Ramp Setting Research Articles

Capturing and modelling metre‐scale spatial facies heterogeneity in a Jurassic ramp setting (Central High Atlas, Morocco)

AbstractEach simulation algorithm, including Truncated Gaussian Simulation, Sequential Indicator Simulation and Indicator Kriging is characterized by different operating modes, which variably influence the facies proportion, distribution and association of digital outcrop models, as shown in clastic sediments. A detailed study of carbonate heterogeneity is then crucial to understanding these differences and providing rules for carbonate modelling. Through a continuous exposure of Bajocian carbonate strata, a study window (320 m long, 190 m wide and 30 m thick) was investigated and metre‐scale lithofacies heterogeneity was captured and modelled using closely‐spaced sections. Ten lithofacies, deposited in a shallow‐water carbonate‐dominated ramp, were recognized and their dimensions and associations were documented. Field data, including height sections, were georeferenced and input into the model. Four models were built in the present study. Model A used all sections and Truncated Gaussian Simulation during the stochastic simulation. For the three other models, Model B was generated using Truncated Gaussian Simulation as for Model A, Model C was generated using Sequential Indicator Simulation and Model D was generated using Indicator Kriging. These three additional models were built by removing two out of eight sections from data input. The removal of sections allows direct insights on geological uncertainties at inter‐well spacings by comparing modelled and described sections. Other quantitative and qualitative comparisons were carried out between models to understand the advantages/disadvantages of each algorithm. Model A is used as the base case. Indicator Kriging (Model D) simplifies the facies distribution by assigning continuous geological bodies of the most abundant lithofacies to each zone. Sequential Indicator Simulation (Model C) is confident to conserve facies proportion when geological heterogeneity is complex. The use of trend with Truncated Gaussian Simulation is a powerful tool for modelling well‐defined spatial facies relationships. However, in shallow‐water carbonate, facies can coexist and their association can change through time and space. The present study shows that the scale of modelling (depositional environment or lithofacies) involves specific simulation constraints on shallow‐water carbonate modelling methods.

Biochemostratigraphy of the Upper Frasnian in the Namur–Dinant Basin, Belgium: Implications for a global Frasnian–Famennian pre-event

The Upper Frasnian sequence of the Namur–Dinant Basin in southern Belgium consists of mixed siliclastic–carbonate succession of a ramp setting, where the sequence spans the rhenana– linguiformis conodont zones. Earlier studies investigated the chemostratigraphic variations during the Frasnian–Famennian event, but little has been yet known about the nature of the counterpart variations that immediately preceded that time interval. Despite the scarcity of well-preserved brachiopods, sixty-one calcitic shells were collected mainly from beds of the Neuville and Les Valisettes formations (Lower and Upper rhenana zones), to investigate biochemostratigraphic profiles of oxygen-, carbon-isotope and rare earth element (REE) variations of the time interval immediately before the Frasnian–Famennian boundary. The δ 18 O and δ 13 C values of the well-preserved shells range from − 9.5 to − 5.6‰ VPDB (− 7.7 ± 1.1, n = 33) and from − 1.8 to 3.8‰ VPDB (1.1 ± 1.7, n = 33), respectively, which are within the documented global values. The C- and O-isotope profiles exhibit parallel shifts, particularly at the top of the Neuville Formation (top of the Lower rhenana Zone), which are associated with a sea-level rise and shrinkage in the brachiopod community. Also, the Th/U (0.9 ± 0.6, n = 16) and Ce/Ce* (2.2 ± 0.5, n = 16) ratios suggest deposition under reducing conditions consistent with sea transgression.

Evolution of Cretaceous to Eocene alluvial and carbonate platform sequences in central and south Jordan

