Lake Rotorua has become increasingly eutrophic over the past 2 to 3 decades. The sediments of the lake have been shown to exert an important influence on this eutrophication process. Chemistry of the sediments has been studied to determine the nature, composition and distribution of elements, through a 1.5 year coring programme. A geophysical survey together with sub-bottom profiling has provided stratigraphic information related to the bathymetry of the lake. Lake Rotorua has two types of sediments: coarse, dense (density c. 0.5 g/cm3) sediments comprised of clastic erosion products and coarse rhyolitic airfall components covering approximately 60% of the lake area; and fine, low-density (approximatly 0.02 g/cm3) diatomaceous ooze that covers the remaining 40% of the lake, accumulated from deposition of biota, predominantly diatom frustules of Aulacoseira granulata. The sediment contains a record of volcanic eruptions, with the Tarawera Tephra typically found 0.5 m below the sediment water interface and Kaharoa Tephra typically between 2 to 3 m depth, in water depths of 10-15 m. Phosphorus concentration in Lake Rotorua sediments decreases with sediment depth. In the centre of the lake phosphorus concentrations in the top 2 cm can exceed 2500 g/tonne and decline to 800 g/tonne at 20 cm depth. Accumulation rate of phosphorus in the sediment based on the nutrient budget is approximatly 29.6 t/yr. Iron and manganese concentrations in the sediment depend on the availability of the element and the sedimentation rate of diatom frustules, and are controlled by the redox conditions in the sediment. The average concentration of iron and manganese in the sediment is approximately 8000 g/tonne and between 300 and 400 g/tonne, respectively. Iron accumulates in the sediment at a rate of 385 t/yr and manganese at 17.9 t/yr. Maximum concentrations of arsenic in the sediment are 250 g/tonne but are generally between 50-100 g/tonne, depending on the water depth. Lead concentrations are typically below 15 g/tonne. Sediment concentrations of both arsenic and lead are highly correlated with iron and manganese concentrations in the sediment and mimic the respective concentration profiles. Arsenic and lead accumulate in the sediment at a rate of 3.71 and 0.49 t/yr, respectively. All elements show a peak in concentration in the tephra layers. The bathymetry of Lake Rotorua is dominated by a curved depression extending from Sulphur Point and almost reaching the Ohau Channel. This depression is probably a structural feature likely associated with the collapse of the caldera, but could be an ancient drainage channel. A series of conical depressions clustered to the north of Sulphur Point and to the east of Mokoia Island are likely to be hydrothermal explosion creators. In the north in the lake at water depths less than 10 m, a series of near-shore terraces are preserved in the sediment. Sub-bottom echo-sounding shows no return of sonic and seismic signals from most of the lake floor, indicating total absorption by the methane gas-filled sediment. In the shallow lake margin environments, generally less than 10 m water depth, gas is absent and a detailed stratigraphy of multiple reflectors from tephra layers was observed with sub-bottom profiles. The basin sediments of Lake Rotorua are significantly pockmarked, with deep, circular flat-bottomed depressions c. 20-60 m diameter and 0.5-6 m depth. The pockmarks are located on the lake floor in areas where the sediment is saturated with gas.

Cette étude doctorale a consisté à décrire la dynamique géochimique de deux pockmarks à hydrates de gaz de la marge africaine en considérant deux approches différentes. La première zone d’étude, appelée Preowei, est située au large du Nigéria. Elle est caractérisée par un grand nombre de pockmarks de tailles différentes, plus ou moins proche les uns des autres. Les analyses géochimiques des échantillons de fluides interstitiels, combinées aux données géophysiques (séismiques) ont permis de mieux comprendre le schéma de migration des hydrocarbures pour un ensemble composé de quatre pockmarks très rapprochés. L’utilisation de ces données géochimiques dans un modèle de transport- réaction a conduit à une datation de plusieurs séquences de libération de gaz au sein de ces structures. Un schéma conceptuel décrivant les processus de formation et d’évolution temporelle des pockmarks a été proposé pour synthétiser les conclusions obtenues. Finalement, cette étude a montré que l’ensemble des pockmarks étudiés sont actifs depuis 2700 ans, qu’ils sont en phase de formation d’hydrates pour certains, et de carbonates pour d’autres. La deuxième structure étudiée est le pockmark Regab. Il est situé au large du Gabon, au nord du canyon sous-marin alimenté par le fleuve Congo. Il est caractérisé par la présence d’hydrates affleurant et une faune abondante et très variée sur toute sa surface. L’originalité de ce travail a été d’étudier la distribution de la mégafaune présente sur ce pockmark en fonction de la nature des fluides qui migrent dans le sédiment superficiel, et qui est libérés dans la colonne d’eau. Une attention particulière a été portée au méthane car c’est un élément central dans le cycle