No description available.

No description available.

No description available.

No description available.

Using the lattice-Boltzmann method, simulations of fluid flow have allowed the effects of moldic dissolution and cementation on porosity and permeability to be studied. Moldic dissolution was simulated by the removal of whole and partial spheres to generate vugs and uniform cement by increasing the radius of the spheres so they coalesce. For an uncemented domain, the evolution effects of increasing moldic dissolution showed that the permeability is initially controlled by the intergranular porosity and the moldic pores are essentially isolated. As porosity increases, with approximately 9% dissolution the moldic vugs start to form connections across the domain (percolation) which agrees with existing empirical relationships. The permeability upon percolation is dominated by the connected pore-space and controlled by the moldic porosity which agrees with other work on field and simulation studies of carbonate rock fabrics. With increasing cementation, the sphere packs show consistent decreases in permeability as porosity is occluded and correlates well to existing analytical solutions of coalesced sphere packs and empirical relationships. With the addition of moldic porosity, which leaves a cement shell intact, initial permeability is controlled solely by the intergranular porosity until percolation occurs and then connected moldic porosity dominates. It was found that connected moldic porosity and intergranular permeability can be treated as separate additive contributions an allow data sets of contrasting sphere arrangements to be correlated. Partial moldic dissolution was found to represent an earlier stage of dissolution that leads to the porosity and permeability of values seen for the whole-sphere dissolution. Modification of the cement by adding in holes reconnects the two pore systems and gives an automatic 54% permeability increase that occurs when a small connection is made for just a 1 % removal of cement. This study highlights the importance of the controlling pore type and the petrophysical property of pore-throat diameter. An additional field study of carbonate rock samples collected from Eleuthera, Bahamas allowed rocks to be reconstructed using micro-CT. Simulations of fluid flow of these samples and a quantitative image analysis of the micro-CT samples indicates that permeability can be effectively described and characterised by percentage contributions of pore-length to porosity, pore-area, a macropore shape factor and the number of data points measured in the sample.

ABSTRACT This study used "TOUGHREACT" to simulate the interactions between subsurface multiphase fluid and heat flow and solute transport resulting from water-rock interaction in carbonate platforms. A high-resolution 2D RTM constructed to evaluate the sensitivity of geothermal convection and brine reflux to variations in extrinsic and intrinsic controls. The model was also modified to investigate the diagenetic potential of "latent reflux". Simulations suggested that refluxing brines rapidly restricts geothermal convection to the platform margin, with flow focussed in the more permeable shallow carbonates. Complete dolomitisation occurs beneath the brine pool then extends downward forming partial dolomitisation. This enhanced porosity at shallow depth, however, anhydrite precipitation occluded porosity within the partial dolomitisation area and limited the depth of reflux circulation. In contrast, geothermal:c~nvection forms a smaller partially dolomitized body over a longer time. Higher rates of fluid flow in circular platform increased the depth of alteration beneath the brine pool compared to linear baseline. Temperatures vanations affected dolomitisation rate to different extends. For accurate estimation of the diagenetic potential of different salinities brines, two versions of TOUGHREACT code were used with pre-estimating ionic strength of brines. The HKF model used for brines characterized by < 3 molal ionic strength, while for> 3 the HMW model was used. "latent reflux" caused only minor dolomitisation due to prior Mg2+ consumption at shallow depth. With geothermal circulation re-established, dolomitisation became focussed towards the platform margin. The sedimentary textures variation in strati graphic sequence, secular variation of seawater and precursor mineralogy were key factors affecting dolomitisation and were explored through a ID high resolution model that represent a complex strati graphic sequence of upper Jurassic Arab-D Formation. Observations indicate preferential alteration of the finer-grained beds within the stratigraphic sequence. Simulations indicate that this phenomenon representative of individual carbonate cycles formed in response to eustatic sea level rises.