ABSTRACT The Cretaceous to Eocene succession in central and south Jordan is characterised by passive continental margin depositional sequences, which pass upward from alluvial/paralic to carbonate shelf and pelagic ramp settings. Detailed section logging and outcrop mapping have produced robust lithostratigraphic and lithofacies schemes that can be correlated throughout the region and in the subsurface. These schemes are set in a sequence-stratigraphic context in relation to the evolution sedimentation on the Arabian and Levant plates. Three major megasequences are described (Kurnub, Ajlun and Belqa), and these are further subdivided into large-scale depositional sequences separated by regional sequence boundaries that represent maximum flooding surfaces. There is close correspondence between maximum flooding surfaces recording major sea-level rise with those derived for the Arabian and Levant plates, although there are some discrepancies with the precise timing of global sea-level fluctuations. An upward change from braided to meandering stream fluvial environments in central and south Jordan during the Early Cretaceous, reflects a decreasing geomorphological gradient of the alluvial plain, declining siliciclastic sediment flux, and increased floodplain accommodation, associated with a regional Late Albian (second-order) rise in relative sea-level. The Late Albian to Early Cenomanian marine transgression across the coastal alluvial plain marks a major sequence boundary. During Cenomanian to Turonian times a rimmed carbonate-shelf was established, characterised by skeletal carbonates showing small-scale, upward-shallowing cycles (fourth- to fifth-order parasequences) ranging from subtidal to intertidal facies, arranged into parasequence sets. Rimmed carbonate shelf sequences pass laterally to coeval coastal/alluvial plain facies to the south and east. Eustatic (third-order) fluctuations in relative sea level during the Cenomanian and Early Turonian resulted in deposition of ammonite-rich wackestones and organic-rich marls, during high sea-level stands (maximum flooding surfaces). Progradational sabkha/salina facies passing landwards to fluvial siliciclastics were deposited during an Early Turonian sea-level low stand, marks a regional sequence boundary, above which a highstand carbonate platform was established. A second-order, regional rise in sea level and marine transgression during the Early Coniacian marks a Type 2 sequence boundary, and subsequent drowning of the rimmed carbonate shelf by Late Coniacian times. Sedimentation during the Santonian to Maastrichtian was characterised by a hemi-pelagic chalk-chert-phosphorite lithofacies association, deposited in shallow to moderate water depths on a homoclinal ramp setting, although thicker coeval sequences were deposited in extensional rifts. The marked change in sedimentation from rimmed carbonate shelf to pelagic ramp is attributed to Neo-Tethyan mid-oceanic rifting, tilting, intracratonic deformation and subsidence of the platform; this is reflected in changes in biogenic productivity and ocean currents. Oceanic upwelling and high organic productivity resulted in the deposition of phosphorite together with giant oyster banks, the latter developing within oxygenated wave-base on the inner ramp. Chalk hardgrounds, sub-marine erosion surfaces, and gravitational slump folds indicate depositional hiatus and tectonic instability on the ramp. In the Early Maastrichtian, deeper-water chalk-marl, locally organic-rich, was deposited in density-stratified, anoxic basins, that were partly fault controlled. Pulsatory marine onlap (highstand sequences) during the Eocene is manifested in pelagic chalk and chert with a paucity of benthic macro-fauna, indicating a highly stressed, possibly hypersaline, and density-stratified water column. Comparison with global and regional relative sea-level curves enable regionally induced tectonic factors (hinterland uplift and ocean spreading) to be deduced, against a background of global sea-level rise, changing oceanic chemistry/productivity and climatic change.

Paleoceanographic conditions following the end-Permian mass extinction recorded by giant ooids (Moyang, South China)

Early Triassic oceans were characterized by deposition of a number of “anachronistic facies”, including microbialites, seafloor carbonate cement fans, and giant ooids. Giant ooids were particularly prevalent in Lower Triassic sections across South China and exhibit unusual features that may provide insights into marine environmental conditions following the end-Permian mass extinction. The section at Moyang (Guizhou Province) contains abundant giant ooids ranging in size between 2 and 6mm (maximum 12mm) and exhibiting various cortical structures, including regular, deformed, compound, regenerated and “domed”. Preservation of ooid cortical structure is generally good as indicated by petrographic observations, and trace element and carbon isotope analyses suggest that diagenesis occurred in a closed diagenetic system. All ooids exhibit fine concentric laminae, frequently alternating between light-colored coarsely crystalline and dark-colored finely crystalline layers probably reflecting variation in organic content or original mineralogy. Under scanning electron microscope, biomineralized filaments or biofilms and tiny carbonate fluorapatite (CFA) crystals are commonly found in the finely crystalline layers. We infer that the precipitation of CFA was related to adsorption of P via microbial activity on the surfaces of ooids following episodic incursions of deep waters rich in CO2, H2S and phosphate into shallow-marine environments. Giant ooid precipitation may have been promoted in shallow ramp settings during these events by increased watermass agitation and supersaturation with respect to CaCO3, as well as reduced carbonate removal rates through biotic skeletal formation. Spatio-temporal distribution data reveal that giant ooids were widespread in the Tethyan region during the Early Triassic, and that they were most abundant immediately after the end-Permian crisis and disappeared gradually as metazoans repopulated marine environments.

Characterization and interpretation of discontinuity surfaces in a Jurassic ramp setting (High Atlas, Morocco)

AbstractDiscontinuity surfaces are widely recognized but often poorly understood features of epeiric carbonate settings. In sedimentary systems, these features often represent hiatus surfaces below biostratigraphic resolution and may represent a considerable portion of the time contained in the sediment record. From an applied perspective, discontinuities may represent horizontal flow barriers and result in reservoir compartmentalization. Here, a total of 80 condensed surfaces (S1), firmgrounds (S2) and hardgrounds (S3) from a Jurassic (Middle and Upper Bajocian Assoul Formation) ramp setting of the High Atlas in Morocco are carefully documented with respect to their morphology, their secondary impregnation by Fe and Mn oxides and phosphates and their palaeoecological record. A statistical frequency distribution of two surfaces of the S1 type, 1·1 surfaces of the S2 type and 0·4 surfaces of the S3 type per 10 section metres is observed along a 220 m long carbonate succession. Based on two stratigraphically and spatially separated study windows and correlative sections, the stratigraphic frequency distribution, the lateral extent and the nature of facies change across discontinuities are documented in a quantitative manner. Specific features of the study site include the considerable stratigraphic thickness of the Assoul Formation and the conspicuous absence of subaerial‐exposure‐related features. Based on the data presented here, firmground and hardground surfaces are best interpreted as maximum‐regression‐related features. Relative sea‐level lowstand results in a lowered wave base, and wave orbitals and currents result in sea floor omission and lithification. Care must be taken to avoid overly simplistic interpretations, as differences in bathymetry and carbonate facies result in marked changes in discontinuity characteristics in proximal–distal transects. The data shown here are of significance for those concerned with the interpretation of shoal water carbonate environments and are instrumental in the building of more realistic carbonate reservoir flow models.

