Located in the midst of the northern Andes, the Mercaderes-Rio Mayo xenolith occurrence presents a unique insight into the crustal and mantle make up of the Andes in this area. The xenoliths are found in the Granatifera Tuff, which is a small tuff cone or tuff ring exposure. The Mercaderes-Rio Mayo xenolith suite contains heterogeneous assemblages of mantle-derived, crustal and igneous rocks, which attest to the heterogeneity of the crust and mantle in the area. The mantle at Mercaderes comprises mainly garnet peridotite at deeper levels, and shows a transition to websterites and pyroxenites towards the crust-mantle boundary. These are probably a product of intrusions of melts originated within a mantle modified by fluids released from a subducting slab. The lower crust comprises a variety of hydrated (Hbl + Bio) (e.g. hornblendites and pyribolites) and dehydrated (Cpx) (e.g. pyroxenites and granulites) rocks. These rocks represent the transition from amphibolite facies to granulite facies. Petrographical, geothermobarometric and geochemical evidence indicates that this involves the dehydration and probable melting of the hydrated rocks to produce the dehydrated rocks (restites). The protoliths for the lower crustal rocks include rocks that were tectonically accreted (lateral accretion) and rocks that were intruded into the crust (vertical accretion). The igneous rocks in Mercaderes are volcanic lamprophyric rocks and andesites, and diorites. The lamprophyric rocks are possibly the host for the mantle xenoliths, suggesting that their source lies within the mantle. The andesites show similarities with adakites, and it is likely that their magma formed with considerable crustal contribution, possibly from the dehydration of lower custal rocks. The magma sources in Mercaderes lie, therefore, within the mantle and the crust, and are possibly the result of mixing between these two. From the xenoliths from Mercaderes-Rio Mayo three processes involved in the crust and mantle formation beneath Mercaderes can be identified: Lateral tectonic accretion; intrusion of magmas into the upper mantle and lower crust and possible melting from restites. The model presented here suggests that the latter two are predominant in the deeper levels of the crust in the area.

Alkalic basaltic volcanism has occurred throughout Mongolia since the Cretaceous. Early Cenozoic lavas were erupted ~30 Ma, the in the Gobi, southern Mongolia, whereas younger lavas (6 to 0.005 Ma) were erupted in Hangai, central Mongolia (based on new 40Ar/39Ar dating). Previously proposed as plume-related, the Hangai lavas differ from Mesozoic basalts, but are remarkably similar to the 30Ma Gobi lavas. Focusing on all the Cenozoic lavas, chemical and isotopic studies (Sr, Nd, Pb, Hf and He) were undertaken to establish magmatic sources and melting processes. Analysis of crustal xenoliths indicate that the basalts have not been affected by crustal contaminations, and a strong heavy rare earth element depleted signature indicates that the lavas originated from within the garnet stability field. Petrogenetic studies show that the lavas are enriched in K, Nb, Sr and P. I propose that this enrichment is due to the break down of hydrous phases during the Cenozoic melting event. Alternatively, it originated from small degree partial melts ahead of an advancing melt front. The enrichment may have occurred at any time in the past; both ancient and modern metasomatic amphiboles occur within Cenozoic hosted mantle xenoliths. Low 143Nd/144Nd data in the basalts, support an old enrichment event, potentially in response to Palaeozoic subduction. In addition to an enriched lithospheric component, isotopic ratios suggest a depleted lithospheric mantle reservoir also contributed to melt compositions. Melt initiation most likely occurred in response to an asthenospheric thermal input, as there is no evidence for extensive lithospheric extension during the Cenozoic. Therefore, and asthenospheric component may have contributed to overall melt compositions, but is overprinted by lithospheric contributions. A thermal anomaly underlying Mongolia need only be slightly hotter than ambient mantle temperatures, as preferential melting of hydrous-bearing assemblages in the lithosphere could lead to alkalic melt generation.

Within the oil and gas industry it is of the utmost importance to determine the value of a hydrocarbon accumulation. Key to this process is the prediction of reservoir flow and performance. Measurements on core are the main source of information that underpin this crucial prediction; however core is not always available and it is necessary to find another means to estimate the required flow properties. Inspection of core data often reveals the presence of a simple relationship between the core porosity and the permeability to gas. Where this is the case, it is common practice to establish a relationship and then use estimates of porosity from wireline logs to predict permeability. However, for the reservoir examined in this thesis, like many others, this porosity-permeability relationship is neither linear, nor straight-forward. A plug porosity-permeability relationship for this reservoir was derived from the analysis of 199 sandstone plugs. In addition 63 of these plugs have image analysis data; the end-trims of these plugs were impregnated, sectioned and polished. Back-scattered electron microscope images at both high (x150) and low (x30) magnifications were taken, captured as 256 grey scales and numerically analysed. This analysis enabled the pore geometry to be quantified. The images data led to an improved understanding of the controls on permeability and porosity. This understanding was achieved by the demonstration that micro-porosity is an ineffective porosity in terms of fluid flow. Micro-porosity is identified as pores <13 m2 and mostly occurs within the clay (defined as the grey scale range 50-170 on the SEM images). Image porosity was calculated and observed to be less than plug porosity and has a stronger linear relationship with permeability; this is a result of micro-porosity being included in the measurement of plug porosity but not in the image porosity calculation. A non-linear relationship between gas permeability and mean pressure in the calculation of Klinkenberg permeability is present; a possible explanation for this observation is given.

