidoux Formation, with sandstones accounting for up to 60% of sedimentation. The Gasconade, Jefferson City, and Cotter dolomites exhibit an increased occurrence of chalcedonic chert nodules in very similar shape and texture to the gypsum and anhydrite nodules common on modern sabkha supratidal flats. Casts of halite and ghosts of gypsum laths also exist in the Jefferson City and Cotter strata but are rarely found in the underlying units. Facies analysis from drill cores and outcrop sections provides the basis for identifying two major meter-scale cycle types. Type I cycles consist of algal stromatolites, tidal-flat laminites (mechanical and algal), ooid grainstones, wavy peloidal wackestones, and quartz sandstones interpreted as peritidal facies. They are the dominant components of the Roubidoux Formation, Jefferson City Dolomite, and Cotter Dolomite. Type II cycles consist mostly of subtidal facies such as strongly burrowed mudstone, thrombolite boundstone, and stromatolites. Type I cycles are thinner and represent highstand systems tracts, whereas the thicker type II cycles represent transgressive systems tracts and are dominant in the Gasconade Dolomite. The cycle stacking patterns, facies changes, and the intrabasinal correlatability of Fischer plots made from the widely spaced sections argue for a eustatic control on sea-level fluctuation on the platform. Interbasinal correlation with other North American basins is possible using biostratigraphic information and comparison of Fischer plots. Five Missouri sequences correlate with those described for other regions. The continent-wide uniformity in cycle stacking patterns indicates a primarily eustatic control on Lower Ordovician meter-scale cycle development. Regional tectonic and autocyclic controls probably account for general differences in sedimentation pattern among the correlated basins.

The upper Moscovian and basal Kasimovian (Pennsylvanian) of Central European Russia: Facies, subaerial exposures and depositional model

The type upper Moscovian-basal Kasimovian argillaceous-carbonate succession of central European Russia contains regionally traced cyclothem-bounded subaerial exposure horizons (geosols) represented mainly by rendzina-type palaeosols. Palaeokarst profiles occurrarely and grade laterally to palaeosols. Composite subaerial profiles divided by one or two thin marine beds are called ‘multiple geosols’. The biofaces structure of the studied succession is defined by brachiopod and fusulinoid biofaces. The heterogeneous Choristites biofacies characterizes openmarine intervals, which constitute the bulk of the succession, and is defined by presence of Choristites. The Meekella biofacies with monospecific concentrations of Meekella shells and extreme rarity of other brachiopods characterises restricted peritidal intervals which commonly constitute the terminal regressive parts of major cyclothems. Three fusulinoid biofacies defined by Baranova and Kabanov (2003) include restricted peritidal Biofacies 1 with only small fusulinoids Fusiella and Schubertella present, open shoal-to-subtidal Biofacies 2 with the richest fusulinoid assemblages, and the most offshore Biofacies 3 with less diverse, sometimes Hemifusulina-dominated, fusulinoid assemblages. Bioturbation patterns and ichnofossils allow recognition of deeper subtidal Zoophycos and shallower non-Zoophycos ichnofacies. Among the latter, shallowest subtidal facies are characterized by presence of thalassinoid burrows. Intertidal laminated lithofacies with suppressed bioturbation contain Skolithos burrows. Seventeen lithofacies are recognized. Terrestrial lithofacies include topclays (upper clayey palaeosol horizons) and aeolian grainstones. Restricted peritidal lithofacies include cross-stratified skeletal-peloidal grainstones, fine-grained laminated grainstones-mudstones, and lagoonal mudstones. Open shoal lithofacies include ooidal grainstones (rare, only in Podolskian) and coarse skeletal-peloidal grainstones. The open subtidal lithofacies include skeletal packstones-rudstones, shallow subtidal packstones-wackestones, deeper subtidal packstones-wackestones, Ivanovia boundstones (only in Podolskian), proximal tempestites, distal tempestites, and skeletal wackestones-mudstones. The fossiliferous shale lithofacies is a miscellaneous group of marine shales lacking distinct features of the above-listed lithofacies. Conglomerates of cyclothem bases that are regarded as early transgressive lithofacies are variable in their palaeoenvironmental position and are characterized by concentrated pebbles derived from palaeosol reworking. The shallowest subtidal lithofacies of fine packstones-grainstones is considered as transitional between open subtidal and restricted peritidal lithofacies. The origin of stratiform dolostones is shown to be early diagenetic in the subsurface. The depositional model involves a shallow and broad epicontinental ramp, where through water circulation prevented stratification of the water column and allowed large skeletal benthos to colonize the entire spectrum of depositional environments. Storms are thought to be the principal water-mixing agent. The anti-estuarine circulation carrying oxygenated waters down-ward may explain the lack of anoxic features in the deepest facies that may have formed below storm wave base.