Corticospinal Excitability is Dependent on the Parameters of Peripheral Electric Stimulation: A Preliminary Study

Chipchase LS, Schabrun SM, Hodges PW. Corticospinal excitability is dependent on the parameters of peripheral electric stimulation: a preliminary study. Objective To evaluate the effect of 6 electric stimulation paradigms on corticospinal excitability. Design Using a same subject pre-post test design, transcranial magnetic stimulation (TMS) was used to measure the responsiveness of corticomotor pathway to biceps and triceps brachii muscles before and after 30 minutes of electric stimulation over the biceps brachii. Six different electric stimulation paradigms were applied in random order, at least 3 days apart. Setting Motor control research laboratory. Participants Healthy subjects (N=10; 5 women, 5 men; mean age ± SD, 26±3.6y). Interventions Six different electric stimulation paradigms with varied stimulus amplitude, frequency, and ramp settings. Main Outcome Measure Amplitudes of TMS-induced motor evoked potentials at biceps and triceps brachii normalized to maximal M-wave amplitudes. Results Electric stimulation delivered at stimulus amplitude sufficient to evoke a sensory response at both 10Hz and 100Hz, and stimulus amplitude to create a noxious response at 10Hz decreased corticomotor responsiveness (all P<0.01). Stimulation sufficient to induce a motor contraction (30Hz) applied in a ramped pattern to mimic a voluntary activation increased corticomotor responsiveness ( P=0.002), whereas constant low- and high-intensity motor stimulation at 10Hz did not. Corticomotor excitability changes were similar for both the stimulated muscle and its antagonist. Conclusions Stimulus amplitude (intensity) and the nature (muscle flicker vs contraction) of motor stimulation have a significant impact on changes in corticospinal excitability induced by electric stimulation. Here, we demonstrate that peripheral electric stimulation at stimulus amplitude to create a sensory response reduces corticomotor responsiveness. Conversely, stimulus amplitude to create a motor response increases corticomotor responsiveness, but only the parameters that create a motor response that mimics a voluntary muscle contraction.

Late Ordovician (Sandbian) ostracods from the Ardwell Farm Formation, SW Scotland

Synopsis The lower part of the Late Ordovician Ardwell Farm Formation of the Girvan district in SW Scotland yields a low-diversity ostracod fauna comprising six species. Many of these ostracods are biostratigraphically short-ranging North American taxa limited to an interval equivalent to the gracilis and bicornis graptolite biozones. These species include ‘ Ctenobolbina ’ ventrospinosa and Baltonotella parsispinosa that permit inter-regional correlation with the Sandbian strata of the USA and Arctic Canada. This chain of correlation is also confirmed by chitinozoans, such as Belonechitina hirsuta , that occur with the ostracods in the Ardwell Farm Formation. Although allochthonous and transported by turbidity currents into a deep-water clastic-rich basin, the ostracod fauna of the Ardwell Farm Formation resembles other Ordovician deep-water ostracod biotopes, characterized by podocope ostracods and low species diversity. Alternatively the ostracods might simply have been transported from a carbonate ramp setting.

Using an Artificial-Neural-Network Method To Predict Carbonate Well Log Facies Successfully

Summary The Maastrichtian (Upper Cretaceous) reservoir is one of five prolific oil reservoirs in the giant Wafra oil field. The Maastrichtian oil production is largely from subtidal dolomites at an average depth of 2,500 ft. Carbonate deposition occurred on a very gently dipping, shallow, arid, and restricted ramp setting that transitioned between normal marine conditions to restricted lagoonal environments. The average porosity of the reservoir interval is approximately 15%, although productive zones have porosity values up to 30–40%. The average permeability of the reservoir interval is approximately 30 md. Individual core plugs have measured permeability up to 1,200 md. Efforts to predict sedimentary facies from well logs in carbonate reservoirs is difficult because of the complex carbonate sedimentary facies structures, strong diagenetic overprint, and challenging log analysis in part owing to the presence of vugs and fractures. In the study, a workflow including (1) core description preprocessing, (2) log- and core-data cleanup, and (3) probabilistic-neural-network (PNN) facies analysis was used to predict facies from log data accurately. After evaluation of a variety of statistical approaches, a PNN-based approach was used to predict facies from well-log data. The PNN was selected as a tool because it has the capability to delineate complex nonlinear relationships between facies and log data. The PNN method was shown to outperform multivariate statistical algorithms and, in this study, gave good prediction accuracy (above 70%). The prediction uncertainty was quantified by two probabilistic logs—discriminant ability and overall confidence. These probabilistic logs can be used to evaluate the prediction uncertainty during interpretation. Lithofacies were predicted for 15 key wells in the Wafra Maastrichtian reservoir and were effectively used to extend the understanding of the Maastrichtian stratigraphy, depositional setting, and facies distribution.

