Le but général de ce travail fut de quantifier la silice biogène (BSiO2) dans les sédiments du continuum de l'Escaut (estuaire – zone côtière), ainsi que son taux de recyclage - rétention. Le coefficient de diffusion moléculaire de la DSi a été déterminé pour différentes valeurs de température et deux salinités et une relation empirique reliant le coefficient de diffusion à la température et à la viscosité de la solution a été établie. La distribution longitudinale de la BSiO2 dans les sédiments de surface, ainsi que les profils verticaux de BSiO2 et de silice dissoute ont été déterminés durant différentes saisons en 2004 et 2005. Les flux de DSi ont été également estimés via des expériences d’incubations et par modélisation des profiles verticaux de DSi. Des expériences de dissolution des sédiments ont permis de déterminer (via modélisation) les constantes cinétiques de vitesse de la BSiO2 et une première évaluation du taux de recyclage de la BSiO2 (5 – 70%). Sur base des calculs à partir des profiles de DSi dans l’eau interstitielle, le taux de recyclage de la BSiO2 varie entre 8 et 92% dans l’estuaire, ce qui correspond à une rétention moyenne de la BSiO2 supérieure à 60%, et souligne ainsi l'efficacité du filtre estuarien par rapport à la silice. En revanche, le recyclage de BSiO2 dans la zone côtière apparaît comme très intensif, avec des valeurs souvent supérieures à 40%. Ces résultats montrent l'importance d'inclure les estuaires dans le calcul des budgets globaux de Si. / The general objective of this study was to quantify the BSiO2 in the sediments of the Scheldt continuum, together with its recycling and retention in this area. First an empirical relation linking the diffusion coefficient of DSi to temperature and salinity was determined, via diffusion experiments. The longitudinal BSiO2 distribution in surface sediments and the vertical BSiO2 and DSi profiles in sediment cores where then measured during 2 years and different seasons, together with other parameters characterizing pore waters, such as alkalinity, pH, sulphate… DSi fluxes were also estimated either directly through whole core incubations measurements or via modelling of interstitial water profiles. BSiO2 dissolution experiments allowed the evaluation of the kinetic rate constants also through modelling of the results, and gave preliminary information over the BSiO2 recycling rates (5 – 70%). The recycling rate as obtained from pore water profiles vary between 8 and 92%, with an average value of about 32%, which highlights the importance of the estuarine filter for silica. Recycling is much more intense in the coastal area, with values up to 40%, which we explain by higher salinity and higher sediment permeability. The different experiments and results also indicated that secondary mineral precipitation might be and important process affecting BSiO2 in the Scheldt sediments.

Diagenetic studies of carbonate rocks focused for a long time on photozoan carbonate assemblages deposited in tropical climates. The results of these investigations were taken as models for the diagenetic evolution of many fossil carbonates. Only in recent years the importance of heterozoan carbonates, generally formed out of the tropics or in deeper waters, was realized. Diagenetic studies focusing on this kind of rocks are still scarce, but indicate that the diagenetic evolution of these rocks might be a better model for many fossil carbonate settings ("calcite-sea" carbonates) than the photozoan model used before.<br><br> This study deals with the determination of the diagenetic pathways and environments in such shallow-water heterozoan carbonate assemblages. Special emphasis is put on the identification of early, near-seafloor diagenetic processes and on the evaluation of the amount of constructive diagenesis in form of cementation in this diagenetic environment.<br><br> As study area the Central Mediterranean, the Maltese Islands and Sicily, was chosen. Here two sections were logged in Olio-Miocene shallow-water carbonates consisting of different kinds of heterozoan assemblages. The study area is very suitable for the investigation of constructive early diagenetic processes, as the rocks were never deeply buried and burial diagenetic pressure solution and cementation as cause of lithification could be ruled out. Nevertheless, the carbonate rocks are well lithified and form steep cliffs, implying cementation/lithification in another, shallower diagenetic environment. To determine the diagenetic pathways and environments, detailed transmitted light and cathodoluminescence petrography was carried out on thin sections. Furthermore the stable isotope (δ¹⁸O and δ¹³C) composition of the bulk rock, single biota and single cement phases was determined, as well as the major and trace element composition of the single cement phases.