Facies analysis of the Trypali carbonate unit (Upper Triassic) in central‐western Crete (Greece): an evaporite formation transformed into solution‐collapse breccias

AbstractThe Trypali carbonate unit (Upper Triassic), which crops out mainly in central‐western Crete, occurs between the parautochthonous series (Plattenkalk or Talea Ori‐Ida series, e.g. metamorphic Ionian series) and the Tripolis nappe (comprising the Tripolis carbonate series and including a basal Phyllite–Quartzite unit). It consists of interbedded dolomitic layers, represented principally by algally laminated peloidal mudstones, foraminiferal, peloidal and ooidal grainstones, as well as by fine‐grained detrital carbonate layers, in which coarse baroque dolomite crystals and dolomite nodules are dispersed. Baroque dolomite is present as pseudomorphs after evaporite crystals (nodules and rosettes), which grew penecontemporaneously by displacement and/or replacement of the host sediments (sabkha diagenesis). However, portions of the evaporites show evidence of resedimentation. Pre‐existing evaporites predominantly consisted of skeletal halite crystals that formed from fragmentation of pyramidal‐shaped hoppers, as well as of anhydrite nodules and rosettes (salt crusts). All microfacies are characteristic of peritidal depositional environments, such as sabkhas, tidal flats, shallow hypersaline lagoons, tidal bars and/or tidal channels. Along most horizons, the Trypali unit is strongly brecciated. These breccias are of solution‐collapse origin, forming after the removal of evaporite beds. Evaporite‐related diagenetic fabrics show that there was extensive dissolution and replacement of pre‐existing evaporites, which resulted in solution‐collapse of the carbonate beds. Evaporite replacement fabrics, including calcitized and silicified evaporite crystals, are present in cements in the carbonate breccias. Brecciation was a multistage process; it started in the Triassic, but was most active in the Tertiary, in association with uplift and ground‐water flow (telogenetic alteration). During late diagenesis, in zones of intense evaporite leaching and brecciation, solution‐collapse breccias were transformed to rauhwackes. The Trypali carbonate breccias (Trypali unit) are lithologically and texturally similar to the Triassic solution‐collapse breccias of the Ionian zone (continental Greece). The evaporites probably represent a major diapiric injection along the base of the parautochthonous series (metamorphic Ionian series) and also along the overthrust surface separating the parautochthonous series from the Tripolis nappe (Phyllite–Quartzite and Tripolis series). The injected evaporites were subsequently transformed into solution‐collapse breccias.

Facies variability in Lower Liassic carbonate successions of the Western Dinarides (Croatia)

In the Western Dinarides the Lower Liassic carbonates are underlain by Upper Triassic “Hauptdolomit”, whereas the first appearance of the foraminiferOrbitopsella praecursor (Gumbel) marks the beginning of the Middle Liassic. Their composition, observed at several localities in Western Croatia, shows a correlation of sedimentation events, which took place during Early Liassic on the Adriatic-Dinaridic carbonate platform. Facies variability is interpreted as result of autocyclic sedimentary processes on which the carbonate platform reacted by periodical oscillations of sea-bottom near the fair-weather wavebase. As a consequence, the Lower Liassic carbonate successions in the Dinarides is characterized by stacking of two main types of coarsening-upward parasequences: (1) the basal part of the Lower Liassic succession is represented by parasequences composed of mudstones or pelletal-bioclastic wackestones as their lower members, and peloidal-bioclastic wackestone/packstones to grain-stones as their upper members; and (2) the upper part of the Lower Liassic succession with parasequences consisting of mudstones or pelletal-bioclastic wackestones overlain by ooid grainstones. Judging from the composition of parasequences and thickness relations of their members, the first type is interpreted to comprise late transgressive system tract (ITST) and/or early highstand system tract (eHST), while the second type corresponds to a late highstand system tract (1HST) and/or early lowstand system tract (eLST) of a third-order sequence.