A simple mathematical model of a sandstone aquifer has been constructed based on a local example, the Sherwood Sandstone of the East Midlands, UK. Simple seismic reflectivity calculations show that the air-water interface should theoretically produce a detectable seismic reflected wave for sandstone porosities as low as 10%. A synthetic seismic reflection dataset was constructed for a typical field survey geometry, and processed using the Promax system to produce a stacked section. The final section clearly shows the water table reflector. A field dataset from a subsequent survey has also been processed using the same sequence which also imaged a clear reflector at 30m depth. This is important evidence that the method has uses in identifying water table as a part of progress in shallow seismic reflection survey. The methods currently employed are (1) to define the optimum field, and (2) to define the optimum processing sequence, so that water table reflection can be imaged in a variety of geological situations. The application of Amplitude versus Offset (AVO) analysis to CMP gathers from the field data shows a characteristic increase of amplitude with increasing angle of incidence for super-critical reflection. In this way the water table reflector is clearly identified with the amplitude increasing by 30% over the range of incident angle from 28° to 34°. AVO analysis has also been applied to other field data that has a similar geological setting, but with a lithological reflector over the same super-critical angle. The resulting AVO curve shows a decrease in amplitude of over 90% with increasing offset, clearly differentiating from the water table reflection. Both water table and lithological results closely agree with theoretical predictions.

High resolution micro-CT images of porous rocks provide a very useful starting point to the development of pore-scale models of fluid flow and transport. Following a literature review covering recent results on the applicability of tomographic imaging to study reaction phenomena at the pore and core scales, this thesis presents a pore-scale streamline-based reactive transport model to simulate rock dissolution. The focus is on carbonate dissolution in CO2-saturated fluids. After injecting CO2-rich fluids into carbonate reservoirs, chemical reactions between the acidic fluid and the host rock are to be expected. Such reactions may cause significant variations in the flow and transport properties of the reservoir, with possible consequences for field development and monitoring. The interplay between flow and reaction exhibits a very rich behaviour that has not yet been fully understood, especially in the case of carbonate rocks, which possess a complex pore structure. The model is developed within a Lagrangian framework, where the advective displacement employs a novel streamline tracing method which respects the no-flow boundary condition at the pore walls. The method is implemented in the pore-space geometry reconstructed from micro-CT images of sedimentary rocks. Diffusion is incorporated with a random walk and fluid-solid reactions are defined in terms of the diffusive flux of reactants through the grain surfaces. To validate the model, simulation results are compared against a dynamic imaging experiment where a carbonate sample was flooded with CO2-saturated brine at reservoir conditions. The agreement is very good and a decrease of one order of magnitude in the average dissolution rate, compared to the rate measured in an ideal reactor, is explained in terms of transport limitations arising from the flow field heterogeneity. The impact of the flow heterogeneity in the reactive transport is illustrated in a series of simulations performed in rocks with different degrees of complexity. It is shown that more heterogeneous rocks, in the sense of flow heterogeneity, may exhibit a decrease of up to two orders of magnitude in the sample-averaged reaction rates, and that the flow rate is also an important factor when studying carbonate dissolution.

