Carbonate diagenesis encapsulates a wide range of water rock interactions that can occur within many environments and act to modify rock properties such as porosity, permeability, and mineralogical composition. These rock modification processes occur by the supply of reactant-laden fluids to areas where geochemical reactions are thermodynamically and kinetically favoured. As such, understanding the development of diagenesis requires an understanding of both palaeohydrology and geochemistry, both of which have their own complexities. However, within geological systems, both the conditions that control fluid migration and the distribution of thermodynamic conditions can change through time in response to external factors. Furthermore, they are often coupled, with rock modification exercising a control on fluid flow by altering the permeability of sediments. Numerical methods allow the coupling of multiple complex processes within a single mathematical formulation. As such, they are well suited to investigations into carbonate diagenesis, where multiple component subsystems interact. This thesis details the application of four separate types of numerical forward modelling to investigations of diagenesis within two Carboniferous carbonate platforms, the Derbyshire Platform (Northern England) and the Tengiz Platform (Western Kazakhstan). Investigations of Derbyshire Platform diagenesis are primarily concerned with explaining the presence of Pb-mineralisation and dolomitisation observed within the Dinantian carbonate succession. A coupled palaeohydrology and basin-development simulation and a series of geochemical simulations was used to investigate the potential for these products to form as a result of basin-derived fluids being driven into the platform by compaction. The results of these models suggest that this mechanism is appropriate for explaining Pb-mineralisation, but dolomitisation requires Mg concentrations within the basin-derived fluids that cannot be attained. Geothermal convection of seawater was thus proposed as an alternative hypothesis to explain the development of dolomitisation. This was tested using an advanced reactive transport model, capable of considering both platform growth and dolomitisation. The results of this suggests that significant dolomitisation may have occurred earlier on in the life of the Derbyshire Platform than has previously been recognised. An updated framework for the development of diagenesis in the Derbyshire Platform is proposed to incorporate these new insights. The Tengiz platform forms an important carbonate oil reservoir at the northeastern shore of the Caspian Sea. The effective exploitation of any reservoir lies in an understanding of its internal distributions of porosity and permeability. Within carbonate systems, this is critically controlled by the distribution of diagenetic products. A model of carbonate sedimentation and meteoric diagenesis is used to produce a framework of early diagenesis within a sequence stratigraphic context. The studies mentioned above provide a broad overview of the capabilities and applicability of forward numerical models to two data-limited systems. They reveal the potential for these methods to guide the ongoing assessment and development of our understanding of diagenetic systems and also help identify key questions for the progression of our understanding in the future.

The North Wales Platform, UK, represents a lower Carboniferous carbonate platform that developed during back-arc extension on the northern margin of the Wales-Brabant Massif. This succession was faulted and folded during the Late Carboniferous Variscan Orogeny and again during the Late Jurassic extension-Tertiary Alpine Orogeny, resulting in multiple reactivations of Caledonian structural trends (N-S, NE-SW and NW-SE) and basin inversion. The platform underwent deformation, several episodes of fluid-flow, and multiple phases of diagenetic overprinting. The products of fluid circulation in this area consist of the Mississippi Valley-type (MVT) mineralisation and dolomitisation, mostly affecting the carbonates of the lower Carboniferous (Dinantian) succession. This study presents a combined regional sedimentological, diagenetic and structural framework through multiscale, interdisciplinary techniques. Techniques include field observation, transmitted light and cathodoluminescence analysis, in-situ and bulk major and trace element analysis including rare earth elements, stable isotope (oxygen/carbon), and strontium isotope analysis. The North Wales Dinantian (Asbian-Brigantian) succession developed from a ramp to rimmed platform geometry and records a range of depositional and non depositional environments including platform margin, subtidal, peritidal and emergent. Early diagenesis comprises a series of marine and meteoric calcite cements. These are volumetrically the most important cements and occlude nearly all primary interparticle porosity on the North Wales Platform. Consequently, burial calcite cements and MVT mineralisation was precipitated within fractures and dissolution-enhanced secondary porosity. Dolomitisation on the North Wales Platform occur as pods along the current day coastline/palaeo platform margin and eight dolomite phases have been identified. These are present as matrix replacive and cement phases that are spatially and temporally related to deep seated structural lineaments. It is proposed that early diagenesis resulted from the establishment of meteoric aquifers, influenced by tectono-eustatic fluctuations. Subsequently, small volumes of fluid were released following compaction and during the waning stages of lower Carboniferous extension. The onset of the Variscan compression during the mid – Late Carboniferous led to the main stage of basin de-watering on to the platform via faults/fracture systems and the development of pockets of overpressuring. Circulating marine pore-waters provided the necessary magnesium required for dolomitisation within select fault/fracture systems. A second phase of tectonic deformation with associated copper mineralisation occurred during the Triassic-Jurassic extension and Alpine uplift. Fluids and metals for the copper mineralisation were derived from the adjacent siliciclastic Permo-Triassic and Jurassic East Irish Sea Basin succession. Compared to the adjacent and time equivalent Derbyshire and Askrigg Platforms, the North Wales Platform displays a more complex paragenesis as a result of differing burial histories and fluid sources. This study highlights the importance of understanding palaeo-fluid flow and diagenesis in platform carbonates and is directly relevant to hydrocarbon production, mining and resource containment in reservoirs.