Porosity Loss, Fluid Flow, and Mass Transfer in Limestone Reservoirs: Application to the Upper Jurassic Smackover Formation, Mississippi1

Ooid grainstones of the Upper Jurassic Smack over Formation are buried to a depth of over 6 km and exposed to temperatures in excess of 200°C at Black Creek field, Mississippi. Combined effects of mechanical compaction, intergranular pressure solution, and cementation have reduced intergranular porosity of these ooid grainstones to 0%, indicating that porosity reduction has gone to completion. Modal analysis of 24 samples lacking preburial cements indicates that from the original 40% porosity, 13 porosity units (range: 4 to 21) were lost by mechanical compaction, 15 porosity units (range: 8 to 23) were reduced by intergranular pressure solution, and 12 porosity units (range: <1 to 26) were destroyed by cementation. Intergranular pressure solution caused an average of 28% (range: 15 to 51%) vertical shortening in Smackover ooid grainstones. Under ideal conditions, the 28% vertical shortening will generate enough calcium carbonate to precipitate 10% calcite cement. This is close to the measured volume of cements in Smackover grainstones (12%), suggesting that intergranular pressure solution provided most of the calcite cement present. No external sources of calcium carbonate are required. Fine-grained samples that experienced high degrees of intergranular pressure solution and contain only small amounts of cement occur at the top of the reservoir, whereas coarse-grained samples with abundant cement and low degrees of pressure solution occur in the middle and basal parts of the reservoir, suggesting that the fine-grained intervals acted as sources and the coarse-grained intervals as sinks for calcium carbonate. Mass transfer of pressure solution-generated calcium carbonate from the top of the unit to precipitation sites in the middle and basal parts of the reservoir could have occurred by a non-Rayleigh-type convection cell. Due to calcite's reverse solubility with respect to temperature, the cooling, upward-moving limb of the convection cell would become progressively more undersaturated, and hence able to transport more dissolved calcium carbonate released by intergranular pressure solution in the upper portion of the reservoir. Pore fluids descending in the downward-moving limb of the cell would become progressively more supersaturated, and calcium carbonate would tend to precipitate as cement in the middle and basal parts of the reservoir as fluids become progressively hotter.

Hydrocarbon Potential of the Mississippian Carbonate Platform, Bechar Basin, Algerian Sahara

The western Bechar basin contains a 1500-2000-m-thick Mississippian succession of carbonates consisting of superposed shoaling-upward sequences, whose porosity or diagenesis have never been investigated to establish the reservoir potential of these carbonates. In this paper, we present a diagenetic and porosity evolution of the Visean part of the Mississippian carbonate platform that may lead to the development of new exploration fairways in the region. A diagenetic model is proposed in which ooid and crinoid grainstones may have a reservoir potential if hydrocarbon migration occurred before the end of the Serpukhovian, prior to complete occlusion of porosity.