High‐resolution sequence stratigraphy of the Maastrichtian‐Ypresian succession along the eastern scarp face of Kharga Oasis, southern Western Desert, Egypt

AbstractA thick Maastrichtian‐Ypresian succession, dominated by marine siliciclastic and carbonate deposits of the regionally recognized Nile Valley and Garra El‐Arbain facies associations, is exposed along the eastern escarpment face of Kharga Oasis, located in the Western Desert of Egypt. The main objectives of the present study are: (i) to establish a detailed biostratigraphic framework; (ii) to interpret the depositional environments; and (iii) to propose a sequence stratigraphic framework in order to constrain the palaeogeographic evolution of the Kharga sub‐basin during the Maastrichtian‐Ypresian time interval. The biostratigraphic analysis suggests the occurrence of 10 planktonic zones; two in the Early Maastrichtian (CF8b and CF7), four in the Palaeocene (P2, P3, P4c and P5) and four in the Early Eocene (E1, E2, E3 and E4). Recorded zonal boundaries and biostratigraphic zones generally match with those proposed elsewhere in the region. The stratigraphic succession comprises seven third‐order depositional sequences which are bounded by unconformities and their correlative conformities which can be correlated within and outside Egypt. These depositional sequences are interpreted as the result of eustatic sea‐level changes coupled with local tectonic activities. Each sequence contains a lower retrogradational parasequence set bounded above by a marine‐flooding surface and an upper progradational parasequence set bounded above by a sequence boundary. Parasequences within parasequence sets are stacked in landward‐stepping and seaward‐stepping patterns indicative of transgressive and highstand systems tracts, respectively. Lowstand systems tracts were not developed in the studied sections, presumably due to the low‐relief ramp setting. The irregular palaeotopography of the Dakhla Basin, which was caused by north‐east to south‐west trending submerged palaeo‐highs and lows, together with the eustatic sea‐level fluctuations, controlled the development and location of the two facies associations in the Kharga Oasis, the Nile Valley (open marine) and Garra El‐Arbain (marginal marine).

SEQUENCE STRATIGRAPHY AND RESERVOIR CHARACTERISTICS OF THE TURONIAN‐CONIACIAN KHASIB FORMATION IN CENTRAL IRAQ

Bioturbated chalky limestones of the Khasib Formation (Upper Turonian – Lower Coniacian) form potential reservoir rocks at oilfields and structures in central Iraq. Core and cuttings samples and wire‐line logs from wells in the East Baghdad, Balad, Samarra and Tikrit fields (wells EB‐77, EB‐57, Ba‐1, Ba‐2, Sr‐1, Sr‐2, and Ti‐1) were used to investigate microfacies types and porosity evolution. Facies modelling was applied to predict the relationship between facies distribution and reservoir characteristics to construct a predictive geologic model which will assist future exploration in central Iraq.Microfacies analysis and electrofacies identification and correlations indicate that the limestones of the Khasib Formation were deposited in a ramp setting. The ramp developed over the distal margin of the Upper Cretaceous proforeland basin, adjacent to the evolved forebulge. Inner ramp facies are characterized by carbonate bank bioclastic packstones intercalated with lagoonal green shales. Middle ramp facies dominate the Khasib Formation and consist of bioturbated, chalky, dolomitic and bioclastic limestones. Bioclasts include benthic and planktonic foraminifera. Intense Thalassinoides and less common Palaeophycus bioturbation has enhanced the porosity of this facies. Outer ramp deposits consist of alternating mid‐ramp bioturbated bioclastic chalky limestones and argillaceous and marly limestones. The latter contain basinal bioclasts including planktonic foraminifera, oligosteginids, dwarf rotaliids, and sponge spicules.Sequence stratigraphic analyses of the Khasib Formation indicate that it represents a third‐order depositional cycle. At the base is a type 1 sequence boundary which separates it from the underlying LST deposits of the Kifl Formation. Shaly and argillaceous limestones in the lower part of the Khasib Formation represent the early TST. These are overlain by a thick, deepening‐upwards succession of outer ramp facies. The maximum flooding surface is represented by a thin and extensive horizon of Oligosteginal limestones with Palaeophycus bioturbation. HST deposits are represented by aggradational build‐ups of bioturbated chalky dolomitic limestones, followed by progradational late HST shales and limestones. The boundary with the overlying Tanuma Formation is a type 2 sequence boundary. Bioclastic packstones and intensively bioturbated (Thalassinoides) bioclastic limestones of the mid‐outer ramp are the primary source of fabric‐selective porosity which is greatly enhanced by diagenetic overprints. These two units constitute the prime target for future exploration in central Iraq.