<br><br> Petrographically three (Sicily) to four (Maltese Islands) cementation phases, two phases of fabric selective and one of non-fabric selective dissolution, one phase of neomorphism and one of chemical compaction could be distinguished. The stable isotope measurements of the single cement phases pointed to cement precipitation from marine, marine-derived and meteoric waters. The trace element analysis indicated precipitation under reducing conditions, (A) in an open system with low rock-water interaction on the Maltese Islands and (B) in a closed system with high rock-water interaction on Sicily. For the closed systems case, aragonite as cement source could be concluded because its chemical composition was preserved in the newly formed cements.<br><br> By integrating these results, diagenetic pathways and environments for the investigated locations were established, and the cement source(s) in the different environments were determined. The diagenetic evolution started in the marine environment with the precipitation of fibrous/fibrous-bladed and epitaxial cement I. These cements formed as High Mg Calcite (HMC) directly out of marine waters. The paleoenvironmentally shallowest part of the section on the Maltese Islands was also exposed to meteoric diagenetic fluids. This meteoric influence lead to the dissolution of aragonitic and HMC skeletons, which sourced the cementation by Low Mg Calcitic (LMC) epitaxial cement II in this part of the Maltese section. Entering the burial-marine environment the main part of dissolution, cementation and neomorphism started to take place. The elevated CO2 content in this environment, caused by the decay of organic matter, lead to the dissolution of aragonitic skeletons, which sourced the cementation by LMC epitaxial cement II, bladed and blocky cements. The earlier precipitated HMC cement phases were either partly dissolved (epitaxial cement I) or neomorphosed to LMC (fibrous/fibrous-bladed and epitaxial cement I). In the burial environment weak chemical compaction took place without sourcing significant amounts of cementation. In a last phase the rocks entered the meteoric realm by uplift, which caused non-fabric selective dissolution. This study shows that early diagenetic processes, taking place at or just below the sediment-water-interface, are very important for the mineralogical stabilization of heterozoan carbonate strata. The main amount of constructive diagenesis in form of cementation takes place in this environment, sourced by dissolution of aragonitic and, to a lesser degree, of HMC skeletons.<br><br> The results of this study imply that the primary amount of aragonitic skeletons in heterozoan carbonate sediments must be carefully assessed, as they are the main early diagenetic cement source. In fossil heterozoan carbonate rocks, aragonitic skeletons might be the cement source even when no relict structures like micritic envelops or biomolds are preserved. In general, the diagenetic evolution of heterozoan carbonate rocks is a good model for the diagenesis of "calcite-sea" time carbonate rocks. / Diagenetische Untersuchungen an Karbonatgesteinen beschränkten sich lange Zeit auf photozoische Karbonatvergesellschaftungen der tropischen Breiten. Die Ergebnisse dieser Untersuchungen wurden als modellhaft für den Diageneseverlauf vieler Karbonatgesteine angesehen. Erst in den letzten Jahren wurde die Bedeutung heterozoischer Karbonatvergesellschaften, die sich im Allgemeinen außerhalb der Tropen oder in tieferem Wasser bilden, erkannt. Diagenetische Untersuchungen an dieser Art von Karbonatgesteinen sind immer noch selten, deuten aber an, dass der Diageneseverlauf in diesen Karbonaten ein besseres Model für viele fossile Karbonatgesteine ("Calcit-Meer"-Karbonate) darstellt als das bisher benutzte photozoische Diagenesemodel. Ziel dieser Studie war die Bestimmung des Diageneseverlaufs in solch flachmarinen Karbonaten mit heterozoischer Biogenvergesellschaftung. Die Milieus, in denen die diagenetischen Veränderungen stattfanden, sollten bestimmt werden. Besonderes Augenmerk lag auf der Art und Menge von Veränderungen, die frühdiagenetisch, nahe dem Meeresboden, stattfanden. Dabei war vor allem der Anteil an konstruktiver Diagenese in Form von Zementation von Interesse.