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The aeromagnetic anomaly high at Bigspruce lake N.W.T. is located over an alkaline and carbonatite complex. Approximately two-thirds of this complex is submerged, but sufficient rock is exposed on the shores and islets to reconstruct the original intrusive form and sequence. The rock-types are distributed in five geographic and petrographic centres showing a successive southerly migration. These are -</p> <dl> <dt>Centre 1</dt> <dd>saturated diorite, gabbro, olivine-magnetite-pyroxenite.</tdd> <dt>Centre 2</dt> <dd>saturated and oversaturated syenites.</tdd> <dt>Centre 3</dt> <dd>undersaturated foyaites and ijolites.</tdd> <dt>Centre 4</dt> <dd>saturated (high potassic) shonkinites, minettes, biotite-pyroxenites.</tdd> <dt>Centre 5</dt> <dd>silico-carbonatite and carbonatite, sovite and dolomite carbonatite.</tdd> </dl> <p>Centre 1 is located to the northeast of the main lake. It is composed of an outer diorite, and a later inner gabbro, layered intrusion, surrounded by arcs of olivine-magnetite-pyroxenite, and syenite intruded along a marginal ring fracture. Both the diorite and the gabbro possess broad wall-rock zones in rapid gradational contact with central laminated differentiates. The diorite lacks significant cryptic variation, but the gabbro is divided into anorthositic and gabbroic series showing the crystal accumulation of olivine, apatite and calcic plagioclase towards the base of the intrusion. The operation of continuous convection currents is evidenced by the radial dip of the lamination and the winnowing of the mafic minerals. Stronger intermittant currents in the gabbro are evidenced by radial trough banding, and rhythmic banking. The olivine-magnetite-pyroxenites are composed of olivine Fo_66-68, clinopyroxene Ca_44.6Mg_55.8Fe_9.6, magnetite and ilmenite in the approximate modal proportions 19 : 59 : 17 : 5. Minor barkevikite, biotite and apatite occur as an alkaline matrix. Quenching to magnetite-pyroxenite at the margins, remobilization of the contact granite or gabbro, and comparison of the modal composition with the field boundaries in the synthetic system ol-cpx-iron oxide demonstrate that these rocks crystallized from an ultramafic magma containing a small percentage of olivine and pyroxene phenocrysts. A temperature of intrusion of 1100-1200° is deduced. Although no descriptions of comparable rocks have been found these are regarded as the alkaline, ilmenite-rich, equivalents of the Alaskan-type zoned ultramafic intrusions. Centre 2 spans the northern half of the lake. It contains two concentric ring dykes of quartz syenite to the south, and two quartz free syenites around the margin of centre 1. The intrusion of these syenites followed an episode of transcurrent faulting which bisected centre 1. Mineralogically and in geochemistry the syenite series forms a continuation of the gabbro-diorite series, and spans the transitions pyroxene-alkali amphibole, and plagioclase → alkali feldspar (An₅₂ → An₂₈ → Or₃₈). A few late syenite dykes are peralkaline, and include aegirine bearing comendites. Centre 3 occupies the southern half of the lake. It is dominated by a 3.4 andtimes; 2.1km body of pegmatitic foyaite rimmed by chilled vertically laminated foyaite, and 10-100m of zoned fenite, A finer grained more mafic foyaite covers Bigspruce and Littlespruce islands. Both foyaites are composed of cryptoperthite Or₆₆, nepheline Ne₆₉, and aegirine and show a differentiation into feldspathic, nephelinic, and ijolitic horizons with low angle contacts, and a concentric distribution. This form is interpreted in terms of the preferential crystal settling of feldspar plates in the absence of convection currents. Both the foyaites show a chill composition lying within the low temperature "foyaite sink" in the system Qtz-Ne-Ks. Breccia pipes of sodic ijolite transitional into cancrinite foyaite are marginal to the carbonatite pegmatite focus of Bigspruce island. A possible origin through breccia-fluid exchange reactions involving the loss of K₂O and SiO₂ in the presence of abundant H₂O and Na₂CO₃ is suggested. High potassic rocks occur predominantly within a subsiduary complex 5km to the southeast of the lake, but identical types occur in close association with the carbonatites. They are composed of pyroxene, olivine, biotite, oxide, apatite and cryptoperthite Or₅₀, with