Changes in size and growth rate of ‘Lilliput’ animals in the earliest Triassic

Marine invertebrate taxa that survived the late Permian (latest Changhsingian) mass extinction event are all much smaller than they were prior to the event, an example of the Lilliput effect. New taxa that first appeared in the immediate extinction aftermath are also small compared to their size in younger strata. The Lilliput effect is a temporary phenomenon, with most surviving taxa returning to pre-extinction size in the first two conodont zones of the Triassic. In this study, the growth rates, of some of the Early Triassic Lilliput animals, estimated by growth-line analysis, were compared from different stratigraphic levels to determine how this variable fluctuated as body size returned to pre-extinction values after the event. Specimens were collected from the Mazzin and Siusi members of the Werfen Formation in northern Italy, from facies deposited in a tropical, mixed siliciclastic-carbonate ramp setting. Measurements of the bivalves Unionites and Claraia, the gastropod Bellerophon and the brachiopod ‘ Lingula’ were made in the field, with subsequent growth-line analysis conducted on well-preserved specimens of ‘Lingula’. Specimens collected from the basal Triassic Mazzin Member were smaller than those from the younger Siusi Member. Growth-line analysis indicates that the smaller Mazzin Member specimens had a greater number of more closely spaced growth lines than the larger Siusi Member specimens. The small, ‘Lilliput’ animals living in the immediate aftermath of the Late Permian event therefore suffered more interruptions to growth and had overall slower growth rates than larger individuals living in later recovery times. Frequent interruptions to growth and an overall slower growth rate in specimens from the Hindeodus parvus and Isarcicella isarcica zones of the Mazzin Member (lower Induan) reflects suboptimal environmental conditions, such as the frequency or severity of low-oxygen episodes, temperature extremes, and/or disruptions to primary productivity.

Taphonomic analysis of fossil concentrations from La Manga Formation (Oxfordian), Neuquén Basin, Mendoza Province, Argentina

Fil: Bressan, Graciela Susana. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Geologia; Argentina

A new Burgess Shale–type assemblage from the “thin” Stephen Formation of the southern Canadian Rockies

Research Article| September 01, 2010 A new Burgess Shale–type assemblage from the “thin” Stephen Formation of the southern Canadian Rockies Jean-Bernard Caron; Jean-Bernard Caron 1Department of Natural History-Palaeobiology, Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada Search for other works by this author on: GSW Google Scholar Robert R. Gaines; Robert R. Gaines 2Pomona College, Geology Department, 185 E. Sixth Street, Claremont, California 91711, USA Search for other works by this author on: GSW Google Scholar M. Gabriela Mángano; M. Gabriela Mángano 3Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada Search for other works by this author on: GSW Google Scholar Michael Streng; Michael Streng 4Department of Earth Sciences, Uppsala University, Villavägen 16, 75236 Uppsala, Sweden Search for other works by this author on: GSW Google Scholar Allison C. Daley Allison C. Daley 4Department of Earth Sciences, Uppsala University, Villavägen 16, 75236 Uppsala, Sweden Search for other works by this author on: GSW Google Scholar Geology (2010) 38 (9): 811–814. https://doi.org/10.1130/G31080.1 Article history received: 31 Jan 2010 rev-recd: 09 Apr 2010 accepted: 14 Apr 2010 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jean-Bernard Caron, Robert R. Gaines, M. Gabriela Mángano, Michael Streng, Allison C. Daley; A new Burgess Shale–type assemblage from the “thin” Stephen Formation of the southern Canadian Rockies. Geology 2010;; 38 (9): 811–814. doi: https://doi.org/10.1130/G31080.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract A new Burgess Shale–type assemblage, from the Stephen Formation of the southern Canadian Rocky Mountains, is described herein. It occurs near Stanley Glacier in Kootenay National Park, 40 km southeast of the type area near Field, British Columbia. While at least a dozen Burgess Shale localities are known from the “thick” Stephen Formation, the Stanley Glacier locality represents the first discovery of Burgess Shale–type fossils from the “thin” Stephen Formation. The Cathedral Escarpment, an important regional paleotopographic feature, has been considered important to the paleoecologic setting and the preservation of the Burgess Shale biota. However, the Stanley Glacier assemblage was preserved in a distal ramp setting in a region where no evidence of an escarpment is present. The low-diversity assemblage contains eight new soft-bodied taxa, including the anomalocaridid Stanleycaris hirpex n. gen., n. sp. (new genus, new species). Nektonic or nektobenthic predators represent the most diverse group, whereas in relative abundance, the assemblage is dominated by typical Cambrian shelly benthic taxa. The low diversity of both the benthic taxa and the ichnofauna, which includes diminutive trace fossils associated with carapaces of soft-bodied arthropods, suggests a paleoenvironment with restrictive conditions. The Stanley Glacier assemblage expands the temporal and geographic range of the Burgess Shale biota in the southern Canadian Rockies, and suggests that Burgess Shale–type assemblages may be common in the “thin” Stephen Formation, which is regionally widespread. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Extracting the Silicate Mineral Fraction from Ancient Carbonate: Assessing the Geologic Record of Dust

