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Fluvial channel sandstones form some of the major hydrocarbon reservoirs throughout the world. They are also the most difficult for reservoir geologists and engineers to deal with, because of the variety of sedimentary environments and complex sand body geometries and relationships which directly contribute to the reservoir quality and distributions. Successful geological reservoir modelling of fluvial channel sandstones involves proper understanding of the geological background of the target area, choosing a suitable method for each particular modelling purpose, and making available appropriate well data and analogue data. It is also the key to providing reservoir engineers with essential information for further oil exploitation and production activities. Two 3D modelling methods for fluvial channel sandstone reservoirs have been explored and contrasted in this Ph.D. project: Vertical Sequence Generation (VSG) and Geometry Based Generation (GBG). Computer software had to be written specially for each as it is not available, commercially or otherwise. Markov stochastic theory is employed in VSG in two aspects of geological reservoir characterisation. The first is to analyse the vertical statistical transition characteristics of different reservoir types or geological units of interest; and the second is to generate pseudo-vertical sequences of unsampled reservoirs by using the obtained Markov transition properties. This is a further development of the use of Markov statistics in geological description, and is a new approach to geological reservoir modelling developed in this Ph.D. project. The GBG method takes channel sand body dimensions and orientation into consideration and generates each channel sand body, three-dimensionally in the area of interest, with its location, thickness, width, length and orientation.

This dissertation is an investigation into the geochemical and geodynamic evolution of the Upper Zone of the Bushveld Complex. The Bushveld Complex is one of the few large layered intrusions in which the entire cumulate stratigraphy is preserved and well-exposed from its base to its roof. Despite this unique feature, relatively little is known about the nature of magmatic differentiation in the uppermost portions of the Bushveld. As a first order, I quantify the chemical composition of the preserved stratigraphy (i.e. the bulk composition) from the geochemical base of the Upper Zone (the Pyroxenite Marker) to the contact with the roof. On the basis of major element modeling and trace element equilibria I show that the bulk composition is not representative of the original magma composition, and therefore some magma must be missing. I propose that the Rooiberg Group lavas and/or Rashoop granophyres, which make up the immediate roof of the intrusion, represent the missing magma. A further test of the magma-loss hypothesis comes from the trace element contents of apatites in the uppermost 625 m of the Upper Zone stratigraphy. Comparison of the equilibrium liquid compositions calculated from these apatites with the Rooiberg and/or Rashoop roof rocks shows that they are a geochemical match. My results also indicate a role for large-scale (>625 m) liquid immiscibility at the top of the Bushveld. This is the first documented evidence for liquid immiscibility based on the compositions of mineral phases, not melt inclusions; and it is the first quantitative evidence for large-scale immiscibility in the Bushveld Complex. Quantification of the parent magma composition at the Pyroxenite Marker allows me to not only estimate the `outputs' from the magma chamber, but also to constrain the `inputs'. A geochemical record of magma input and mixing is recorded in the cumulate stratigraphy for approximately 350 m below the Pyroxenite Marker. Using the evolution in mineral compositions I calculate the composition and proportion of incoming magma to the Upper Zone, as well as the style of input. The composition of the incoming magma is then compared to other known pulses of magma into the Bushveld Complex in order to put constraints on the source contributions and formation dynamics of the intrusion as a whole.