The Chalk is a fractured rock with a fine-grained porous matrix, where the matrix provides most of the porosity and storage capacity and the fractures greatly enhance the permeability. For the unsaturated zone, the role of the fractures and the matrix in flow and solute transport is not well understood. Therefore a tracer test was carried out in the unsaturated chalk at the Fleam Dyke research site in Cambridgeshire. The experiment, under natural rainfall conditions, involved distributing deuterium and bromide on a grass-covered lysimeter (a cube of volume 125 m3) and on an adjacent 4x4 m plot. Up until 15 months after the tracer application, none of the tracer was detected in the lysimeter drainage at 5 m depth. Repeated core profiles on the adjacent plot revealed significant differences in vertical tracer distribution, indicating lateral heterogeneity of transport pathways. The results of the tracer test were modelled with MACRO 5.0, a numerical dual permeability model that was initially developed for macroporous soils. The model suggested that fracture flow is important at the site, but that it is only initiated below a depth of about 1 m. The extent of fracture flow appeared to be highly variable in different layers of the profile, mainly depending on the saturated hydraulic conductivity of the matrix. Regarding exchange of solutes between the fractures and the matrix, the model indicated that advective exchange could be important and might even outweigh diffusive exchange. The results suggested that the Chalk aquifer at the Fleam Dyke site was only moderately vulnerable to pollution, but it was concluded that even for moderate rainfall conditions some bypass flow was possible. When modelling recharge without solute transport, it was shown that a simple water balance model called DFIDGWR could perform similar or better than MACRO 5.0.

Limestone Pavements are highly significant components of the physiographic and ecological landscapes of the UK. As relict glacial features they are subject to destruction by natural processes but also by human intervention. This thesis identifies the most effective methods to monitor such change at a variety of temporal and spatial scales, based on the Morecambe Bay pavements at Hutton Roof and Farleton Fell. The starting point for such a study is a methodology to define the baseline on which to base change detection and the key to this is the development of a. suitably detailed scene model. This must reflect the environment at the macro-, meso- and micro- scales and also incorporate considerations of the dynamics involved in the landscape evolution. The scene model (the land Surface Classification Hierarchy (lSCH)) was developed by field measurement of the reflectance spectra of the main elements, biotic and abiotic, with measurements of the pavement surface in terms of the scale of karren development and toe texture of the limestone itself. Study of the OEM allowed a fractal dimension to be established and also the nature of ice-flow and its contribution to pavement development, with extending flow, entraining fractured limestone blocks above a plastic, impermeable shale band, being the main mechanism. At the meso scale pavements were classified according to clint form derived from intra-pavement trends in grike direction calculated by Preferred Direction Analysis. Measurements of the key karren forms, runnels, solution pits and pipes and grikes allow assessment of their contribution to the variability of the pavement surface as an element of the scene model through the identification of solution domains.' Identification of different lithologies allowed an investigation of spatial variation across the study area, although lithological control on karren form and magnitude is weaker than variability from age of exposure as shown by statistical analysis of karren morphometry using univariate comparative methods and Link diagrams, bivariate and multivariate regression, discriminant analysis, cluster analysis, multi-dimensional scaling and star diagrams with the derived Star Index. Pavements were classified according to karren morphometry. The traditional view of pedestals asan indicator of solution rates, and hence the concentration of solution at the surface, is challenged through the investigation of water flow over the pavement surface and the consideration of the role of lichen as a protective agent as we" as the size of solution pits and grike width. It is suggested that only 10% of solution potential is achieved at the surface with 43% in the immediate epikarst. From this solution rate diagrams were developed, allowing the dating of exposure of pavements. These were shown to be within the period when human impact in the area was becoming significant and confirms an early anthropogenic impact on this element of the landscape. Further to this the development of grikes as emergent features was confirmed and this linked to' the concept of breakthrough, a"o~ing a model of grike development to be proposed, an important consideration in the dynamics of pavement change. At the micro scale texture analysis allowed the calculation of fractal measures which are related to variations in reflectance. The radiometric response of biotic and abiotic elements of the scene model was analysed confirming the facility of the baseline scene reflectance model of the pavement. Remotely sensed images from the Airborne Digital Camera were linked to ATM, CASI and TM images assessing the effect of scale on change detection and the evaluation of the pavement environment.