Abstract The climatic influence of eolian dust (particles < 63 μm) is well recognized in the Cenozoic earth system, but less so for the pre-Cenozoic. In ancient carbonate rocks that formed isolated from fluvio-deltaic influx, the silicate mineral fraction can, in some cases, represent atmospheric dust. Here we present a methodology for sequential extraction of carbonate, organic matter, pyrite, and iron oxide from pre-Cenozoic carbonate strata to isolate the eolian fraction, and illustrate the application to stratigraphic sequences from a late Paleozoic carbonate buildup and ramp setting.

Components, facies and ramps: Redefining Upper Oligocene shallow water carbonates using coralline red algae and larger foraminifera (Venetian area, northeast Italy)

Chattian mixed carbonate-siliciclastic deposits from two palaeogeographic units of Southern Alps, the Lessini Shelf and Venetian foreland basin of northeast Italy, are studied with respect to facies distributions and controlling ecological factors on dominant biogenic components. These ramp depositional systems are located in the western Southern Alps on the Lessini Shelf, and in the eastern Southern Alps in the Venetian foreland basin. The successions show an extensive geographic distribution of at least 150 km × 40 km at the southern margin of the Southern Alps. Palaeoenvironments are inferred following microfacies characteristics and floral and faunal indicators, especially those of coralline red algae and larger foraminifera. An inner- to middle homoclinal ramp morphology is reconstructed based on facies distributions and taxonomic and coralline algal growth-form proxy data. The distribution along depositional strike shows variation from thinner, carbonate dominated sequence in the west (Monti Berici and Monti Lessini) to a more variable, mixed carbonate-siliciclastic dominated environment to the east (Vittorio Veneto, Alpago). Facies character and occurrence are controlled by local hydrodynamic conditions and varying water turbidity of the studied ramp depositional system. The general facies succession shows a transgressive sequence from the proximal inner- to proximal middle ramp. A shoal belt of trough cross-bedded sandstones occurs in a proximal inner ramp position in all of the studied areas. In a gentle-dipping homoclinal ramp, most benthic organisms inhabited the relatively quite environments off these shoals in a distal inner ramp setting. Basinward environments (proximal middle ramp) constitute areas of maximal carbonate production and consist primarily of larger foraminiferal facies and rhodolith pavements. Larger foraminifera, represented by two larger foraminiferal assemblages, are most diversified in the shallower settings (near the fair-weather wave base, distal inner/proximal middle ramp), whereas coralline algae predominate in deeper areas (proximal middle ramp) constructing extensive rhodolith pavements. Rhodolith pavements are distinguished with respect to rhodolith shape, size, growth-forms and coralline algal taxonomic composition. Larger foraminifera are interpreted with respect to shell structures, shell architectures and comparable Recent counterparts. The analysed benthic communities thrived in a mesotrophic regime within trophic gradients constrained by fluvial influence. Coralline algal dominance and a moderately diverse larger foraminiferal assemblage suggest a tropical/warm temperate transition. The considerable stability imposed on sediments by the rhodolith pavement must also have significantly contributed to the maintenance of a middle ramp profile bulge. This study will contribute to a better understanding of Oligocene carbonates which have received less attention than their Eocene or Miocene counterparts.

The Asmari Formation, north of the Gachsaran (Dill anticline), southwest Iran: facies analysis, depositional environments and sequence stratigraphy

The Asmari Formation is exposed at the Dill anticline in the Dezful Embayment of the Zagros foreland basin with 206 m thickness comprising thin, medium and thick to massive bedded carbonates. The Asmari Formation is Late Oligocene (Chattian)–Early Miocene (Burdigalian) in age at the study area. Ten facies characterizing upward gradual shallowing trend of an open marine (MF 1–3), shoal (MF 4), semi-restricted and restricted lagoon (MF 5–9) and near-shore lagoon (MF 10) depositional environments were identified. Based on environmental interpretations, a homoclinal ramp consisting of inner and middle parts prevails. MF 4–10 were characterized by the occurrence of large and small porcelaneous benthic foraminifera representing a shallow-water setting of an inner ramp influenced by wave and tide processes. MF 1–MF 3 with large and small hyaline benthic foraminifera represent a deeper fair water wave base of a middle ramp setting. Three-third-order depositional sequences were recognized. Sequence 1 mostly consists of an open marine to the lower part and is followed by semi to restricted lagoon facies. Sequences 2 and 3 are characterized by semi to restricted lagoon facies. Moreover, the relative sea-level change curves correlate with the global sea-level change curves.