<br><br> Das Arbeitsgebiet wurde im zentralen Mittelmeerraum gewählt und befindet sich auf den Maltesischen Inseln und Sizilien. Hier wurden zwei Profile in Flachwassserkarbonaten oligo-miozänen Alters aufgenommen, die sich aus unterschiedlichen heterozoischen Biogenvergesellschaftungen zusammensetzen. Dieses Arbeitsgebiet ist für die Untersuchung konstruktiver frühdiagenetischer Prozesse besonders geeignet, da die Gesteinsabfolgen niemals tief versenkt wurden und versenkungsdiagenetisch bedingte Drucklösung und Zementation als Ursache für die Lithifizierung ausgeschlossen werden können. Trotzdem sind die untersuchten Karbonatgesteine gut verfestigt und bilden steile Kliffs, was auf Zementation/Verfestigung in einem anderen, flacheren Diagenesemilieu hinweist. Zur Bestimmung der Diageneseabfolge und der diagenetischen Milieus wurden detaillierte petrographische Untersuchungen im Durchlicht und mit Kathodolumineszenz an Dünnschliffen durchgeführt. Außerdem wurden die Verhältnisse der stabilen Isotope δ¹⁸O und δ¹³C am Gesamtgestein, an einzelnen Biogenen und an einzelnen Zementphasen bestimmt. Die Haupt- und Spurenelement Zusammensetzung (Ca, Mg, Fe, Mn und Sr) wurde an den einzelnen Zementphasen ermittelt.<br><br> Petrographisch lassen sich drei (Sizilien), beziehungsweise vier (Maltesische Inseln) Zementationsphasen, zwei Phasen von materialabhängiger und eine Phase von materialunabhängiger Lösung, sowie eine Phase von Neomorphismus und eine chemische Kompaktionsphase unterscheiden. Die Messungen der stabilen Isotopenverhältnisse an den einzelnen Zementphasen deuten auf Zementausfällung aus marinen und meteorischen Wässern sowie aus Fluiden marinen Ursprungs hin. Die Spurenelementanalyse lässt außerdem auf Zementausfällung unter reduzierenden Bedingungen schließen. Diese fand einerseits, im Falle der Maltesischen Inseln, in einem offenen System mit geringer Gesteins-Wasser-Interaktion, andererseits, im Falle von Sizilien, in einem geschlossenen System mit großer Gesteins-Wasser-Interaktion statt. Bei der Zementation in einem geschlossenen System konnte Aragonit als Zementquelle bestimmt werden, da seine charakteristische chemische Zusammensetzung im neu gebildeten Zement erhalten blieb.<br><br> Durch die Integration aller Ergebnisse konnten für die beiden Lokalitäten die Diageneseabfolgen und die diagenetischen Milieus sowie die Zementquelle(n) in diesen Milieus bestimmt werden. Die diagenetische Entwicklung begann im marinen Milieu mit der Ausfällung von fibrösem bis fibrös-blättrigem und epitaxialem Zement I. Diese Zemente wurden als Hoch Magnesium Calcit (HMC) direkt aus marinem Wasser ausgefällt. Die paläogeographisch flachsten Abschnitte des Profils auf den Maltesischen Inseln gelangten dann unter den Einfluss meteorischer Wässer. Dieser meteorische Einfluss führte zur Lösung von aragonitischen und HMC Schalen, was die Zementation mit Niedrig (Low) Magnesium Calcitischem (LMC) epitaxialem Zement II in diesem Profilabschnitt speiste. Im marinen Versenkungsmilieu fand anschließend der Hauptteil an Lösung, Zementation und Neomorphismus statt. Der erhöhte CO2-Gehalt in diesem Milieu, verursacht durch den Zerfall von organischem Material, führte zur Lösung von aragonitischen Schalen, was die Zementation mit LMC epitaxialem Zement II, blättrigem und blockigem Zement speiste. Die vorher ausgefällten HMC Zementphasen wurden entweder teilweise gelöst (epitaxialer Zement I) oder in LMC umgewandelt (fibrös/fibrös-blättriger und epitaxialer Zement I). Im versenkungsdiagenetischen Milieu fand anschließend geringe chemische Kompaktion statt, ohne aber die Ausfällung von größeren Mengen an Zement zu speisen. In einer letzten Phase gelangten die Gesteine durch Hebung wieder ins meteorische Milieu, was materialunabhängige Lösungserscheinungen verursachte. Diese Untersuchung zeigt, dass frühdiagenetische Prozesse, die an der Sediment-Wasser-Grenzfläche oder knapp darunter stattfinden, sehr wichtig für die mineralogische Stabilisierung von heterozoischen Karbonatabfolgen sind. Der Hauptteil der konstruktiven Diagenese in Form von Zementation findet in diesem Milieu statt, gespeist durch die Lösung von aragonitischen und, zu einem geringern Teil, HMC Schalen.<br><br> Die Ergebnisse dieser Studie implizieren, dass der ursprünglich vorhandene Anteil an aragonitischen Schalen in heterozoischen Karbonatsedimenten sehr sorgfältig bestimmt werden muss, da diese Schalen die wichtigste frühdiagenetische Zementquelle darstellen. In fossilen heterozoischen Karbonatgesteinen können aragonitische Schalen die wichtigste Zementquelle darstellen, auch wenn keine Reliktstrukturen wie mikritische Hüllen oder Biomolds erhalten geblieben sind. Im Allgemeinen stellt der Diageneseablauf in heterozoischen Karbonaten ein gutes Modell für die Diagenese von "Calcit-Meer"-Karbonatgesteinen dar.