The Lower Carboniferous ('mid'-Courceyan to early Chadian) Black Rock Limestone ramp succession of SW Britain, accumulated along the southern margin of the Welsh-Brabant Massif during the early stage of development of the SW European foreland basin. Two depositional cycles are recognized which culminate in the subaerial exposure of those sequences in a more landward setting. The lower depositional cycle includes the Barry Harbour Limestone Formation, which contains a variety of storm-related sedimentary structures including hummocky cross-stratification. This is conformably overlain by the tidally influenced, cross-stratified Brofiscin Oolite Formation which locally displays evidence of subaerial exposure. Seven sedimentary lithofacies are identified in these two formations, which record a distal to proximal, shallowing-upward trend that passes from beneath mean wave-base to above fairweather wave-base. The upper depositional cycle of the Friars Point Limestone Formation includes the volcanics at Middle Hope, the Waulsortian facies at Castlemartin and Cannington Park, and is capped by the (herein named) Portishead Paleosol Beds (near Bristol). Above the basal transgressive sequence, the Friars Point Limestone consists of two distinct offshore ramp facies belts. The more proximal inner ramp facies belt was more strongly influenced by storm-related currents and exhibits tubular tempestites (storm sediment-filled burrows). The finer-grained, outer ramp facies belt is extensively bioturbated and preserves few features indicative of deposition from storm-related currents. The effects and relative timing of differential subsidence was elucidated by using the well documented biostratigraphy to separate the succession into isochronous units. The onset of pronounced differential subsidence was signified by volcanic activity at Middle Hope (Weston-super-Mare). The volcanic tuffs form a coarsening- and shallowing-upward sequence which records the development of a localised, probably non-emergent, offshore-volcanic high. Recognition of a major eustatic sea level fall at the end of the Courceyan and during the early part of the Chadian, is revealed by studying the decline of the Waulsortian reefs in an offshore ramp setting and evaluating the regional significance of the Portishead Paleosol Beds. Overall, the development of the Black Rock Limestone ramp was the result of the interplay between eustacy and tectonism. A full petrographic and geochemical analysis of the Waulsortian dolomites at Castlemartin, including the use of stable isotopes revealed that dolomitization occurred during burial, most probably as a result of basin dewatering.

Chalk putties are common in Southern England, occurring naturally and as a result of earthworks in intact chalk. The ease and readiness with which intact chalk breaks down into putty makes establishing a good geotechnical profile of the material necessary. A review of literature indicates that previous laboratory studies on chalk putties are limited, and that testing of the material has proven difficult using standard procedures. This study initially quantifies index parameters such as Atterberg limits, thixotropy and particle size distribution before considering susceptibility to age-related strength gains and shear strength-strain dependency so that subsequent shear test data can be normalised. Contrary to literature, age-related strength gains were found to be minor, whilst shear strength-strain dependencies were found to be significant. Large strain tests in ring shear apparatus (following recommended test procedures) found non linearity in the drained shear failure envelopes, with effective friction angles (Ø') increasing with strain. This non linearity is explained by sample grading evolution. Using these findings the study develops new preparation and testing methodologies to create ‘identical soils’ of known stress history. Testing in a computer governed stress path cell (using ‘Triax’ software) found that chalk putty behaves as a contractive material, ‘wet’ of its critical state, exhibiting failure by liquefaction for mean effective stresses (p') lower than 200kPa. Pre and post yield permeability values in the range 2.5-13 x 10-9m/s were recorded with state parameters indicating a constant a pre shear p' of between 0 and 400kPa.