a maximum feldspar content of circa 50%. The shonkinite forms dykes and breccia pipes of similar form and composition to those of the Navajo area, Arizona. The biotite-pyroxenites also form pipes, are transitional into shonkinite through the loss of feldspar, and are surrounded by zoned fenites. They are closely comparable to the stock of Libby, Montana, and to the "O.B.P." nodules found in the East African potassic provinces. Comparisons of the volume and composition of the fenite, pyroxenite and shonkinite are used to show that the biotite-pyroxenite may have originated from the shonkinite through the loss of normative feldspar to the wall rock in a fluoride-rich aqueous fluid phase. Both the foyaite and pyroxenite fenites resemble those of Fen, Alno and Chishanya, with concentric zones of aegirine-perthite-syenite, arfvedsonite-albite-quartz-syenite and biotite microgranite. The principal geochemical changes are the transfer of silica into the microgranite, and the transfer of feldspar with similar K : Na, ratio to the foyaite or "shonkinite" magma into the syenites. The carbonatites of centre 5 outcrop within two cone sheet and radial dyke foci of dolomitic and calcitic composition respectively. The dolomite focus lies on the east side of the lake within a fault bound structure. It consists of a central breccia plug of biotite-ankerite carbonatite surrounded by inwardly dipping sheets of dolomite defining a focus ~ 300m deep. The silicate content of the rock and iron content of the dolomite decrease away from the centre. There is a complete transition within these rocks from calcite bearing biotite-peridotite to carbonatite. On the basis of these replacement reactions a temperature of intrusion of 500-560°C is deduced. The sovite focus lies on the west side of Bigspruce island. It shows a well-defined radial dyke and cone-sheet distribution of silicate-calcite pegmatites associated with Fe, Ti, Ca, Nb, R.E., CO₂, H₂O, Cl metasomatism. Multiple fracturing and intrusion within these pegmatites shows a sequence of:- Aegirine + magnetite + calcite; Biotite + magnetite + calcite + pyrochlore; Biotite + calcite + bastnasite. Biotite carbonatite, fluorite carbonatite, and bastnasite carbonatite occur as peripheral satellite bodies. Mineral textures in the carbonatites give evidence of chilling, quenching and the crystal settling of apatite within the magmas. The close similarity of the trace element content and volatile content support the derivation of the carbonatites from the high potassic parent magma. The Sr^87/86 ratio of 0.7020 from the calcite and dolomite supports a mantle derivation for this parent. Within the exposed rock types three main differentiation trends may be distinguished. These are -</p> <ol type="1"> <li>Alkali gabbro - alkali diorite - syenite - quartz syenite-comendite.</li> <li>Alkali gabbro - rhomb syenite - sanidine syenite - foyaite.</li> <li>Biotite peridotite - shonkinite - biotite pyroxenite.</li> </ol> <p>The geochemical series 1 and 2, as illustrated by the Ca : Na : K and Mg : Fe : Alk diagrams, more closely resemble those of the Gardar province than those of the Oslo or Monteregian provinces but show a lesser degree of iron enrichment, and a greater enrichment in soda relative to Ca and K. These differences are thought to be due to the early disappearance of olivine as a fractionating phase, and to the dominating influence of feldspar fractionation in both series. Although both the saturated and undersaturated trend may ultimately be derived from the same parent and the composition of the magnetite-pyroxenite shows that it is related, the potassic series is believed to be derived from a distinct high potassic peridotitic parent of mantle origin. The differentiation of these magmas is believed to have taken place simultaneously in a series of independent magma chambers located within the prism of shattered rock at the intersection of a system of N-S and E-W transcurrent faults. The intrusion of the derived magmas was closely related to movement on the N-S fault system. On a regional scale the complex is located at the centre of a dome of granitic rocks close to the border between the Slave and Bear provinces. The age of the complex at 1820 m.y. is similar to the age of uplift of the neighbouring Snare orogenic belt. At the present time these rocks form the oldest known alkali-carbonatite centre.