énergétique des microorganismes qui vivent en symbiose avec cette mégafaune. Trois nouveaux habitats ont été étudiés. Les données obtenues, associées à celle de la littérature ouverte, renforcent les conclusions des travaux antérieurs. Les Mytilidés ont besoin de très fortes concentrations de méthane pour se développer. Elles colonisent les zones de sortie de bulles et celle caractérisées par des hydrates affleurants. Les tapis bactériens sont associés à des zones où l’oxydation anaérobique du méthane se déroule dans le sédiment superficiel, avec une méthanogenèse dans la couche sous-jacente. Les Vésicomydé polychètes vivent dans des zones pauvres en méthane et sont très sensibles à sa variation de concentration. / The present work describes the dynamics of two pockmark areas, off West Africa. The intention is to propose two different approaches to study the relationships between fluid migration and pockmarks. The first investigated area corresponds to a pockmark cluster called Preowei, located off Nigeria. Geochemical analyses and modeling were combined with seismic data to detail the hydrocarbon migration pattern at this area, with implication on both the pockmark formation and the evolution of their morphology. The proposed interpretation seeks to identify the conceptual bases of pockmark evolution over time at this area. It is argued that the cluster has been active for at least 2700 years, and it is still at the stage of hydrate formation for some pockmarks and carbonate formation for other. The second investigated pockmark, called Regab, is located off Gabon. It is a giant pockmark of 800-m diameter, characterized by an ecosystem rich in fauna, with a large variety of living species. The main core of the work done on this pockmark was focused on finding a link between the fluid chemistry and the spatial distribution of the living communities which populate it. This was achieved by combining new geochemical and bathymetric results with a well-compiled dataset from the literature.

Le bassin profond du Bas-Congo est une marge passive affectée par une forte déformation salifère. Elle présente des sorties naturelles d'hydrocarbures lourds qui ont des impacts majeurs sociétaux, biologique, écologique et économique. L'objectif de ce travail est de comprendre les modes de fonctionnement de ces systèmes, depuis la mobilisation des hydrocarbures dans les sédiments, jusqu'à l'exutoire en fond de mer, puis en surface mer. Cette étude combine des analyses de données issues de l’imagerie spatiale, de l’océanographie, de la géochimie et de la géophysique marine afin d'obtenir à une vision intégrée multi-échelle des zones d’expulsion naturelles d'hydrocarbures dans le bassin profond du Bas-Congo. L'analyse de données issues de l'imagerie spatiale montre qu'une centaine de sites sont actifs dans la zone d'étude. Un volume de 4400 m3 d’hydrocarbures est naturellement émis par an. L'intégration des données de courantométrie permet de relier les nappes d'hydrocarbures visibles à la surface de la mer avec les structures fond de mer. Celles-ci correspondent à des chapelets de dépressions localisés sur le pourtour des diapirs de sel et à des regroupements de monticules composés d'asphaltes en périphérie. Dans les sédiments, les anomalies géophysiques caractéristiques correspondent à des cheminées verticales, délimitées par le bas par le réflecteur sismique associé à la base de stabilité des hydrates de gaz. La correspondance spatiale de ces différents critères a permis d'inventorier les sites potentiellement actifs sur les données géophysiques. Seulement 40% de ces sites sont associés à des nappes d'hydrocarbures récurrentes à la surface de la mer. / The Lower Congo Basin is a passive margin, affected by strong salt tectonics. Natural escapes of heavy hydrocarbons observed in the area have major impacts on the society, ecology, biology, and the economy. The aim of this work is to understand the mechanisms of these systems, from the mobilisation in the sediments towards the seafloor outlets and subsequently towards the sea surface. This study combines data analysis from spatial imagery, oceanography, geochemistry and marine geophysics in order to get a multi-scale integrated vision of the natural seepage situation in the Lower Congo Basin. The analysis of spatial imagery data shows that in the study area, the hundred active seeps sites expel a hydrocarbons volume of 4400 m3 per year, following an intermittent mechanism with miscellaneous frequencies from one site to another. We connected visible hydrocarbon slicks at the sea surface with seabed structures by integrating current measurements across the water column. Seafloor structures correspond to clustered pockmarks of high seismic amplitude located at the rim of salt diapirs and to clusters of mounds composed of highly degraded oil outwards. In sediments, geophysical anomalies form vertical chimneys, delimited by the seismic reflector associated with the base of gas hydrates stability downwards. The spatial correspondence of geophysical criteria enabled to inventory the potentially active sites on the geophysical data. Only 40% of these sites are associated with recurring oil slicks at the sea surface.