Detecting Thief Zones in Carbonate Reservoirs by Integrating Borehole Images With Dynamic Measurements: Case Study From the Mauddud Reservoir, North Kuwait

Summary One of the key issues in creating a good reservoir model in carbonate reservoirs is identifying the horizontal permeability conduits—"thief zones"—if there are any. In the Sabriyah field in Kuwait, dynamic measurements showed evidence of thief zones in the Lower Cretaceous (Albian) Mauddud formation. Early water breakthrough has occurred in some wells. Previous studies indicated that it was very challenging to detect the thinly layered thief zones using conventional openhole logs. This paper describes a method of recognizing the different types of thief zones in the Mauddud carbonate reservoirs using high-resolution image logs with calibration from core and dynamic measurements and by integrating image logs with nuclear magnetic resonance (NMR) and conventional openhole logs. The Mauddud carbonates are Early Albian in age and consist of grainstones, wackstones, and mudstones deposited in a ramp setting. Observations from production logging tools (PLTs) and production data indicated that there are a few thief zones in different levels within the vertical Mauddud sequence. A previous core study shows that the fractures in the Mauddud formation are short (&amp;lt;10 cm) and concentrated in diagenetically cemented layers. The fractured thin layers are believed to be the principal type of thief zone. Another type of thief zone is associated with better-developed vuggy porosity. This study shows that both fractured and vuggy porosity-related types of thief zones can potentially be detected through integration of high-resolution image logs with PLT, NMR, and conventional logs. In addition, methods of estimating fracture permeability and porosity-related permeability based on logs are also proposed. The log-estimated permeability determined using this approach fits better with the production profile and can then be used to evaluate the thief zones in a more quantitative manner.

Reservoir description of a mid-Cretaceous siliciclastic-carbonate ramp reservoir: Mauddud Formation in the Raudhatain and Sabiriyah fields, North Kuwait

ABSTRACTThe mid-Cretaceous Mauddud Formation is the main producing carbonate reservoir in the Raudhatain and Sabiriyah fields of northern Kuwait. Historical field information and results from waterflood pilots indicate that reservoir performance in these reservoirs is controlled by geological complexity at several scales. A detailed, integrated sedimentological and biostratigraphic investigation of the reservoirs, combined with dynamic reservoir data, have provided an understanding of Mauddud reservoir heterogeneity and of the principle controls on reservoir matrix behaviour. The largely carbonate Mauddud Formation overlies the Upper Burgan Member, a thick succession of fluvio-deltaic deposits, and consists of a diverse suite of carbonate facies deposited in low to high-energy, shallow-marine ramp settings. The basal part of the reservoir comprises mixed carbonate and siliciclastic sediments and reflects the establishment of a carbonate-dominated regime during waning supply of Burgan siliciclastic sediment. This system was eventually drowned and covered by the Wara Formation, a shaly offshore succession that is also the reservoir seal.Sedimentary facies associations and microfossil assemblages within the reservoir are organised in a broadly upward-shallowing succession constructed of several transgressive-regressive cycles, which are defined by prominent, widely-correlatable flooding surfaces. Each cycle exhibits a characteristic internal stacking pattern of minor depositional cycles. Field-wide mapping and interpretation of facies within each cycle reveals a SW to NE, proximal to distal, trend consistent with regional seismic and palaeogeographic interpretations. The high-energy, inner to mid-ramp carbonate succession in the lower portion of the Mauddud reservoir is punctuated by siliciclastic incursions. Abrupt lateral facies changes, thickness variations, and local intra-reservoir erosion surfaces in this section suggest that deposition was influenced by subtle syndepositional tectonism. The upper part of the reservoir, in contrast, lacks significant siliciclastic influence and is made up of widely-correlatable, lower-energy carbonate facies, although local subtle facies variations show that the Raudhatain-Sabiriyah structures continued as palaeohighs during deposition. The contrast in quality between grain-dominated facies at the crests of the two structures and less grainy facies along their flanks was accentuated by carbonate cementation in the water legs of the reservoirs, largely in the form of calcite concretions of variable abundance. Cementation is most pronounced in low-energy wackestone facies, particularly in proximity to flooding surfaces where nodules may be amalgamated to form laterally continuous, cemented layers which are commonly fractured. Another significant, but contrasting, diagenetic modification within the reservoir was the generation of secondary macroporosity through dissolution of aragonitic skeletal components in a shallow to intermediate burial environment.The stratigraphic evolution of the Mauddud reservoir, and its diagenetic overprint, in addition to post-depositional fracturing and faulting, created reservoir heterogeneities, which are critical to reservoir performance; one of the most significant of these is the relationship between horizontal and vertical permeability. Parasequences dominated by high-energy inner ramp grainstones, thin inner ramp rudist-bearing tempestites, and vuggy and fractured rudist floatstones and rudstones constitute thief zones that represent major challenges to reservoir management. In contrast, some cemented layers and flooding surfaces support pressure differentials of up to several hundred pounds/square inch (psi), thus complicating sweep and promoting reservoir compartmentalisation. The strong facies, diagenetic and stratigraphic controls on the distribution of thief zones and intra-reservoir baffles demonstrates how important it is to comprehensively understand reservoir sedimentology and stratigraphy when devising long-term development plans for reservoirs of this deceptively simple character.More recent 3-D seismic data, production surveillance, and horizontal development wells show faults and fractures to be important heterogeneities in both reservoirs. Due to the immaturity of the water flood in the Mauddud reservoirs, the impact of these features on field and well behaviour is as yet unclear, but it is anticipated that the impact of such features on well and field performance will become more pronounced during later development.