99/02347 Infrastructure and management of mines. Expert system for predicting coal seam splits

The Xinji and Zhushadong formations are the oldest record of Cambrian deposits in the southwestern margin of the North China Platform. The two lithostratigraphic units are carbonate-dominated in lithology and are Cambrian Series 2 in age. The basal unit, the Xinji Formation yields diverse skeletal fossils, including sponge spicules, chancelloriid sclerites, hyoliths, micromolluscs, trilobites, echinoderm ossicles, cambroclaves, tianzhushanellids, protoconodonts and other small shelly fossils. The fossil assemblage, as the earliest fauna of the Cambrian in North China, provides an important window to investigate the Cambrian explosion in the region and contains fossil elements with biostratigraphical significance. Seven microfacies types of the Xinji and Zhushadong formations are recognized: (1) phosphorized and selectively dolomitized bioclastic grainstone, (2) dolomitized ooid grainstone, (3) dolomitized peloid wackstone, (4) lime mudstone preserved as dolomitic microsparite, (5) siliciclastic wackstone, (6) calcareous siltstone, and (7) breccia. The different microfacies are traced to three successive depositional environments, beginning with a bioclastic shoal, then a mixed-depositional back-shoal and finally an oolitic shoal. The system of the shoal and back-shoal is interpreted as representing the mixed siliciclastic-carbonate deposition during the initial transgression of the North China Platform as well as the early development of an epeiric environment.

99/02355 State-of-the-art and potential for development of raw mineral resources in northern Kazakhstan

The Xinji and Zhushadong formations are the oldest record of Cambrian deposits in the southwestern margin of the North China Platform. The two lithostratigraphic units are carbonate-dominated in lithology and are Cambrian Series 2 in age. The basal unit, the Xinji Formation yields diverse skeletal fossils, including sponge spicules, chancelloriid sclerites, hyoliths, micromolluscs, trilobites, echinoderm ossicles, cambroclaves, tianzhushanellids, protoconodonts and other small shelly fossils. The fossil assemblage, as the earliest fauna of the Cambrian in North China, provides an important window to investigate the Cambrian explosion in the region and contains fossil elements with biostratigraphical significance. Seven microfacies types of the Xinji and Zhushadong formations are recognized: (1) phosphorized and selectively dolomitized bioclastic grainstone, (2) dolomitized ooid grainstone, (3) dolomitized peloid wackstone, (4) lime mudstone preserved as dolomitic microsparite, (5) siliciclastic wackstone, (6) calcareous siltstone, and (7) breccia. The different microfacies are traced to three successive depositional environments, beginning with a bioclastic shoal, then a mixed-depositional back-shoal and finally an oolitic shoal. The system of the shoal and back-shoal is interpreted as representing the mixed siliciclastic-carbonate deposition during the initial transgression of the North China Platform as well as the early development of an epeiric environment.

Blackriveran carbonates from the subsurface of the Lake Simcoe area, southern Ontario: stratigraphy and sedimentology of a low-energy carbonate ramp

The lowest Upper Ordovician (Blackriveran) strata in the Lake Simcoe area represent the base of a major transgressive sequence of peritidal carbonates and siliciclastics. From detailed petrographic study of 12 cores that penetrate, in ascending order, the Shadow Lake, Gull River, and lower Bobcaygeon formations, seven distinctive lithofacies are recognized: (1) sandstone, siltstone, and shale; (2) dolostone and calcareous dolostone; (3) lime mudstone; (4) peloidal bioclastic wackestone and packstone; (5) peloidal bioclastic grainstone; (6) ooid grainstone; and (7) intraclastic bioclastic wackestone and packstone. Metre-scale cycles, like those reported from numerous other peritidal deposits, are apparently absent in this succession. In addition, these strata are commonly interrupted by a variety of hardgrounds. The carbonate strata can be grouped into four lithofacies associations that represent deposition in generally low energy, tropical paleoenvironments along a southward-facing, homoclinal ramp that was frequently influenced by storms. Rapid facies changes along depositional strike likely resulted from a combination of an irregular basement topography and coastline geometry. The coexistence of Heterozoan and Photozoan carbonates in the lower Bobcaygeon Formation probably reflects the increasing incorporation of cooler oceanic waters as the ramp gradually deepened during the Tippecanoe transgression rather than the onset of more temperate climatic conditions.

