Geophysical surveys allowed characterization of gas-charged sediments and targeting of seepage sites. Acoustic and seismic records revealed that at these locations the release of the hydrocarbon-rich fluids mainly occur from reservoirs sited below the seepage sites or from the dissolution of gas hydrates. Both the size of the minerals and the shape of the authigenic carbonate deposits appear to be related to the seepage conditions and the lithology of the enclosing host sediment. The slabs retrieved on the seafloor or on the subsurface from mud volcano structures consisted of micrite-cemented mud breccia and hemipelagic sedimentary units present. At these sites, finer grained sediments (or gas hydrates) appear to buffer the fluid seepage thereby enhancing the precipitation of authigenic carbonate in the more porous layers, forming slab shaped deposits. Most of the irregularly shaped deposits form the studied pockmarks showed micritic/sparitic calcite- and occasionally aragonite-cemented hemipelagic sediments with associated chemosynthetic fauna. These chemoherms arise when fluids seep through the sediment allowing a diffuse colonization of chemosynthetic fauna on the seafloor that becomes gradually cemented. The formation of tubular features in the subsurface seems to occur when methane-saturated impermeable sediments include weakness zones where the seepage of hydrocarbon-rich fluids is concentrated. Irregularly shaped deposits consisting of sparitic, zoned carbonate, devoid of hemipelagic sediments, are typical of settings where fluids seep through open fractures in more consolidated sediments. There is an association of seepage carbonate deposits, and injected sandstones above hydrocarbon reservoirs. The modelling of this plumbing system suggests that similar pathways can be used for the seepage of hydrocarbon-rich fluids and sand injections.

Gravity-driven sediment transport is a major mechanism occurring on active and passive margins. Past research on outcrops was limited by the incomplete preservation and limited 2D exposure. Over the last few decades geophysical techniques have made it possible to map modern sea-floors and to obtain subsurface images from seismic surveys. Each of the data has advantages and disadvantages; hence investigating and comparing gravity-driven sediments using complementary approaches was undertaken in this project to gain a more complete picture. Proximally, slide blocks form a planform jigsaw-pattern with an elongated axis parallel to the headscarp. Underperformed slide bodies (10s km) are found in the lower slope. Exposures of small-scale (meters) slide blocks show that the initial break-up stage of sand sheets forms a jigsaw-pattern of rafts, which down-dip transform into deformed and tilted blocks. At the base of slumps the end-member facies is typically a ‘rolled-up’ sandstone body. Preservation of the internal stratigraphy within slides is observed even in only partly consolidated sediments. Slide blocks may displace over long distances without loosing their internal geometry and deformation is only observed close to the shear surface, at the front and at the lateral margins. This makes the identification of slides difficult on seismic data and also at outcrop. Slide blocks are overlooked as petroleum reservoirs since it is commonly inferred that the internal geometries are discontinuities and because the sediment is overconsolidated. This may be a premature assumption as the lack of internal deformation implies continuity and minimal overconsolidation, especially in slides, which have only moved a little.

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No description available.

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The Upper Triassic of North Africa contains a number of extensive sandstone units, which are important hydrocarbon reservoirs in Algeria, Libya, Morocco and Tunisia. The stratigraphic subdivision of the Triassic reservoirs (TAGI- Triassic Argilo-Gréseux Inférieur) in the main Algerian fields is currently adequate for initial field development. However, doubts exist with regard to chronostratigraphic equivalence of reservoir zones and the identification of key bounding surfaces from field to field. A major problem is the definition of sequence boundaries in continental settings that are detached from direct effects of sea-level changes lacking biostratigraphic control. An accurate method of dating and correlating these sediments is therefore required in order to provide a refined stratigraphic and sedimentological model. Attempting to address these correlation problems, the influence of climate on sedimentation and cyclicity was introduced as an alternative method in order to link continental sequence. The influence of climate has increasingly been recognised as a control on runoff and sediment input into siliciclastic basins (e.g. Perlmutter & Matthews, 1989; 1990; Olsen, 1990; Clemmensen et al., 1994). Furthermore, the Upper Triassic documents an era when vast amounts of land were elevated and subjected to the direct effect of extreme climatic regimes driving sedimentation on the mega-continent. This study of Carnian-Norian continental fluvio-aeolian successions therefore tests the hypothesis whether sediment deposition in low palaeolatitudes of around 10 to 25° N on Pangaea was driven by mega-monsoonal oscillations (sensu Kutzbach & Gallimore, 1989). The study is based on the comparison of time- and facies-equivalent siliciclastic deposits within geographically separate basins integrating outcrop and subsurface data from the Agadir-Essaouira Basins (SW Morocco), the Fundy Basin (E Canada) and subsurface data from the Berkine Basin (Central Algeria). Comparable outcrop and subsurface studies have been undertaken with the ultimate aim of (1) providing analogues for the prolific TAGI (Triassic Argilo-Gréseux Inférieur) of the main Algerian fields; (2) correlating major climatic periods on a regional scale; in order to introduce a semi-chronostratigraphic component; and to (3) consequently assess the temporal and spatial distribution of these prolific fluvio-aeolian reservoir sandstones on a regional scale.