It is well established that more than half of the world's hydrocarbon reserves are contained in carbonate reservoirs. In a global context, which is characterized by an increasing demand in energy, population growth and overall economic development, it is very important to unlock potential carbonate resources while mitigating the effects of climate change. Moreover, significant volumes of carbon dioxide - the major greenhouse gas contributor to global warming - can be stored in carbonate subsurface formations such as carbonate depleted reservoirs and deep saline aquifers. Therefore, better understanding of carbonate porous media has a wide range of major industrial and environmental applications. However, because of complex pore structures, including the presence of micro-porosity, heterogeneities at different scales, combined with high chemical reactivity, it remains very challenging to describe flow and transport in carbonates. In this thesis, we focus on carbonate porous media and aim to better describe flow, transport and reaction in them. The main application of this work is related to carbon storage in deep saline carbonate aquifers. More particularly, we address fluid-rock interactions e.g. wettability alterations and reactive transport, that occur in carbonate formations. First, we investigate the impact of wettability alteration on multi-phase flow properties. We use pore-network modelling to analyze the impact of wettability alteration by modelling water-flood relative permeability for six different carbonate samples with different connectivity. Pore-scale multi-phase flow physics is described in detail and the efficiency of water-flooding in mixed-wet carbonates is related to the wettability and pore connectivity. We study six carbonate samples. Four quarry samples - Indiana, Portland, Guiting and Mount Gambier - and two subsurface samples obtained from a deep saline Middle Eastern aquifer. The pore space is imaged in three dimensions using X-ray micro-tomography at a resolution of a few microns. The images are segmented into pore and void and a topologically representative network of pores and throats is extracted from these images. We then simulate quasi-static displacement in the networks. We represent mixed-wet behaviour by varying the oil-wet fraction of the pore space. The relative permeability is strongly dependent on both the wettability and the average coordination number of the network. We show that traditional measures of wettability based on the point where the relative permeability curves cross are not reliable. Good agreement is found between our calculations and measurements of relative permeability on carbonates in the literature. The work helps establish a library of benchmark samples for multi-phase flow and transport computations. The implications of the results for field-scale displacement mechanisms are discussed, and the efficiency of waterflooding as an oil recovery process in carbonate reservoirs is assessed depending on the wettability and pore space connectivity. Secondly, we investigate at the laboratory column scale (50 cm), fluid-rock interactions that occur through the injection of an acidic solution into carbonate porous media. Laboratory columns are packed with crushed and sieved porous Guiting carbonate grains. Therefore a homogenous porous medium at the Darcy scale is created and the effect of micro-heterogeneities on transport and reactive transport properties is highlighted. We first conduct a series of passive tracer experiments. Salinity is used as a non-reactive tracer as brine is injected at a constant flow rate into columns pre-saturated with equilibrated deionised water. Solute breakthrough curves are experimentally obtained by measuring the conductivity of collected effluent samples. Subsequently, by solving the advection-dispersion equations using PHREEQC geochemical software, we compare the experimental measurements with numerical predictions of breakthrough curves. A good match is obtained for a dual porosity model and a dispersion coefficient is estimated. We then investigate reactive transport by injecting at constant flow rate acidic brine (hydrochloric acid diluted in saline brine with an overall pH of 3) into columns pre-saturated with equilibrated brine. We measure the effluent concentrations using ICP-AES (inductively coupled plasma atomic emission spectroscopy) Moreover; scanning electron microscopy (SEM) is used to determine single grain-scale changes. We assess the impact of flow rate on the resident time distribution of solutes and reaction profiles along the columns. We discuss challenges encountered regarding the reproducibility of the results and we highlight the implications of such phenomenological studies on carbon storage in carbonates. Finally, we experimentally examine fluid-rock interactions that are induced by the injection of supercritical CO2 (sc-CO2) in carbonate formations at the pore scale. I designed and built a novel experimental apparatus that allows the