Samples from three well cores in the Pipe Creek Junior Reef (Niagaran-Cayugan) in the' Wabash Formation of northeastern Indiana revealed an indirect and imperfect relationship between dolomite and clay content. A microscopic examination of the larger than two micron fractions of residues that were insoluble in acetic acid, and semi-quantitative X-ray analyses of the smaller than two micron fractions revealed illite to be the only clay mineral present in 91 samples. Illite comprised approximately 50 percent of the insoluble residue and more than 90 percent of the aluminum oxide-containing minerals. No magnesium silicates were found.Illite and dolomite contents were calculated from spectrochemical analyses of Al203 and MgCO3 respectively in 94 reef-influenced samples from three Indiana Geological Survey well cores. The regression and linearity of the dolomite/illite relationship revealed an imperfect correlation in two of the three cores.Nineteen thin sections of textural' varieties, from the core that did not reveal a strong illite/dolomite correlation, and eight thin sections from a portion of interbedded limestones and dolostones in the pipe Creek Junior quarry reef flank revealed the authigenetic origin of dolomite and the probable detrital origin of illite. The concentration of illite and other insolubles by post-micritization microstylolitization occurred along seams of carbonate solution and high permeability which developed selectively within the micritic matrices. Dolomite selectively crystallized in those more permeable sediments following the complete or nearly complete lithification of the reef.Ball State UniversityMuncie, IN 47306

Linking diagenesis to depositional facies and sequence stratigraphy enables better prediction of spatial and temporal distribution of diagenetic alterations, and thus of evolution of reservoir quality in sandstones. This thesis demonstrates that employing this approach is possible because depositional facies and sequence stratigraphy can provide useful information on parameters controlling the near-surface diagenesis, such as changes in: (i) pore-water chemistry, (ii) residence time of sediments under certain geochemical conditions, (iii) detrital composition and proportion of extra- and intra-basinal grains, and (iv) types and amounts of organic matter. Evidence from four case studies enabled the development of conceptual models for distribution of diagenetic alterations and of their impact on evolution of reservoir quality in sandstones deposited in paralic, shallow marine and fluvio-lacustrine environments. Diagenetic alterations that have been constrained within the context of depositional facies and sequence stratigraphy include: (i) carbonate cement (microcrystalline and equant calcite spars dolomite over poikilotopic calcite), pyrite and glaucony are most abundant in progradational braid-delta fan sequences, particularly along the topsets (i.e. maxiumum flooding surface, MFS) and along parasequences boundaries in the deltaic facies of the early highstand systems tract HST, (ii) cementation by coarse spar calcite, dolomite, and the formation of moldic porosity by the dissolution of framework carbonate grains are most abundant in the aggradational fan deltas sequences, (iii) eogenetic kaolinitization of framework silicates is largely restricted to the fluvial and paralic HST sandstones, whereas telogenetic kaolinite may occur in the transgressive systems tract TST sandstones too, (iv) formation of goethite ooids in the TST sediments, (v) formation of glaucony, siderite spherules, and extensive grain-coatings, grain-replacing and ooidal berthierine, more in the TST than in the HST sediments, particularly below the transgressive surface TS and MFS, (vi) cementation by calcite with (δ18OV-PDB = -11.5‰ to -5.4‰) and Fe-dolomite/ankerite (δ18OV-PDB = -10.8‰ to -9.6‰) occurs in both TST and HST sandstones, (vii) syntaxial quartz overgrowths are most extensive in the HST sandstones owing to the presence of incomplete grain-coating berthierine/chlorite, (viii) greater amounts of micro-porosity in the TST sandstones than in the HST sandstones are related to the greater amounts of berthierine/chlorite in the former sandstones, and (ix) chlorite rims around quartz grains retarded the precipitation of quartz overgrowths, and hence prevented a greater loss of primary intergranular porosity in fluvio-lacustrine sandstones. Therefore, constraining the distribution of diagenetic alterations in the contexts of depositional facies and sequence stratigraphic context is a powerful approach to be used in hydrocarbon exploration.