The isotopic composition and diagenetic history of pleistocene carbonates, North Qatar

Field, petrographic, and geochemical data for Pleistocene oolitic carbonates from North Qatar suggest that the ooids formed in a marine environment separating the open Gulf from the coastal sabkhas. Locally, the more homogeneous granulometry and abundant cross-bedding of these rocks reveal an aeolian depositional environment. Petrographic examination shows that theserrocks are ooid grainstones with low-Mg calcite cements. The sequence of petrographic features suggests a progression of diagenetic fluids from more marine to more meteoric. Early marine cementation was followed by partial dissolution of the ooids and cements. Dissolution and cementation processes redistributed the primary porosity. After mineralogical stabilization was completed, subsequent meteoric flow caused very localized cementation. The final diagenetic phase precipitated was a low-Mg calcite spar cement, representing diagenesis in a fresh-water zone. There is no indication for the presence of dolomites. This sequence of diagenetic features was the result of a fall in eustatic sea-level following deposition.

Jurassic aeolian oolite on a palaeohigh in the Sundance Sea, Bighorn Basin, Wyoming

ABSTRACTAeolian limestones are widespread in the Quaternary record and have been identified in outcrops and cores of late Palaeozoic strata. These rocks have been interpreted as a low latitude signal of glacio‐eustatic sea level fluctuations and have not been previously reported from the Mesozoic or from other episodes of earth history generally believed to have been non‐glacial. Numerous lenticular bodies of cross‐stratified oolite lie near the contact between the lower and upper members of the mudstone‐dominated lower Sundance Formation (Middle and Upper Jurassic) in the Bighorn Basin of north‐central Wyoming, USA. The lenses, up to 12 m thick, contain sedimentary structures diagnostic of aeolian deposition. Inversely graded laminae within thick sets of cross‐strata were deposited by climbing wind ripples. Adhesion structures and evenly dispersed lag granules are present in flat‐bedded strata at the bases of several of the oolite bodies. Thin sections reveal abundant intergranular micrite of vadose origin. The lenses appear to represent virtually intact, isolated aeolian bedforms that migrated across a nearly sand‐free deflation surface. When the Sundance Sea transgressed the dunes, a thin (&lt;1 m thick), wave‐rippled, oolite veneer formed on the upper surface of the aeolianite. Previous workers, primarily on the basis of sedimentary structures in the veneer, interpreted the oolite lenses as tidal sand bodies.The dunes provide clear evidence of widespread subaerial exposure on the crest and north flank of the Sheridan Arch. This structural high was delineated by previous workers who demonstrated thinning of pre‐upper‐Sundance Formation strata and localized development of ooid shoals. Ooids that formed in shoals on the windward (southern) side of the palaeohigh were exposed and deflated during lowstand. Thin, scour‐filling ooid grainstone lenses that crop out in the southern part of the study area represent remnants of the marine beds that sourced the aeolianites. Farther north (down‐wind), oolitic dunes prograded over thinly laminated lagoonal silts. When relative sea level began to rise, the uncemented dunes were buried under fine‐grained marine sediment as the lee side of a low‐relief island was inundated.

Stratigraphlc Hierarchy and Cycle Stacking, Facies Distribution, and Interwell-Scale Heterogenelty: Grayburg Formation, New Mexico: ABSTRACT

The Grayburg Formation (middle Guadaluplan) of southeastern New Mexico is a mixed carbonate-siliciclastic succession that represents predominantly shallow water sedimentation on the Northwest Shelf of the Delaware Basin. Exceptional Grayburg outcrops in the Brokeoff Mountains permit high-resolution correlation and mapping of interwell-scale facies complexity both along depositional dip and across strike. Three orders of stratigraphic hierarchy are recognized using vertical facies successions and transgressive-regressive relationships expressed along a dip cross section. This stratigraphic organization cannot be fully defined from a vertical section. Cycles (3-30 ft thick) are the smallest-scale, upward-shoaling facies successions that can be correlated across facies tracts and constitute the basic chronostratigraphic unit. Cycles are organized into transgressive-regressive cycle sets (15-40 ft thick). High-frequency sequences (100-160 ft thick) are unconformity-bound successions composed of 10 to 20 cycles arranged into transgressive and highstand systems tracts. One-dimensional cycle thickness stacking patterns do not reflect the long-term accommodation trends imposed by high-frequency sequences because most cycles did not fully aggrade to sea level. Interwell-scale heterogeneity (hundreds of ft) is high in transgressive ooid grainstones, which formed dip-elongate tidal channels and shoals. Highstand ooid grainstones formed strike-elongate shoals that are laterally continuous along dip and across strike (thousands of feet) due tomore » stacking and amalgamation of grainstone bodies in an accommodation-limited highstand setting. The well-defined relationship between stratigraphic hierarchy and facies composition and heterogeneity can be used to improve correlations and better predict lateral dimensions of reservoir facies.« less