injection of brine enriched with sc-CO2 at typical CO2 storage conditions. In our experiments the temperature is 500C and the injecting pressure is 9MPa. A novel methodology that combines pore-scale imaging, core flooding and pore-scale modelling is applied in the context of CO2-carbonate-brine interactions. We experimentally use a high pressure and temperature mixing vessel to generate brines enriched with sc-CO2.The mixture is then injected using high precision piston pumps at a constant flow rate (Q=0.1 ml/min) into carbonate micro samples (5 mm diameter and 20 mm length) saturated with pre-equilibrated high salinity brine. We measure the permeability changes in real time during the injection of reactive fluids, In addition, dry high-resolution micro-computed tomography scans are obtained prior to and after the experiments and the pore structure, connectivity and computed flow fields are compared using image analysis and pore-scale modelling techniques. We perform direct simulations of transport properties and velocity fields on the three-dimensional scans and we extract representative pore-throat networks to compute average coordination number and assess changes in pore and throat size distributions. Moreover, we assess the impact of reaction rate on reactive transport. We alter the reaction rate and hence the Damköhler number by under saturating the sc-CO2/brine mixture with crushed and sieved carbonate grains. Two regimes of dissolution are experimentally observed: dominant wormholing and a more uniform dissolution regime. High resolution 3D scans of the dissolution patterns confirm these observations. Permeability increases over several order of magnitude with wormholing whereas for the uniform dissolution, the increase in permeability is less pronounced. Overall, fewer pore and throats are present after dissolution while the average coordination number does not change significantly. Flow becomes concentrated in the wormhole regions after reactions although a very wide range of velocities is still observed. We then compare the observed results for single phase flow (wormholing induced by the injection of single phase brine saturated with sc-CO2) to two-phase flow reactive flow experiments (co injection of sc-CO2 and brine). Results show that wormholing is also seen in the two-phase experiments. Directions for future research in the area of fluid-rock interactions are then discussed.

Tidal heterolithic sandstone reservoirs comprise millimetre- to centimetre-scale intercalations of mudstone and sandstone. Their effective flow properties are poorly predicted by data that do not sample a representative elementary volume (REV), or models that fail to capture the complex 3D architecture of sandstone and mudstone layers. The aim of this dissertation is to develop improved predictions of effective single and multiphase permeabilities in heterolithic tidal sandstones. A surface-based modelling approach has been developed, which honours the geometry of geologic heterogeneity surfaces, integrating easily measurable outcrop derived dataset as input parameters. The workflow uses template surfaces to represent heterogeneities classified by geometry rather than length-scale. The surface-based methodology has been applied to generate generic, 3D mini-models of trough and planar cross-bedded tidal sandstones with differing proportions of sandstone and mudstone. The models closely capture the bedform architectures observed at outcrop and are suitable for flow simulation. Quantitative geometrical input data to construct the models was extracted from an outcrop analogue which records deposition in a tide-dominated deltaic and estuarine setting. The model results demonstrate that effective single-phase permeability of tidal crossbedded sandstones varies with sample volume. A REV of 1 m3 was identified, confirming that the model volume of 9 m3 (> REV) yields representative values. The impact on effective permeability of seven geometric parameters controlling the density of mudstone drapes was determined: mudstone drape coverage, foreset thickness, toeset dip angle, dune climb angle, foreset to toeset ratio, style of cross-bedding (trough or tabular), size of the mudstone drape patches. The impact of seven geometric parameters on effective permeability was determined. Effective permeability decreases as the mudstone fraction increases, and is highly anisotropic. Each parameter investigated can significantly impact on effective permeability, depending upon the flow direction and sandstone fraction, causing considerable spread of the values. During oil/water flow, relative permeabilities are not affected by mudstone drapes, but the irreducible oil saturation varies depending on flow direction and mudstone content. The workflow presented here can be used with subsurface data, supplemented by outcrop analog observations, to generate effective single and multi-phase permeability values to be derived for use in larger-scale reservoir models.

No description available.