Reservoir wettability is controlled by crude oil/rock/brine interactions. Wettability plays an important role in oil recovery because of its effect on fluid location, flow behaviour and residual oil distribution. In mature oil fields, a development programme is initiated to maintain oil production in economic size. Understanding reservoir wettability is a central issue in such field development plans which can be optimised to boost oil production rate. However, there is a considerable uncertainty in understanding the reservoir wettability especially in terms of effect of brine composition and temperature. This thesis serves to minimise such uncertainty. In the present work, several experimental techniques are employed to investigate crude Oil/brine/rock interaction in mineral surface. A protocol of contact angle measurements is developed to mimic the drainage of oil thorough receding contact angle and imbibition of water through advancing contact angle. Experiments are designed carefully to investigate the role of water film, brine composition, asphaltene content, crude oil solvency and temperature in wettability of mineral surface. The consequence of wettability alteration is studied in how fluid behaviour is developed and affects the trapping fluids. A two dimensional glass micromodel gives new visual insights into the type of flow behaviour as a function of brine salinity and temperatures. Furthermore, the role of capillary number is investigated in three distinctive wettability scenarios that exist in oil fields. Real rock surfaces, which are extracted from underground reservoirs in Oman, are studied extensively to understand the role of mineralogy in rock surface. A novel experimental combination between Nuclear Magnetic Resonance (NMR) and capillary imbibition test are utilised to track the wettability alteration in core samples as brine salinity and temperature manipulated. For the first time, the introduction of optical interferometry to petroleum recovery literature is achieved successfully by using vertical scanning interferometry. A detailed investigation of surface profile of crude oil adsorption gives new information on nanoscale wettability. Application of the above-mentioned techniques to different crude oils on different reservoir conditions allows the establishment of fundamental wettability understanding to current enhance oil recovery techniques such as smart water flooding and steam injection. To our best knowledge, some of these finding are reported for, the first time.

The interpretation of geophysical field measurements of seismic anisotropy is presently limited by our knowledge of the controls of the elastic anisotropy of sedimentary rocks in the subsurface. Traditionally, laboratory ultrasonic velocity measurements have been used to provide important information on bulk aggregate seismic anisotropy, however, they do not allow the discrimination of the contribution from the various microstructural parameters (e. g., crystallographic lattice preferred orientation (LPO), preferentially aligned porosity, aligned fractures and the non-random spatial distribution of mineral phases). In this study the results from scanning electron microscope-electron backscattered diffraction (SENI-EBSD), quantitative X-ray diffraction (QXRD), image analysis, ultrasonic velocity measurements, palaeomagnetism, anisotropic magnetic susceptibility, and numerical modelling are combined to elucidate the controls of the elastic anisotropy of siliciclastic sedimentary rocks from an oil reservoir. SEM-EBSD was used to measure both the overall and individual constituent mineral phase LPO (Maddock et al. 2004). As phyllosilicates are both very fine-grained, with a high aspect-ratio and low crystallinity, their LPO contribution was established via a combination of image analysis and numerical modelling (Bingham approximation). These analytical and predictive methods for determining phyllosilicate fabric intensity produced consistent results. For the first time, the azimuthally preferred orientation of elongate grains within sedimentary rocks was determined using anisotropic magnetic susceptibility of ferrous minerals and were compared to those predictions obtained using EBSD. The strength of the fabric-texture (J), as determined by EBSD, is proportional to the maximum compressional and shear-wave anisotropy, as calculated from the Christoffel equation, by taking a Hill average of the bulk aggregate elastic constants. The quartz and feldspar velocity maxima aligned in a constructive fashion throughout most of the samples. It is possible that the preferred alignment of crystals detected by EBSD reflects the palaeoflow direction. The predicted symmetries of velocity anisotropy ranged from orthorhombic in the phyllosilicate-free, well-sorted, mature sandstones to strong vertical transverse isotropy in the unfractured phyllosilicate-rich mudstones. Vertical transverse isotropy is predicted to be oriented, such that, the plane of azimuthal isotropy is aligned parallel to bedding i. e., parallel to the horizontally aligned clays and micas. Similarly, orthorhombic symmetry is predicted to be oriented, such that, one plane of symmetry is aligned approximately parallel to bedding whilst the other symmetry plane is aligned parallel to the single most dominant fracture set. The results from this study provide the input needed for a general mathematical model for the reservoir allowing the prediction of seismic anisotropy for any rock in the reservoir given accurate modal proportions. The resulting model is an advance on the empirical correlations that are usually used to determine how seismic velocities are affected by factors such as clay content and porosity. In particular, the bulk aggregate elastic stiffness tensor obtained during this study can be integrated with high-pressure ultrasonic measurements to enable the prediction of the additional contribution from grain-scale effects such as shape-preferred orientations, and grain boundary compliances (Hall et al. 2007). The results from this study have also provided the basic data to allow field seismic data to be inverted to obtain estimates of in situ fracture density and orientation (Kendall et al. 2006). In summary, analysis of a suite of siliciclastic hydrocarbon reservoir rocks has shown that the LPO of constitutive minerals can offer information about the nature of a reservoir. The results suggest that seismic anisotropy is not only indicative of lithology but can also be an indicator of reservoir quality and palaeoflow direction.

