Sediment diagenesis in aquatic systems is usually understood to be controlled by the concentrations of both organic carbon and the oxidant. However, the concept that sediment respiration may be limited by the supply of organic carbon, even in systems with moderate concentrations of organic carbon in the water column, has yet to be fully explored. Typically we assume that a direct coupling between water column and sediment diagenesis processes occurs and the chemical evolution of porewater and surface water are linked through fluxes of chemical species across the sediment-water interface. While the dynamics of supply of particulate organic carbon (POC) to the sediments via plankton deposition and resuspension, has previously been examined, the fate of dissolved organic carbon (DOC) once in the sediments, has rarely been investigated. A series of experiments comprising batch tests, microcosms and sediment cores were conducted on sediment and water from four diverse field sites in which sediment respiration was considered to be carbon limited. Three sites were oligotrophic, acidic lakes and the fourth an oligotrophic coastal embayment. During each experiment dissolved organic carbon was added and measurements were undertaken of solutes that were considered participants in diagenetic processes. While each system differed in its chemical, biological and geological makeup, a key commonality was the rapid onset of anoxic conditions in the sediments irrespective of the overlying water oxygen concentrations, indicating lack of direct coupling between biogeochemical processes in the water column and sediments. Also, similar apparent DOC remineralisation rates were observed, measured solute fluxes after the addition of DOC indicated adherence to the ecological redox sequence, and increased ammonium concentrations were measured in the overlying waters of the acidic microcosms. In marine system experiments it was noted that diagenetic respiration, as indicated by decreasing concentrations of oxygen in the overlying water, increased rapidly after labile DOC was added. To explore the influence of geochemical processes on sediment respiration, a diagenetic model was tested against the laboratory data. The model was able to capture the rapid changes observed in the microcosms after addition of DOC in both the marine and acidic systems experiments. The model has the potential to serve as an essential tool for quantifying sediment organic matter decomposition and dissolved chemical fluxes. This work has focussed our attention on the control of DOC availability on sediment respiration and thus its ultimate control on solute fluxes across the sediment water interface. The results highlight the need to understand and quantify the supply of DOC to the sediment (as POC or already as the dissolved form), its transport through the sediment and its eventual remineralisation. This understanding is critical for improved management of aquatic systems, possibly even in systems where water column organic carbon is plentiful but sediment respiration is constrained by high organic carbon turnover rates in the water column and a resulting low flux of organic carbon to the sediment.

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published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

The Jurassic (Toarcian) Posidonienschiefer of southern Germany is famous for its well preserved vertebrate fossils and its high organic content. The majority of the Posidonienschiefer (10 meters thick in the study area) consists of the Bituminous Shale, a fossiliferous laminated illite claystone. Two thin (30-40 mm) clayey pyritic biomicrosparites, the Upper and Lower Schlacken, interrupt the Bituminous Shale. Geologists who have studied the Posidonienschiefer disagree about the exact nature of its depositional environment. The argument centers on the interpretation of an impoverished benthic fauna and whether or not the water column directly above the sediment-water interface was anoxic or normally oxygenated. Kauffman (1981) proposed that an algal/fungal mat located at or near the sediment/water interface marked the boundary between aerobic and anaerobic conditions during deposition. The purpose of my research was to investigate the geologic conditions during deposition and early diagenesis of the Bituminous Shale and the Schlacken and to search for evidence of microbial activity. A detailed petrologic investigation of these two lithologies found no evidence of an algal/fungal mat, but did reveal the important contribution of microbial activity in the formation of pyrite and calcite cement. The Bituminous Shale was deposited in a low-energy tropical seaway. The upper water-column supported a diverse marine fauna. The aerobic/anaerobic boundary in the water column may have been located several millimeters above the sediment/water interface. Pore waters of the ocean-floor mud were dysaerobic to anaerobic. Occasional oxygenation events allowed opportunistic benthic organisms to colonize the sea-floor. Compaction of the Bituminous Shale occured prior to cementation of original porosity. Framboidal pyrite was formed during sulfidic diagenesis under anaerobic, but open, sediment/pore water conditions. Euhedral pyrite formed later as communication between pores became restricted during sediment compaction. The skeletal grains of the Schlacken formed as a winnowed lag deposit of Bituminous Shale sediment. During the early stages of sulfidic diagenesis the winnowed beds were rapidly cemented in a concretion-like sheet. Early cementation preserved delicate algal spores and clay fabric. Fossil bacteria were discovered in the calcite cement of the Schlacken by modified petrographic techniques, and confirmed with the scanning electron microscope. Experiments in which live bacteria were gradually entrapped in halite produced a crystal fabric identical to that of the fossiliferous calcite cement of the Schlacken. The microbial production of bicarbonate and ammonia during sulfidic diagenesis played a significant role in altering local geochemical conditions in the Schlacken sediment and initiated the precipitation of calcite cements. Fossil bacteria in the cements of the Schlacken are direct evidence of the presence and entrapment of bacteria during cementation, but do not conclusively prove their active role in the formation of calcite. / text