This thesis focuses on the effective hydraulic transmissivity of two-dimensional fracture networks in rocks. The main simulation tool used in this work is the discrete fracture network code NAPSAC. There are four main topics in this thesis: (1) estimating permeability from network properties, (2) comparing discrete fracture network with effective continuum models, (3) using DFN for hydro-mechanical coupled modelling, and (4) solute transport simulations. For fracture networks with uniform aperture, the permeability can be estimated using segment density, fracture density, and fracture lengths of the fracture network. For fracture networks with apertures directly proportional to their lengths, the individual conductance of each of the fracture segments was used to calculate an effective conductance for the whole network. The arithmetic mean of the segment conductance gives a good approximation for the effective conductance of the whole network. A series of effective continuum models of a fracture network were created using different element sizes, and their flow behaviours were compared against results obtained from discrete fracture network model. The permeability tensors of each of the elements in the effective continuum meshes were calculated using discrete fracture network methods. It was found that the flow through effective continuum model with any element size gave good agreement with the discrete fracture network results. Hydro-mechanical coupled simulations were carried out using NAPSAC, where the applied far field stresses are applied to each fractures independently. Simulations were then carried out using the distinct element code UDEC to justify the simplified physics used in NAPSAC. It was shown that for random 2D fracture networks under a range of loadings, NAPSAC and UDEC seem to predict similar overall flows. Different ways for modelling the effects of rock matrix diffusion were explored. The significance of rock matrix diffusion, as well as the diffusion distance, was linked to the magnitude of the pressure gradient across the fracture network. A semi-analytical method for estimating the diffusion distance was proposed: using the perimeter and the area of each of the matrix blocks, it is possible to estimate the diffusion distance using the 'shape factor' concept.

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The palaeomagnetism and magnetic properties of Scottish Permo-Carboniferous quartz dolerite dykes at 22 sites were investigated. The measurements were performed using apparatus constructed and tested for this project. Alternating field (AF) demagnetization and thermal cleaning reveal, in all of the samples studied, the presence of a stable reversed NRM. The mean AF cleaned direction is 181.5 N,-13.3 which corresponds to a north pole position of 41 N, 174 E while the mean direction after thermal cleaning is 179.8 N,-15.5 which gives a north pole position of 42 N, 176 E. Baked country rocks were collected, from 45% of the sites and there is general agreement between the cleaned (AF and thermal) directions of baked and baking rocks at each site. This suggests that in both cases the remanence is the primary thermo-remanent magnetization (TRM) acquired during the dyke formation. Microscopic observations and Gurie point measurements indicate that titanomagnetite is the remanence carrier (dyke sample). A stable normal component of remanence almost antiparallel to that of the primary component (reversed) was isolated in 90% specimens from one of the sites (site 5) during AF treatment. The physical properties of dyke specimens collected from this site (with possible exceptions of Gurie temperature and direction of NRM) show no significant difference to those of dyke specimens at the other sites. Possible origins of the normal component were discussed. It is suggested that the normal component arises as a consequence of remagnetization (possibly by chemical process) over a time period during which the earth's magnetic field had changed its polarity. An anomalous magnetic profile (i.e. negative trough on the north) across an unexposed dyke was also investigated by drilling samples 3 metres below the soil surface and the results suggest that its primary stable remanence (reversed direction) is small compared with the component of magnetization induced by the present earth's magnetic field.

Six sites formed from serpentinite parent material on Greenhill, Strathdon and Unst, Shetland were excavated and the soil profiles sampled and described. After removal of organic material and separation of minerals into particle size fractions, the fractions obtained were studied by X - ray Diffraction, Differential Thermal Analysis and elemental analysis employing Atomic Absorption Spectrometry, using a dissolution procedure especially developed for conditions occuring here. Further investigations were performed using IR spectrometry and chemical dissolution techniques. Weathering tended towards three distinct types, these being (i) podzolization, characterized by bleached upper horizons due to leaching of Fe (III) down the profile, (2) laterization, not of tropical intensity but characterized by accumulation of hydrated iron oxides at the surface and iron rich clay minerals at depth and (3) gleying, characterized by reducing conditions virtually throughout the profile but with mottles containing Fe (III). The factors involved in the development of these profiles are discussed and possible mechanisms for weathering of serpentine are suggested. The dominant clay, minerals were smectite and vermiculite in all sites and the ratio of smectite to vermiculite increased with a decrease in particle size and with poorer drainage of the site. Two types of smectite were present, nontronite and saponite. The former occurs in freely drained sites and the latter under conditions of poor drainage. Chlorite, kaolinite and illite were also present. High levels of Chromium, Manganese, Nickel and Cobalt were found in the parent serpentinite and the clay minerals. The effect of particle size and drainage on the distribution of these elements was studied and their concentrations were lower in the clay fractions than in the parent serpentinite indicating that they are released into solution during weathering. Chromium is associated with chromite and nickel with serpentine. A method for the estimation of Chromium (III) and Chromium (VI) was developed and applied to the whole soils. Only Chromium (III) was found to occur naturally in the sites investigated.