Understanding and predicting the size, shape and internal architecture of sandbodies is of fundamental importance in hydrocarbon exploration. High quality hydrocarbon reservoirs are often formed in deltaic environments where there is a complex interplay between changes in relative sea level, sediment input and climate. When combined with the intrinsic sedimentary variability of deltas, this makes prediction of the internal facies distributions and architectures of deltaic sandbodies a challenging task. The aim of this thesis is to conduct a detailed qualitative and quantitative field study of fluvial and deltaic sandbody architecture and facies distribution, and to assess the usefulness of the data thus obtained in predicting the architecture and sedimentary characteristics of reservoir sandstones in the subsurface. The El Wastani Formation of the Nile Delta, Egypt, has previously been identified as an interval of reservoir quality sandstones within the Plio-Pleistocene deltaic succession. Limited core data, and poor seismic imaging due to gas seepage effects, hindered past attempts to assess the internal architecture and facies of the sandbodies. Therefore it was considered appropriate to use an outcrop analogue to aid understanding of the El Wastani Formation reservoir characteristics. From a review of literature, the Upper Carboniferous fluvial and deltaic sandstones of the Central Clare Group, County Clare, western Ireland, were found to be suitable analogues for the El Wastani Formation sandstones. Controls on the two sedimentary systems were similar; both were fluvial-dominated and wave-influenced, and both show evidence for fluctuating relative sea-level. Comparisons of facies observed in outcrop (Co. Clare) and interpreted from image logs (Nile Delta) show similar facies and sedimentary successions in the two systems, improving confidence in the choice of analogue. Fieldwork carried out on the Upper Carboniferous (Namurian) coastal outcrops of Co. Clare produced detailed measurements of facies distributions and bed geometries, which, together with sedimentary logs, palaeocurrent studies and outcrop-scale photomontages, enabled interpretation and quantification of channel dimensions, internal architectures and stacking patterns. Based on these data, the Tullig Sandstone, a major sandbody within the Central Clare Group, is interpreted to be a low-sinuosity, braided fluvial system that flowed to the north-northeast. This sandbody shows decreasing amounts of erosion and conglomeratic facies in both downstream and vertical directions, interpreted to reflect the combined effects of delta subsidence and sea-level rise over time, influencing the downstream reaches of the system first. The mean sand to non-sand ratio for the Tullig Sandstone is 97% by area, and connectivity of sandstone facies within this sandbody is 93%. In contrast, mouthbar sandbodies that were studied have a mean sand to non-sand ratio of 90%, and greatly reduced sandstone connectivity, at 65%. The data that characterise the field outcrops can be taken as indicative of the probable characteristics of the El Wastani sandbodies. The data generated from the quantitative field studies were used to construct computer models of the outcrops, in order to see how well the modelling software was able to reproduce the outcrop architectures and facies distributions, and also to test the sensitivity of the models to different scales of data. One large-scale model was built to include all the Tullig Sandstone outcrops along the coastline, with a vertical resolution (cell height) of 1m. A second smaller model was constructed to cover just the Trusklieve outcrop, and was built using a vertical cell height of O.1m. Each model was designed to fit the sedimentary log data, and was conditioned to reflect the facies percentages and channel dimensions measured and calculated respectively from the outcrops. The results showed that although the larger modelling grid, with lower vertical and horizontal data resolution, showed significant differences in finegrained facies distribution from the outcrops, it was reasonably successful at reproducing the channel shapes and stacking patterns seen in outcrop. In addition, the high sand to non-sand ratio meant that sandstone connectivity was not reduced compared with either the outcrops or the small, high-resolution model. The small model was better at reproducing the geometries of beds of fine-grained facies, but lacked the ability to accurately simulate the channel architectures and stacking patterns.