>Magister Scientiae - MSc / This contribution engages in the evaluation of offshore sandstone reservoirs of the Central Bredasdorp basin, Block 9, South Africa using primarily petrophysical procedures. Four wells were selected for the basis of this study (F-AH1, F-AH2, F-AH4, and F-AR2) and were drilled in two known gas fields namely F-AH and F-AR. The primary objective of this thesis was to evaluate the potential of identified Cretaceous sandstone reservoirs through the use and comparison of conventional core, special core analysis, wire-line log and production data. A total of 30 sandstone reservoirs were identified using primarily gamma-ray log baselines coupled with neutron-density crossovers. Eleven lithofacies were recognised from core samples. The pore reduction factor was calculated, and corrected for overburden conditions. Observing core porosity distribution for all wells, well F-AH4 displayed the highest recorded porosity, whereas well F-AH1 measured the lowest recorded porosity. Low porosity values have been attributed to mud and silt lamination influence as well as calcite overgrowths. The core permeability distribution over all the studied wells ranged between 0.001 mD and 2767 mD. Oil, water, and gas, were recorded within cored sections of the wells. Average oil saturations of 3 %, 1.1 %, and 0.2 % were discovered in wells F-AH1, F-AH2, and F-AH4. Wells F-AH1 to F-AR2 each had average gas saturations of 61 %, 57 %, 27 %, and 56 % respectively; average core water saturations of 36 %, 42 %, 27 %, and 44 % were recorded per well.

The northwestern (NW) continental shelf of the Black Sea undergoes seasonal hypoxia. The benthic environment, the exchanges at the water-sediment interface and the diagenetic reactions are influenced by this phenomenon. In the framework of the BENTHOX project, two field cruises were conducted on the shelf in spring 2016 and in summer 2017.The first part of this investigation concerned the study of the impact of low oxygen levels in bottom waters on the diagenetic reactions. The microprofilings of geochemical parameters and the flux measurements showed both spatial and temporal variabilities in the benthic compartment of the NW continental shelf for the two seasons studied. The areas closest to the coasts exhibited the most important fluxes of oxygen consumption and of sulfate. These regions were strongly influenced by riverine inputs inducing a higher productivity and in turn resulting in an increase in the fluxes of organic matter deposited on the seabed.The diagenetic reactions were impacted by seasonal deoxygenation in bottom waters. The oxic respiration was less important in the summer as reflected by the shallower penetration depth of dissolved oxygen. Since 1995, the diffusive oxygen uptake (DOU) reported during hypoxic period indicated that the concentration of dissolved oxygen played an important role in the benthic exchange fluxes. Furthermore, a shallower reduction of sulphate and of Mn- and Fe-oxides observed in the sediments evidenced the impact of low oxygen levels on the diagenetic cascade. As a consequence, the benthic mineralization of organic matter was affected. During summer 2017, the oxic mineralization of organic carbon was less important and the contribution of the reducing species to oxygen consumption increased. The anaerobic mineralization of organic matter became thus the dominant process during the period of deoxygenation.The second part concerned the study of the sulfur and iron cycling in the shelf sediments during the low oxygen event of summer 2017. The sediments from the station close to the mouth of the Danube and that near the Dnieper exhibited a signature of detrital material different from that of the upper crust. The inputs of organic matter, in particular transported by the rivers, could influence the signal. The sediments of the NW shelf investigated were however of marine origin.Following the sulfate reduction already observed and mentioned above, the hydrogen sulfides produced were rapidly consumed as suggested by the low concentrations of acid volatile sulfide (AVS) and of the dissolved sulfide. The presence of pyrite in the upper layers of the sediments close to the water-sediment interface was plausible because the formation of pyrite in the water column had been reported due to the higher resistance of pyrite to oxidation compared to AVS (Wijsman et al. 2001). A non steady-state deposition was observed as suggested by Wijsman et al. (1999). It was characterized by an abrupt drop in the pyrite content caused possibly by fluctuations in salinity, dissolved O2 concentrations and organic matter fluxes.The non-reactive iron constituted the principal fraction of the total iron