Lower Cisco Formation (Pennsylvanian- Virgilian) Paleokarst, Wolf Flat Field, Palo Duro Basin, Texas: ABSTRACT

Wolf Flat field produces from karstified Lower Cisco Formation at the rimmed northeast shelf margin. Mounds, peripheral skeletal debris, and oolitic facies form the vertical/lateral succession. Mounds are composed of crinoids, bryozoans, corals and algae. Skeletal packstone/grainstones surround mounds, With back-mound wackestone. Ooid grainstone formed in high-energy passages onto the shelf and cap shallowing-upward cycles. Karstification resulted from two 100+ ft. (30+ m) sea level drops, that dropped in a step-by-step manner to form [approximately]20 porous intervals that extend through all facies and lithologies in the field. Aragonitic mound and skeletal grainstone/wackestones experienced intense dissolution. Calcitic ooid grainstones dissolved less and form cave roofs and floors. Dissolution was in two phases: first, small vertically oriented pipes formed, resembling a [open quotes]swiss cheese[close quotes] texture; second, was intense lateral dissolution that created a [open quotes]sponge[close quotes] texture, with porosities up to 60-70%. Eventually, collapse breccias formed, with some clasts transported in the karst system. Porosity was reduced by vadose silt, breccia clasts, speleothem cements, transgressive shale infilling, and burial saddle dolomite. Portions of the field nearest the rimmed margin were dolomitized. [delta][sup 18]O is within the range of early meteoric diagenesis and [delta][sup 13]C is negative and becomes more negative towardmore » the exposure surface.« less

High-Resolution Sequence Stratigraphy of Late Mississippian Carbonates in the Appalachian Basin, Implications for Compartmentalization of Reservoir Facies: ABSTRACT

The Late Mississippian Newman/Greenbrier carbonates were deposited in the Appalachian foreland basin whose depocenter lay to the south and east of Kentucky, West Virginia and Virginia. Over 50 closely spaced detailed measured sections along with numerous wireline logs, biostratigraphic data, and lithologic markers are used to construct detailed facies cross-sections. In the Newman limestone along the Cincinnati Arch, four sequences bounded by regional unconformities can be recognized. The lower three sequences contain one to eight oolite bearing disconformity bounded parasequences. Parasequences within sequences one to three, progressively onlap the Waverly and Cincinnati arches. These are dominated by shoal water ooid grainstone and lagoonal skeletal wackestone/mudstone facies, while eolianite quartzose peloidal grainstone facies are restricted to the lower two sequences. Sequence four is thicker and capped by a disconformity, but is internally conformable. It contains thick oolite units in the lower part (up to 10 m), open ramp skeletal packstone and shale (2nd order maximum flooding of the studied interval). In the much thicker foreland basin sections in West Virginia, four sequences also can be defined. The lowstand deposits are characterized by red beds up-dip, locally thickened tidal flat facies down-dip, and thin grainstone tongues extending into the slope/basin facies. Withinmore » the sequences, parasequences lack bounding disconformities, and are dominated by open ramp skeletal packstone and shoal water ooid grainstone facies. Eolianite facies are common in landward parts of the lower two sequences. The complex regional distribution and vertical compartmentalization of these multilateral oolitic reservoirs in both areas on this tropical ramp reflect tidal bar morphologies, differential regional subsidence patterns, coupled with 4th order moderate amplitude eustacy.« less