Grain assemblages in organic-rich mudrocks of the Eagle Ford Formation of South Texas are assessed to determine the relative contributions of intra- and extrabasinal sediment sources, with the ultimate goal of producing data of relevance to prediction of diagenetic pathways. Integrated light microscopy, BSE imaging, and X-ray mapping reveal a mixed grain assemblage of calcareous allochems, biosiliceous grains (radiolaria), quartz, feldspar, lithics, and clay minerals. Dominant fossils are pelagic and benthic foraminifers and thin-walled and prismatic mollusks; echinoderms, calcispheres, and oysters are present. Early-formed authigenic minerals, including calcite, kaolinite, dolomite, albite, pyrite, quartz, and Ca-phosphate, some reworked, add to the overall lithologic heterogeneity. Point counting of images produced using energy-dispersive X-ray mapping in the SEM provides observations at a scale appropriate to classifying the mudrocks based on the composition of the grain assemblage, although grains and other crystals of clay-size cannot be fully characterized even with the SEM. Each sample is plotted on a triangle, whose vertices correspond to terrigenous and volcanic grains (extrabasinal components), calcareous allochems, and biosiliceous grains. As a subequal mix of grains of intrabasinal and extrabasinal origins the detrital grain assemblage of the Eagle Ford, presents a formidable challenge to the task of lithologic classification of this unit, as neither conventional limestone nor sandstone classifications can be readily applied. The abundant marine skeletal debris in the Eagle Ford is accompanied by abundant calcite cementation and the dissolution and replacement of biosiliceous debris is accompanied by authigenic quartz, suggesting that mudrock grain classification has potential for yielding diagenetic predictions. / text

The Laurentine Shore is the Imperial Roman palaeo-shoreline preserved up to 1km inland of the southern, distal edge of the Tiber Delta coastline of Lazio, western Central Italy. The progradation of the delta is recorded on the site as a series of shore-parallel relict dune ridges. High-status villas developed along the roman period coastline, with a service village (Vicus Augustanus), and other infrastructure such as roads, aqueduct, piscinae and several baths (thermae), these structures have been examined using a multi-scale geoarchaeological approach. A sea level reconstruction based on multi-proxy palaeo-environmental analysis of a silt/peat sedimentary transition from the base of a Roman piscina suggests that the sea level at ca. 2400 ± 40 BP was around 1.25 ± 0.2 m below modern sea level. This analysis provides further context for assessing the development of the site during the late Holocene in relation to the progradation of the Tiber delta and for the important Imperial Roman period occupation of the Laurentine Shore and other important sites such as Portus and Ostia Antica in the central part of the Tiber delta. At several key periods in the late Holocene, the palaeo-shoreline has been reconstructed using a geochronological framework of optical luminescence dates and geomorphological survey of the Tiber Delta dune ridge record. In particular, during the Imperial Roman period, ca. 2000 BP) it has been shown that the Laurentine Shore was settled during a period of significant Tiber delta shoreline progradation. Two-major building phases at the Vicus Augustanus occur within this progradation phase. By the abandonment of the site in the 5th century AD, the shoreline was around 70 m seaward of the shoreline during the 1st building phase of the Vicus. This rate of shoreline change could be noticeable by the population over decadal timescales and may have driven the alteration of coastal building and property plots during the 500 year lifetime of the settlement. A combined methodology incorporating sedimentology, geochemistry and petrological analysis of diagenetically altered sediments found that early vadose diagenesis may have a deleterious effect upon luminescence dating dosimetry, inducing age underestimation, especially of reddened dune sands. Petrological analysis has also shown that a lack of anomalous fading in luminescence behaviour observed in K-feldspars may be due to a lack of complex microstructure in the mineral grains driven by the metamorphic, Alpine origin of these minerals. An assessment of the geoarchaeological approach used in this thesis shows that a scale-driven context provides a useful structure for examining the various processes and factors affecting the geomorphological and sedimentological records improving confidence in the examination of the archaeological record.