This study utilises seismic data to improve understanding of the subsurface fluid flow behaviour in the Taranaki and Great South Basins offshore New Zealand. The aim of this study is to characterise fluid flow features and to investigate their genesis, fluid origins and implications for subsurface fluid plumbing system by integrating seismic interpretation and 3D petroleum systems modelling techniques. After an early phase studying Pliocene pockmarks in the Taranaki Basin, this study has been focused on the subsurface fluid plumbing system and on the fluid expulsion history in the Great South Basin. The Taranaki Basin lies on the west coast and offshore of the North Island, New Zealand. The seismic interpretation revealed that paleo-pockmark formation in the study area relates to fluid escape due to a rapid sediment loading environment in a distal fan setting. Seismic analysis rules out any links between the paleo-pockmarks and faulting. The relationship between paleo-pockmark occurrence and fan depositional thickness variations suggests that pore-water expulsion during overburden progradation is the most likely cause of the paleo-pockmarks. The rapid sediment loading generated overpressure which was greatest on the proximal fan due to a lateral gradient in overburden pressure. Fluids were consequently forced towards the fan distal parts where, eventually, the pore pressure exceeded the fracture gradient of the seal. The Great South Basin lies off the southern coast of the South Island of New Zealand and is located beneath the modern shelf area. Evidence for past and present subsurface fluid flow in this basin is manifested by the presence of numerous paleo-pockmarks, seabed pockmarks, polygonal fault systems, bright spots and bottom simulating reflections (BSR), all of which help constrain aspects of the overburden plumbing system and may provide clues to deeper hydrocarbon prospectivity in this frontier region. The various types of fluid flow features observed in this study are interpreted to be caused by different fluid origins and mechanisms based on evidences from seismic interpretation in the study area. The possible fluid origins which contribute to fluid flow features in the Great South Basin are compactional pore water as well as biogenic and thermogenic hydrocarbons. Using 3D seismic attribute analysis it was possible to highlight the occurrence of these features, particularly polygonal faults and pockmarks, which tend to be hosted within fine-grained sequences. Paleo- and present-day fluid flow features were investigated using 3D basin and petroleum systems modelling with varying heat flow scenarios. The models predict that thermogenic gas is currently being generated in mid-Cretaceous sedimentary sequences and possibly migrates along tectonic faults and polygonal faults feeding present-day pockmarks at the seabed. The models suggest that biogenic gas was the main fluid source for the Middle Eocene paleo-pockmarks and compactional pore fluid may be the main fluid contributor to the Late Eocene paleo-pockmarks. Different heat flow scenarios show that only mid-Cretaceous source rocks have reached thermal maturity in the basin, whilst Late Cretaceous and Paleocene source rocks would be largely immature. The observations and interpretations provided here contribute to the ongoing discussion on basin de-watering and de-gassing and the fluid contributors involved in pockmark formation and the use of pockmarks as a potential indicator of hydrocarbon expulsion. It is clear from this study that seismically-defined fluid flow features should be integrated into petroleum systems modelling of frontier and mature exploration areas in order to improve our understanding on fluid phases, their migration routes, timings and eventual expulsion history.

Στο πεδίο κρατήρων του Κόλπου του Ελαιώνα, παρατηρήθηκε διαφυγή γλυκού νερού (Christodoulou et al., 2002). Προκειμένου να διαπιστωθεί αν ο παράκτιος υδροφόρος εκφορτίζεται στη θάλασσα, έγινε δειγματοληψία νερού σε πηγές και γεωτρήσεις στη χέρσο και πραγματοποιήθηκαν υδροχημικές και ισοτοπικές αναλύσεις. / Groundwater seepage was observed in the pockmark field in Eleonas Bay (Christodoulou et al., 2002). In order to find out if the coastal aquifer seeps through the seafloor, water samples were taken from springs and boreholes and hydrochemical and isotopic analyses took place.

Seismic reflection data from a multi-channel streamer deployed offshore Korea reveal evidence of hydrateforming gases being vented into the ocean. Numerous, localised vent structures are apparent from reduced seismic reflection amplitude, high seismic velocities, and reflector pull-up. These structures penetrate upward from the base of the gas hydrate stability zone (GHSZ) and are typically several hundred metres wide, and only a few hundred metres high. Underlying zones of reduced reflection amplitude and low velocities indicate the presence of gas many kilometers below the seabed, which migrates upward through near-vertical conduits to feed the vent structures. Where the local geology and underlying plumbing indicates a high flux of gases migrating through the system, the associated vent structures show the greatest change of reflector pull-up (the greatest concentration of hydrate) to be near the seabed; where the local geology and underlying plumbing indicates a moderate flux of gases, the greatest change of reflector pullup (the greatest concentration of hydrate) is near the base of the GHSZ. The distribution of gas hydrate in the high-flux gas vent is consistent with the recent salinity-driven model developed for a rapid and continuous flow of migrating gas, while the hydrate distribution in the lower-flux vent is consistent with a liquid-dominated system. The high-flux vent shows evidence of recent activity at the seabed, and it is likely that a substantial amount of gas is passing, or has passed, through this vent structure directly into the overlying ocean.