present in the sediments. Its contribution fell in the range reported for the marine sediments of the continental margins. A spatial variation of the concentration of highly reactive iron was nevertheless observed. Near the Dnieper mouth, the contents of reactive iron were lower and could be attributed to the less important fluvial inputs, to the sorption and/or precipitation processes or yet to the reallocation of the reactive iron of the shelf deposits towards the sediments in deep and euxinic waters.The last part of this thesis concerned the biogeochemistry of metals (Fe, Mn, Zn, Cu, Ni, Hg, Co and Cd) during the early diagenesis and the assessment of enrichment and pollution of these metals in the sediments of the shelf. The results showed that trace metals are probably linked to iron and manganese oxides as well as to sulphides. Ni and Zn could be involved in adsorption onto and co-precipitation with Fe- and Mn-oxides. Cu seemed to be associated preferentially to sulphides but no clear correlation was found. The enrichment and pollution of these metals in the shelf sediments showed spatial variability. The Danube delta area was enriched in Ni, Cu and Zn while the Odessa region was enriched in Co and Cd. The pollution of shelf sediments ranged from “unpolluted” to “very highly polluted”. Since the late 20th century, the heavy metal contents in shelf sediments remained stable in the Odessa region but increased in the Danube delta area. / Le plateau continental du Nord-Ouest de la Mer Noire subit une hypoxie saisonnière. L'environnement benthique, l'échange à l'interface eau-sédiment et les réactions diagénétiques sont influencés par ce phénomène. Dans le cadre du projet BENTHOX, deux campagnes de prélèvement ont été menées sur le plateau au printemps 2016 et à l'été 2017 dans le but de comprendre ces changements.La première partie de cette recherche concerne l’impact des faibles concentrations en oxygène dans les eaux de fond sur les réactions diagénétiques. Les micro-profils de paramètres géochimiques et les mesures de flux ont montré des variabilités spatiales et temporelles dans le compartiment benthique pour les deux saisons étudiées. Les zones les plus proches des côtes ont présenté les flux les plus importants de consommation d'oxygène et de sulfate. Ces régions ont été fortement influencées par les apports fluviaux induisant une productivité plus élevée et entraînant à son tour une augmentation des flux de matière organique déposée sur le fond marin. Les réactions diagénétiques sont affectées par la désoxygénation saisonnière des eaux de fond. La respiration oxique etait moins importante en été comme en témoigne la profondeur de pénétration moins profonde de l’oxygène. Depuis 1995, l’absorption d’oxygène dissous (DOU) rapportée pendant la période limitée en oxygène a indiqué que la concentration d'oxygène dissous jouait un rôle important dans les flux d'échanges benthiques. De plus, une diminution de la profondeur à laquelle la réduction des sulfates et la réduction des oxydes métalliques témoigne de l'impact d'une faible concentration en oxygène sur la cascade diagénétique. En conséquence, la minéralisation benthique de la matière organique a été affectée. Au cours de l'été 2017, la minéralisation oxique du carbone a été moins importante et la contribution des espèces réductrices à la consommation d’oxygène a augmenté. La respiration anaérobie de la matière organique est ainsi devenue le processus dominant pendant la période d'hypoxie.La deuxième partie portait sur l'étude du cycle du soufre et du fer dans les sédiments du plateau lors de l'événement de faible teneur en oxygène de l'été 2017. Les sédiments de la station proche de l'embouchure du Danube et de celle du Dniepr présentaient une signature de matière détritique différente de celle de la croûte supérieure. Les apports de matière organique, notamment transportés par les rivières, pourraient influencer le signal. Les sédiments du plateau nord-ouest étudiés étaient cependant d'origine marine. Suite à la réduction du sulfate déjà observée et mentionnée ci-dessus, les sulfures d'hydrogène produits ont été rapidement consommés comme le suggèrent les faibles concentrations en sulfure acide volatil (AVS) et en sulfure dissous. La présence de pyrite dans les couches supérieures des sédiments près de l'interface eau-sédiment était plausible car la formation de pyrite dans la colonne d'eau avait été signalée en raison de la résistance plus élevée de la pyrite à l'oxydation par rapport à l'AVS (Wijsman et al. 2001). Un dépôt à l'état non stationnaire a été observé comme le suggèrent Wijsman et al. (1999). Elle a été caractérisée par une chute brutale de la teneur en pyrite causée peut-être par des