Characterization of a Complex Carbonate Heavy-Oil Reservoir

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 120423, ’Characterization of Complex Carbonate Heavy Oil Reservoir - A Case Study,’ by Afzal Iqbal,John Smith, Ali Reza Zahedi, Deemer Arthur, and Falah M. Al-Yami, Saudi Arabian Chevron; W. Scott Meddaugh, Chevron Energy Technology Company; Mansoor A. Rampurawala, SPE, Bingjian Li, SPE, and Ihsan Gok, SPE, Schlumberger Oilfield Services; and Talal Al Enazi, Kuwait Gulf Oil Company, originally prepared for the 2009 SPE Middle East Oil and Gas Show and Conference, Bahrain, 15-18 March. The First Eocene reservoir is the shallowest producing interval of the Wafra field in the partitioned Neutral Zone between Saudi Arabia and Kuwait. Characterization of this heavy-oil reservoir is challenging because of observed variations in oil viscosity, heterogeneity related to complex mineralogy, a possible dual-porosity system, and the presence of fractures of varing scales. The full-length paper presents several aspects of an integrated approach to characterize the reservoir. Introduction Currently, a large-scale steamflood pilot (LSP) consisting of sixteen 2.5-acre inverted-5-spot patterns is being installed. This will be the first multipattern steamflood of a carbonate reservoir in the Middle East. Oil was discovered in the First Eocene in 1954, and full-scale development began in 1956. The First Eocene has produced only 4% of original oil in place (OOIP), and primary recovery is not expected to exceed 10%. The oil gravity in this heavy-oil reservoir varies from 14 to 20ºAPI. The First Eocene is a depletion-drive reservoir with partial solution-gas drive and limited aquifer support. The estimated gross OOIP within the currently delineated field limit is more than 10 billion reservoir bbl. The reservoir mainly consists of dolomitized packstones and grainstones deposited under arid-to-semiarid conditions on a shallow, very gently dipping, low- to moderate-energy inner shelf or ramp setting in a gently dipping, restricted ramp environment. The presence of minor interbedded evaporites suggests restriction was occasionally sufficient for the development of hypersaline lagoons and sabkhas. The shallowing-upward cycles are capped by mud-dominated rocks, hardgrounds, and exposure surfaces. The average porosity is 35% and the average permeability is 250 md over the gross interval. Well-log and core-plug porosity values greater than 50% are common, and measured permeability values range up to 5,000 md. The First Eocene reservoir has an average depth of approximately 1,000 ft and a gross thickness of approximately 750 ft. Three wells were used in the study to demonstrate the approach of integrating log and core data to evaluate reservoir heterogeneity.

Sequence stratigraphy of Cenomanian–Turonian carbonate platform margins (Sarvak Formation) in the High Zagros, SW Iran: an outcrop reference model for the Arabian Plate

Abstract A high resolution sequence stratigraphic model has been constructed for the mid-Cretaceous Sarvak Formation (in the High Zagros region of SW Iran) which was deposited close to the eastern margin of the Arabian Plate. The exceptional outcrop quality, displaying the detailed facies patterns in the transition zone from carbonate platform to intra-shelf basin, offers the rare opportunity to distinguish between the relative control of carbonate sediment supply (S) and accommodation (A) on the depositional geometries of third- and fourth-order depositional sequences. Four third-order sequences have been distinguished in the Sarvak Formation, with a duration varying between 1.5 and 3 Ma, and a thickness of 50–150 m. These are in turn composed of fourth- and fifth-order sequences that form the stratigraphic building blocks of this carbonate system. A significant distinction has been made in the third-order sequences between the early transgression (e-TST) when the system was still flat, and corresponds to a ramp setting, and the late transgression (l-TST) when the carbonate platform to intra-shelf basin topography was created. The rate of accommodation creation is identified as the dominant factor controlling the morphology of the depositional profile, and, as such, the driving motor behind the dynamics of this type of carbonate system. The dip angle of the depositional profile has a major influence on: (1) the hydrodynamics of the system; (2) the type of carbonate sediment; and (3) the volume of carbonate sediment produced. A good correlation with the third-order sequences of the Natih Formation in Oman is demonstrated, which supports a dominant control by eustatic sea-level changes and a similar response of the carbonate system to changes in the rate of sea-level rise on the southern part of the Arabian Plate. This outcrop analogue can be considered as a good reference model for the Cenomanian–Turonian carbonate platform margins of the Arabian Plate, but also as a textbook example of the response of carbonate systems to sea-level fluctuations (relative influence of accommodation and sediment supply).