The Savannah River Site (SRS) deposits in the Southeastern US between 30-45 m of depth are calcium carbonate-rich, marine-skeletal, Eocene-aged sediments with varying clastic content and extensive diagenetic alteration, including meter-sized caves that coexist with brittle and hard limestone. An experimental investigation including geotechnical (P- and S-wave velocities, tensile strength, porosity) and geochemical (EDS, XRD, SEM, N2-adsorption, stable isotopes, K-Ar age dating, ICP-assisted solubility, groundwater) studies highlighted the contrast between hard and brittle limestones, their relationship with cave formation, and allowed calculation of parameters for geochemical modeling. Results demonstrate that brittle and hard limestones bear distinct geochemical signatures whereby the latter exhibits higher crystallinity, lower clastic load, and freshwater-influenced composition. Results also reveal carbonate diagenesis pathways likely driven by geologic-time seawater/freshwater cycles, microorganism-driven micritization, and freshwater micrite lithification. The second section of this investigation dealt with SRS surface soils which are largely coarse-grained and rich in iron oxides with various degrees of maturity. These soils were simulated in the laboratory using Ottawa sands that were chemically coated with goethite and hematite. Surface (SEM, AFM, N2-adsorption) and geotechnical properties (fabric, small-strain stiffness, shear strength) were investigated on the resulting "soil analog". Results indicate that iron-oxide coated sands bear distinct inherent fabric and enhanced small-strain stiffness and critical state parameters when compared to uncoated sands. Contact mechanics analyses suggest that iron oxide coatings yield an increased number of grain-to-grain contacts, higher surface roughness, and interlocking, which are believed to be responsible for the observed properties.

The Little Grand Wash normal fault near Green River, eastern Utah, hosts a series of naturally occurring CO₂ seeps in the form of active and extinct CO₂-charged springs distributed along the fault zone. I have studied the association of fault structure with CO₂-related alteration as an analogue for the long-term (1,000- to 10,000-year) effects of leakage through faults in CO₂ sequestration reservoirs. Structure and alteration in a portion of the Little Grand Wash fault zone were mapped at a 1:700 scale in order to determine the association of faulting with CO₂-related diagenesis. I combined structural and diagenetic mapping were combined with laboratory analyses of mineralogical, isotopic and textural changes in order to assess controls on the migration of CO₂ traveling up the fault and its effects on the fault itself. The fault zone is 200 m wide at its widest and contains 4-5 major subparallel fault segments that form multiple soft- and hard-linked relay ramps. The area includes a travertine deposit and related sandstone alteration: outcrop-visible coloration, porosity-occluding calcite cement and veins occasionally so abundant that they obliterate the rock fabric. Structural mapping shows that the travertine is located at an intersection of major fault segments constituting the hard link of a 450-meter-long relay ramp. Sandstone alteration is confirmed to be related to the CO₂ seep by mapping its distribution, which shows a decrease in concentration away from the travertine, and by the unique isotopic signature of calcite cement near the travertine. At distances greater than 25 m from the travertine intense alteration disappears, though scattered fault-subparallel veins and patchy, burial-related calcite cement remain. Intense alteration is limited to major fault overlaps and does not permeate the fault zone along its entire length, nor does it extend outside the zone. This indicates that rising CO₂-laden fluids do not flow uniformly through the entire fault zone, but that vertical flow is channeled at fault intersections. In thin section, porosity near the travertine has been extensively or completely occluded by calcite cement. Permeability in some conduit samples is less than 1 mD, three or four orders of magnitude lower