fluctuations de salinité, des concentrations d'O2 dissous et des flux de matière organiqueLe fer non réactif constituait la fraction principale du fer total présent dans les sédiments. Sa contribution est tombée dans la gamme signalée pour les sédiments marins des marges continentales. Une variation spatiale de la concentration en fer hautement réactif a néanmoins été observée. Près de l'embouchure du Dniepr, les teneurs en fer réactif étaient plus faibles et pouvaient être attribuées aux apports fluviaux moins importants, aux processus de sorption et / ou de précipitation ou encore à la réallocation du fer réactif des dépôts du plateau vers les sédiments en profondeur et eaux euxiniques.La dernière partie de cette thèse concerne la biogéochimie des métaux (Fe, Mn, Zn, Cu, Ni, Hg, Co et Cd) lors de la diagenèse précoce et l'évaluation de l'enrichissement et de la pollution de ces métaux dans les sédiments du plateau. Les résultats ont montré que les métaux traces sont probablement liés aux oxydes de fer et de manganèse ainsi qu'aux sulfures. Ni et Zn pourraient être impliqués dans l'adsorption et la coprécipitation avec les oxydes de Fe et de Mn. Le Cu semble être associé préférentiellement aux sulfures mais aucune corrélation claire n'a été trouvée. L'enrichissement et la pollution de ces métaux dans les sédiments du plateau ont montré une variabilité spatiale. La zone du delta du Danube a été enrichie en Ni, Cu et Zn tandis que la région d'Odessa s'est enrichie en Co et Cd. La pollution des sédiments du plateau allait de « non polluée » à « très fortement polluée ». Depuis la fin du 20e siècle, la teneur en métaux lourds des sédiments du plateau est restée stable dans la région d'Odessa mais a augmenté dans la région du delta du Danube. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

This study considers the chemical alteration of archaeological freshwater shell above the water table at Lyon's Bluff, located in east-central Mississippi, changes in trace element concentrations between unfired and fired shell, and the effect bacteria have on archaeological freshwater shell. Thin-section petrography, X-ray diffraction, cathodoluminescence, and scanning electron microscopy were conducted on archaeological shell from four layers from Unit 20N20W, with a depth of 80 cm spanning 450 years. ICP-MS analysis was also conducted on a modern freshwater shell. Results of the microscopy indicate pristine crystal structure. ICP-MS data show that certain trace elements within the shell maintain their concentration after firing at 500°C. The broader implications are: 1) that diagenetic alteration does not hinder chemical sourcing of shell at Lyon’s Bluff, and 2) that certain trace elements are more reliable than others, namely Sr2+, Al2+, Ba2+, and Mn2+, when conducting provenance studies on shell temper.

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An enhanced coal-bed methane facility in Russell County, Virginia is targeting lower Pennsylvanian coals for CO2 storage, but the shallow sandstone units intercalated with the coals may also prove to be potential CO2 reservoirs, since the injection apparatus is already in place. Using samples from a continuous core in southwestern Virginia, this detailed review of the petrography and local volume of the Breathitt Formation sandstone units examines their diagenetic alterations in order to assess the units' reservoir quality. The high-frequency sequences of immature sandstones, heterolithics, shales and coals in Russell County represent deposits from the transverse fluvial facies association of a broad braided-fluvial drainage system in the central Appalachian Basin. The sandstone units within these sequences are laterally extensive, maintaining similar thickness and gamma ray signature across the study area. Lower Pennsylvanian sandstone units are consistently sublitharenite with a diagenetic mineral assemblage including siderite, chlorite, kaolinite, albite, illite, silica and calcite. Primary porosity is not preserved, but secondary porosity (5 ± 3.1 %) has developed at the expense of feldspars and unstable lithic fragments. Permeability assessments collected in Grimm (2010) measured impervious values (0.005-0.008mD) for the medium-coarse grained sublitharenites. At the temperatures and pressures present within these units, CO₂ is unlikely to react with either the primary or diagenetic mineralogy in a way that negatively impacts continued injection on human time scales. Low pore volume and permeability due to the timing of certain authigenic mineral emplacement are the main hindrance to reservoir quality. Lower Pennsylvanian sandstones are not viable potential reservoirs for carbon sequestration. / Master of Science