Master of Science / Department of Geology / Matthew W. Totten / Shales are generally regarded as organic rich source and seal rocks that are unworthy of the amount of research that has been given to their coarser-grained counterparts, even though shales comprise nearly two-thirds of Earth’s sedimentary record (Potter et al., 1980). The Woodford Shale is acknowledged as a prolific source rock across much of Oklahoma and the midcontinent (Lambert, 1990). Up to 8% world's original hydrocarbon reserves are estimated to have been sourced by the Woodford and its equivalents (Fritz et al., 1991). Study of the heavy-mineral fraction in sedimentary rocks is important because it can indicate provenance and some of the diagenetic changes that occur in sedimentary rocks. This goal of this study is to describe the heavy-mineral fraction of eight Woodford Shale samples from the Greater Anadarko Basin of Oklahoma, and determine whether or not the constituents that make up the heavy-mineral fraction have any impact on the process of thermal maturity within source rocks. This study utilizes a method designed to efficiently separate the heavy-mineral fraction of shale samples. Scanning electron microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) are used in this study to identify mineralogy, grain size, composition and shape. Mineral distributions in the samples have been determined from point counting. The weight percent of the heavy mineral fraction was calculated for each of the samples. This was then compared to their location within the basin, depth, vitrinite reflectance and total organic carbon (TOC). We found that as the thermal maturity increase, the weight percent of heavy minerals also increases. Pyrite (FeS₂) was the most abundant heavy mineral found in the Woodford samples used in this study. From analyzing the different forms of pyrite, it was found that as thermal maturity increases, framboidal pyrite alters to euhedral pyrite.

Caractériser les réseaux de fractures dans les réservoirs de subsurface est un challenge majeur. En effet, les fractures contrôlant en grande partie la perméabilité dans les réservoirs, il est nécessaire aujourd’hui d’en prédire l’organisation. Or, la prédiction des fractures en subsurface est difficilement contraignable du fait (1) de données limitées en termes de résolution (sismique), ou de continuité spatiale (imagerie de puits, carottes), et (2) d’un manque de compréhension des facteurs contrôlant la fracturation. Les analogues de terrain sont une bonne alternative aux données de subsurface, permettant d’accéder à la complexité du réseau de fractures, et aux hétérogénéités des carbonates en 3D. Pour chaque affleurement sélectionné, une étude pluridisciplinaire est réalisée, combinant géologie structurale, sédimentologie, diagenèse et pétrophysique. Ainsi, les conditions, le « timing » et les facteurs contrôlant la fracturation peuvent être précisés à travers l’histoire de l’encaissant carbonaté. La fracturation diffuse dans les carbonates est régie par (1) le faciès de dépôt qui contrôle les hétérogénéités mécaniques et la susceptibilité de la roche à la diagenèse, (2) la diagenèse que contrôle l’acquisition précoce du caractère cassant et l’inhibition de la fracturation tardive, et (3) l’enfouissement, responsable des variations de contraintes verticales et de l’apparition des fractures et stylolites. / Characterizing fracture networks in Naturally Fractured Reservoirs (NFR) is a major challenge for hydrocarbon exploration and production. Because fractures control most of the permeability through the reservoir, there is a necessity to predict their organization. However, at present-day, fracture prediction in the subsurface is poorly constrained because: (1) of limited data from seismic, well imaging and cores, and (2) of lack of understanding of controls on fracture occurrence. Outcrop analogues are good alternative to subsurface data for a full 3D fracture characterization. It enables to access the spatial complexity of fracture patterns, and to the 3D heterogeneities of carbonates. In each selected outcrop, a multidisciplinary approach has been undertaken, There, the conditions, timing and controlling factors on fracturing can be precised through the geodynamic history of the host carbonate. Diffuse fracturing in carbonates is governed by (1) the depositional facies which controls mechanical heterogeneities and rock diagenetic susceptibility, (2) the diagenesis which controls the early rock embrittlement and the late fracture inhibition, and (3) the burial, responsible for vertical stress variation and fracture–stylolite occurrence.