than sandstone away from the travertine. In active CO₂ conduits, such reduction in porosity and permeability would occlude the preferred flow conduit and ultimately restrict upward flow of CO₂-charged water. X-ray diffraction detects small amounts of goethite and hematite and a decrease in chlorite-smectite in altered conduit sandstones. Calcite is abundant, but many authigenic minerals predicted by geochemical models of CO₂ influx into sandstone reservoirs are not observed, including kaolinite, aragonite, dolomite, siderite, ankerite or dawsonite. This difference between observed and predicted mineral occurrence likely results from differences in mineral kinetics between natural and laboratory systems. Prediction of leakage risk based on fault geometry improves the ability to assess the suitability of potential carbon sequestration reservoirs, many of which will be faulted. The point seep nature of leakage through a fault zone limits the amount of CO₂ that can escape over time and also enables targeted surface monitoring for CO₂ escape into the atmosphere--both critical for ensuring the effectiveness of injection projects and earning the trust necessary for carbon sequestration to gain public acceptance. The point seep nature of leakage also accelerates the rate at which conduits may seal through mineralization, since precipitation from a large volume of fluid is focused in a narrow conduit. The presence of multiple fossil and active seep locations along the Little Grand Wash fault, active at different times in the geologic past, indicates that cementation may be effective in sealing single conduits but that fault systems with complex geometry such as Little Grand Wash may continue to leak for a long period of time. / text

In the Permian Basin, strata of Leonardian age typically consist of interbedded carbonates and siliciclastics interpreted to be turbidite deposits. Happy Spraberry Field produces from a 100-foot thick carbonate section in the Lower Clear Fork Formation (Lower Leonardian) on the Eastern Shelf of the Midland Basin. Reservoir facies include oolitic- to-skeletal grainstones and packstones, rudstones and in situ Tubiphytes bindstones. Depositional environments vary from open marine reefs to shallow marine oolitic shoal mounds. Best reservoir rocks are found in the oolitic-skeletal packstones. Diagenesis occurred in several phases and includes (1) micritization, (2) stabilization of skeletal fragments, (3) recrystallization of lime mud, (4) intense and selective dissolution, (5) precipitation of four different stages of calcite cement, (6) mechanical compaction, (7) late formation of anhydrite and (8) saddle dolomite and (9) replacement by chalcedony. Oomoldic porosity is the dominant pore type in oolitic grainstones and packstones. Incomplete dissolution of some ooids left ring-shaped structures that indicate ooids were originally bi-mineralic. Bacterial sulfate reduction is suggested by the presence of (1) dissolved anhydrite, (2) saddle dolomite, (3) late-stage coarse-calcite cement and (4) small clusters of pyrite. Diagenetic overprinting on depositional porosity is clearly evident in all reservoir facies and is especially important in the less-cemented parts of the oolitic grainstones where partially-dissolved ooids were subjected to mechanical compaction resulting in "eggshell" remnants. Pore filling by late anhydrite is most extensive in zones where dissolution and compaction were intense. Finally, a porosity-permeability model was constructed to present variations in oolitic packstone- rudstone-bindstone reservoir rocks. The poroperm model could not be applied to oolitic grainstone intervals because no consistent trends in the spatial distribution of porosity and permeability were identified. Routine core analysis did not produce any reliable value of water saturation (Sw). An attempt to take advantage of wireline log data indicates that the saturation exponent (n) may be variable in this reservoir.

No description available.

No description available.

Thesis (Ph.D.)--McMaster University (Canada), 1988. / Source: Dissertation Abstracts International, Volume: 62-13, Section: A, page: 0000.