Numerical computations have been performed to evaluate the influence of bedform roughness on turbulent transport of sediments in geophysical flows. Special attention is paid to turbidity currents, which are responsible for the transport of sedimentary rocks far into the deep ocean. It has been suggested that enhanced turbulence mixing in flows over rugose topography contributes to the unexpectedly large runout lengths of naturally occurring turbidity currents. One of the objectives of this study is to provide evidence for against this conjecture. We perform computations over a wide range of periodic arrays of rectangular roughness elements, We find that a strong peak in turbulent mixing occurs when the width-to-height ratio equals a critical value of seven. We also find that a strong peak in resistance to flow occurs at the same critical value. These are competing effects, with the former acting to promote, and the latter acting to diminish runout length. So we are not able to conclude definitively that the enhancement of mixing is responsible for long runout lengths. We continue by considering flows over periodic arrays of shapes which are representative of bedforms that occur in the natural environment. We again find a strong correlation between the optimisation of both turbulence mixing and resistance to the flow. We are unable to distinguish bedform shapes that promote long runout length relative to the flat bed case. However, we are able to distinguish those bedform shapes that have large resistance to flow and large turbulence mixing compared to those that have low resistance and low turbulent mixing, with the latter case occurring for widely spaced asymmetric dunes with a long low angled slope facing the flow. Finally, we develop a model for flow and sediment transport which takes into account erosion and deposition from the bottom boundary. We first apply this model to flow over fixed dune shapes, in order to assess the influence of bedform shape on flow capacity, stratification, and the energy budget. An important result of this study is that flow capacity is optimised for the class of bedform shapes that promote low flow resistance and low turbulent mixing. We conclude by applying the model to the two-way coupled flow of a mobile dune, starting from an initially symmetric inherited dune morphology. We find that, for sufficiently large grain sizes, the dune evolves into a sequence of asymmetric dunes, rather than to a flat bed, and that the long-time evolution tends to be towards those dune shapes that promote large relative flow capacity. However, the model has a discrepancy in that it is unable to prevent the dune shape exceeding the maximum angle of repose. Hence, further work is required before these results can be regarded as reliable.

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No description available.

Melt entrapped as inclusions in early-formed phenocrysts provide geochemists with an exceptional opportunity to study sample material from the earliest stages in the formation of a suite of lavas. With a foucus on olivine-hosted melt inclusions, this Ph.D. thesis has explored the potentials for obtaining Sr isotope ratios on individual olivine-hosted melt inclusions, and examined the potentials for Sr isotope studies on melt inclusions to reveal new information on the origin of CFB and OIB. A novel technique is introduced that facilitate precise and accurate Sr isotope and trace element analysis of individual melt inclusions at sub-nanogram levels - thus applicable to typical melt inclusion suites from OIB and CFB, and in general to 'problems' where precise and accurate Sr isotope and trace element information is required on sub-nanogram Sr samples. The technique developed combines off-line sampling by micro-milling, micro Sr column chemistry, Sr isotope determination by TIMS, and trace element analysis by ICPMS. Olivine-hosted melt inclusions from two suites of high (^3)He/(^4)He lavas of the North Atlantic Igneous Province are studied. These reveal that Sr isotope and trace element measurements on individual melt inclusions provide a higher resolution picture of the pre-aggregated melt compositions and the different mantle and crustal components involved in the magma genesis, which otherwise were obscured within the whole-rock data. The Sr isotope and elemental variability recorded by the olivine-hosted melt inclusions contrast the more subtle variations of the host lava suites and raises the question of whether the (^3)He/(^4)He measured in melt inclusions in olivine phenocrysts should be related to the chemistry of melt inclusions rather than the bulk lava chemistry. The study further provides strong evidence that the extreme, high (^3)He/(^4)He signature observed in magmas from the North Atlantic Igneous Province is derived from a depleted component in their source, and hence such He isotopic signature should no longer be regarded as canonical evidence for primitive, lower mantle source.

In carbonates, the petrophysical interpretation is strongly controlled by the porosity exponent used in Archie’s Law due to the variety of the pore types, the complexity of the pore geometry and the topology. Traditionally, the petrophysical model modifies the porosity exponent with consideration of the porosity based on experimental resistivity. Additionally, the porosity models can be used to predict porosity exponent based on the equivalent resistance network, in series or parallel or both. However, these models do not consider the intrinsic factors, affecting the porosity exponent, such as pore geometry and topology and simplify the arrangement of pore types. In order to further build the formulation between porosity exponent and these intrinsic factors for accurately predicting porosity exponent, non-destructive pore scale measurement-CT imaging is used to extract pore geometry and topology, and to determine the main pore types. At the same time measurements of porosity exponent in the lab can be used to calibrate the pore scale modelling and used to investigate the influence of the pore geometry, topology and pore type on the porosity exponent. It is found that the key factors affecting porosity exponent are the ratio of pore cross section area ratio to the pore throat cross sectional area ratio (PTAR) and Euler number representing the connectivity. With the increase of the PTAR and Euler number, the porosity exponent increases. Both of them are related to the pore types. Fracture-like (FT) pores can reduce the PTAR and Euler number resulting in the decrease of the formation factor, porosity exponent and the anisotropy while vuggy-like (VG) pores increase PTAR and Euler number resulting in the increase of porosity exponent with less influence on anisotropy. The effect on formation factor is related to the type of VG, whether connected or non-connected. In coquinas from the Morro do Chaves Formation (Lower Cretaceous, NE Brazil) there are many combinations of pore, pore shape and pore connectivity that make numerical modelling of resistivity in these rocks such a useful addition to help explain the laboratory measurements and add an understanding of the dominant pore structure which was found to vary from sample to sample. Some interesting concavo-convex pore systems have been identified.