Afin de satisfaire à la croissance des besoins énergétiques mondiaux, le domaine des réservoirs carbonatés très enfouis (Deeply Buried Reservoirs) constitue une cible privilégiée pour l'exploration et l'exploitation pétrolière. La préservation, dans les roches carbonatées, de propriétés pétrophysiques favorables (porosité / perméabilité) à grande profondeur peut alors conduire à l'existence d'un réservoir profondément enfoui (Deeply Buried Reservoir, DBR). Les processus chemo-mécaniques impliqués dans la diagénèse d'enfouissement des réservoirs carbonatés sont cependant encore mal contraints et il y a un manque crucial de données expérimentales sur ce sujet. Afin de mieux comprendre ces processus et de déterminer comment la porosité et la perméabilité peuvent être préservées aux profondeurs des DBR (>4000m), un dispositif expérimental novateur et le protocole associé ont été développés. Ce dispositif expérimental permet à la fois de reproduire les conditions de pression / contraintes / température des DBR (80°C, contrainte de confinement de 60 MPa et contrainte déviatorique jusqu'à 40 MPa) et de faire circuler des fluides de diverse nature dans les échantillons. Des échantillons d'un calcaire bioclastique induré (calcaire de Massangis) ont été testés via une approche multidisciplinaire : déformations mécaniques de fluage, suivi de l'évolution de la chimie du fluide de circulation et analyses pétrographiques et pétrophysiques des roches. Il a ainsi été démontré qu'il existe une relation importante entre la contrainte mécanique à laquelle est soumise la roche, les interactions thermodynamiques fluide-roche et l'évolution des propriétés pétrophysiques. Le processus principal responsable des déformations de fluage des roches carbonatées lors de l'enfouissement a été déterminé comme étant la pression-dissolution sous contraintes, processus chemo-mécanique impliqué dans la perte de porosité lors de la diagénèse de profondeur. Les expériences menées l'ont été avec différentes conditions de circulation de fluide (saturation et circuit ouvert) et différentes compositions chimiques du fluide ont été testées (eau météorique, eau météorique additionnée de phosphates et saumure). De même, les effets des hydrocarbures et de leur timing de mise en place ont été étudiés. Les résultats obtenus ont permis d'affirmer que, en présence de fluide météorique, la PSC se produit à l'échelle micritique, et résulte en la précipitation de calcite sur les faces libres de la calcite, provoquant le blocage de la microporosité. La préservation des macropores permet de conserver la perméabilité. L'addition de phosphates dans le fluide de circulation résulte en l'inhibition de la PSC tandis que l'augmentation de la salinité par l'ajout de NaCl accélère ce mécanisme. L'injection précoce d'huile dans les échantillons, précédant la circulation de fluide aqueux provoque l'inhibition de la PSC par un coating des grains, résultant en la préservation de la porosité. Les résultats de cette étude montrent l'importance de la chimie du fluide de circulation et du timing de mise en place des hydrocarbures dans la préservation de la porosité précoce à grande profondeur / The preservation of good petrophysical properties (high porosity / high permeability) at great depth in carbonate rocks may lead to the existence of a deeply buried reservoir (DBR), a target of interest for the oil industry. However, chemo-mechanical processes involved in the burial diagenesis of carbonate petroleum reservoirs are still poorly understood or restrained and few experimental results exists in this domain. To better understand these processes and explain how porosity and permeability can be preserved at the great depth of DBR (burial > 4000m), we developed an innovative experimental device and an associated protocol. This device allows both the simulation of high pressure/stresses/temperature conditions (80°C, 60 MPa of confining pressure and differential stress up to 40 MPa) of DBR and the circulation of different fluids in rock samples. Through a multidisciplinary approach we tested core samples of a cemented bioclastic limestone (Massangis limestone) and we analyzed creep deformations, chemistry of pore fluids, petrographical and petrophysical properties of samples. It has been demonstrated that there exists an important relationship between the mechanical stress affecting the rock, the thermodynamic of water/rock interactions and the evolution of the petrophysical properties. The main process responsible for creep deformation of carbonated rocks during burial has been determined to be Intergranular Pressure Solution (IPS) resulting in Pressure Solution Creep (PSC) which is the main process implied in the porosity reduction of a carbonate rock during deep burial. Different flow conditions (no-flow and flow-through) and chemical compositions (natural meteoric water, adjunction of phosphate ions, brine) were considered. The effects of hydrocarbon and of their timing of set-up have also been investigated. It has been discovered from our study that, in presence of meteoric fluid, PSC happens at the micritic scale, and results in the blocking of microporosity between micrites crystals due to the precipitation of calcite on free pore walls of micrites. The preservation of macropores allows the preservation of permeability. The adjunction of phosphate ions in the circulating fluid results in the inhibition of PSC while the increase of salinity by addition of NaCl leads to its acceleration. An early injection of oil prior to water circulation causes the inhibition of PSC by the coating of the grains, leading to the preservation of porosity. The dataset obtained from this study show the importance of fluid chemistry and of the timing of oil charging in a reservoir in the preservation of early porosity at great depth