Saline aquifers and depleted hydrocarbon fields situated beneath the North Sea are currently being proposed as storage repositories for anthropogenic CO2 captured from point source emitters in the UK and mainland Europe. Two experimental sites are already operating successfully offshore Norway: Sleipner since 1996 and Snøhvit since 2007, collectively storing several million tonnes of CO2/year in the sub-surface. Despite the apparent success of these current projects, one of the major public and scientific concerns is the ability of storage sites to retain CO2 on the millennial timescales required for CO2 plume stabilisation and dissolution. Some areas of the North Sea are also known to contain palaeo-gas seepage pathways within overburden sediments that overlie deeper hydrocarbon reservoirs (e.g. Witch Ground Graben). These areas either need to be avoided for CO2 storage or rigorously assessed in terms of leakage risk. Since the Sleipner storage site lies within such a province, this thesis delivers a detailed evaluation of the Nordland Group overburden and a critical assessment of its long-term sealing capability for CO2. From interpretation and detailed mapping of a baseline 3D seismic dataset (acquired before CO2 injection operations commenced in 1996), we have identified numerous palaeo-migration pathways and high-amplitude seismic anomalies within the Nordland Group overburden sediments deposited above the Sleipner CO2 storage site. We attributed these features to thermogenic or biogenic gas migration, accumulation and bio-degradation over geological time. We also mapped a complex network of sand-filled, glacial channels and tunnel valleys distributed within a few hundred metres below seabed and highlighted their significance as potential fluid migration networks and/or secondary storage containment for leaking CO2. Of further significance, we confirmed that these overburden features also create spatial density variations that impact on the accuracy of seismic time-depth conversions, resulting in the probability of topographic distortions being propagated into seismic interpretations and models. To the best of our knowledge no such detailed mapping of the Nordland Group overburden at Sleipner has been undertaken previously. To determine whether the top layer of the CO2 plume at Sleipner might encounter these relict pathways as it ascends and migrates laterally beneath the caprock, we evaluated the critical column heights required for a CO2 accumulation to enter such a pathway under a range of storage conditions for a CH4/CO2/brine system; assuming that these pathways currently contain methane gas. Risking scenarios were based on a range of phase saturation, pressure, temperature, density, viscosity, interfacial tension and wettability conditions likely to be encountered at depths commensurate with the caprock at Sleipner. We concluded that given certain conditions at the caprock, CO2 could leak more easily into palaeo-migration pathways than CH4 (i.e. at lower entry pressures and therefore smaller column heights), assuming that brine densities and, most importantly, pore radii have not changed significantly over geological time (i.e. no cementation or dissolution has taken place). To further understand the dynamic significance of these palaeo-migration pathways, channels and tunnel valleys, including their ability to form inter-connected leakage/migration networks, we constructed a series of high-resolution 3D models of the Sleipner storage site and overburden, then used stochastic basin modelling and simulation techniques to investigate the processes involved during the introduction of CO2 into the storage site over a prolonged time period. Models were populated with geological, stratigraphic and structural information derived from our seismic interpretation. Flow simulations were calibrated to published data and matched to the present-day plume distribution. The absence of observational reservoir pressure and temperature data from Sleipner introduces significant uncertainty to model outcomes with respect to CO2 density and column height estimates and to surmount this difficulty we constrained the caprock temperature to CO2 density estimates obtained from the most recent gravity data observations at Sleipner. We concluded that the overburden heterogeneity is significant and palaeo-migration pathways, highpermeability channels and tunnel valleys at Sleipner may become potential migration pathways for CO2 as the plume continues to spread laterally over the coming decade, but the possible storage response is difficult to quantify given the absence of sufficient overburden rock property information and accurate pressure and temperature data for the storage site. The overall conclusion from this work is that insufficient information was collected within the Sleipner area prior to storage site development and too many significant studies which should have been performed as a pre-requisite (e.g. obtaining a caprock sample for laboratory testing of potential seal capacity), were actually performed some years after CO2 injection operations had already commenced. The pressure and temperature conditions at the caprock depth for the Sleipner storage site are also marginal in terms of maintaining CO2 above critical point conditions in dense phase and thus maximising storage efficiency. Most significantly, no rigorous overburden mapping and risking was performed for Sleipner (such as the work described in this thesis), thus the fact that no leakage has been detected at Sleipner is more due to good fortune than following best practices. Hopefully, our work has highlighted these key deficiencies so that future CO2 storage site feasibility and development studies will be performed more diligently.