The research presented in this thesis employs spatial and statistical properties of major element variability in basaltic lavas and mantle residues to constrain some of the processes and dynamics occurring beneath ocean ridge magmatic systems. Ocean ridges represent a critical setting for many geochemical fractionation processes involved in the chemical evolution of the silicate Earth, and are fundamental to the plate tectonic cycle. Because of the inherent inaccessibility, it remains an ongoing challenge to interpret the geochemistry of ocean ridge lavas and exposed mantle residues in order to extract information about the petrogenetic and geodynamic workings of ocean ridge magmatic systems. This endeavor continues to require a concerted effort, incorporating field work, laboratory experimentation and quantitative modeling, in which the identification of features in the spatial or statistical distribution of geochemical variability represents an important contribution. In the three main chapters of this thesis, I apply techniques of exploratory data analysis, computational statistics, and petrologic modeling to develop original ideas about the relationship between sampled major element variability and the effects of specific processes, both petrogenetic and scientific: crystallization, melt transport, and sampling. In Chapter 2, I use spatial patterns of mid-ocean ridge basalt (MORB) glass variability to test competing hypotheses about crystallization in the thermal boundary layer beneath ocean ridges. I develop the hypothesis that reactive crystallization (crystallization influenced by chemical exchange with surrounding peridotite) could result in a different geochemical evolution of crystallizing magmas than expected for fractional crystallization. According to this hypothesis, fractionation-corrected MORB variability could be caused largely by sample-to-sample variations in the relative extents of reactive versus fractional crystallization. I demonstrate that MORB major element variability observed within 30-km-scale spatial bins contains 40-70 percent of globally observed variability, consistent with the predicted effects of reactive crystallization, but inconsistent with mantle temperature variations. Chapter 3 considers the effect of spatially heterogeneous sampling on apparent variability in MORB glasses. I demonstrate that MORB variability, as represented by the PetDB MORB glass database, contains large variability in sampling density, leading to significant artifacts in the estimated relative frequency of different MORB compositions. I introduce a method for removing these artifacts, and show that the increase in MORB data availability over the past decades has not been sufficient to increase significantly the resolution with which major element variability systematics can be studied at global or regional length scales, at least in comparison to early syntheses of global MORB data. Chapter 4 examines statistical variability within spatially defined volumes of mantle residue exposed in the Oman ophiolite. I provide a preliminary map of intermediate-scale compositional variability within the southernmost Oman ophiolite massif, in which multiple, spatially coherent, compositionally distinctive, 20-100 sq. km regions are resolved, representing the first mapping of compositional mantle domains at this length scale anywhere in the world. I interpret the observations as the consequence of regionally distinctive internal proportions of different mantle lithologies (e.g., dunite versus harzburgite), in turn reflecting the organization of focused melt transport at mid-ocean ridges into channel-rich and channel-poor zones.

The formation of carbonate minerals during alteration of ultramafic rocks represents a geological analogue of mineral carbon sequestration. In the Oman Ophiolite, these carbonation reactions are manifested in (1) active, on-going low-temperature systems involving meteoric water, which result in serpentinization, carbonate vein formation, and travertine precipitation at alkaline springs, and (2) older, higher- temperature systems, which resulted in completely carbonated peridotite, known as listvenite. Employing electron microprobe analysis, X-ray diffraction, stable and clumped isotope thermometry, Sr isotope geochemistry, and geochemical modeling, this study seeks to constrain the conditions under which natural carbonation has occurred in the Oman ophiolite, with the broader goal of understanding what factors and feedbacks control efficient carbonation of peridotite. Near low-temperature alkaline springs emanating from peridotite in Oman, networks of young carbonate veins are prevalent in highly serpentinized peridotite. A notable feature in some carbonate-veined serpentinite samples is the coexistence of Fe-rich serpentine and quartz. At a given pressure, the formation of iron-rich serpentine at the expense of magnetite should be favored at lower temperatures. Calculations of thermodynamic equilibria in the MgO-SiO2-H2O-CO2 system show that serpentine + quartz is stable assemblage at sufficiently low temperatures (e.g., less than ~15-50℃), and is stabilized to higher temperatures by preferential cation substitutions in serpentine over talc. Thus, the observed serpentine + quartz assemblages could result from serpentinization at near-surface temperatures. Clumped isotope thermometry of carbonate veins yields temperatures within error of the observed temperatures in Oman groundwater, while the d18O of water calculated to be in equilibrium with carbonate precipitated at those temperatures is within error of the observed isotopic composition of Oman groundwater. As groundwater geochemistry suggests that carbonate precipitation and serpentinization occur concomitantly, this indicates that both hydration and carbonation of peridotite are able to produce extensive alteration at the relatively low temperatures of the near-surface weathering environment in Oman. Along some locations near the basal thrust of the ophiolite, hydrothermal alteration of peridotite in the Samail Ophiolite of Oman has resulted in the formation of listvenite, characterized by complete carbonation, in which all of the Mg and much of the Fe has been incorporated into carbonate minerals, resulting in a rock composed primarily of magnesite (and/or dolomite where Ca has been added) + quartz. Mineral parageneses and clumped isotope data from magnesite and dolomite suggest that carbonate phases within the listvenite formed at peak temperatures ~100℃. CO2-enriched fluids were likely derived from underlying calcite-bearing sediment during emplacement of the ophiolite. Initial 87Sr/86Sr values in the listvenite vary from 0.7085 to 0.7135, mostly significantly higher than seawater values, and are consistent with values within the underlying allochthonous and autochthonous metasediments. An internal Rb- Sr isochron from one listvenite sample yields an age of 97 ± 29 Ma, consistent with the timing of emplacement of the ophiolite. Release of pore fluid during compaction of subducted sediments may result in similar carbonation of peridotite in the shallow hanging wall of other subduction/obduction environments. These natural systems demonstrate that significant carbonation of peridotite may occur even at low temperatures, but can be much more efficient at higher temperatures. Furthermore, complete carbonation of peridotite may be achieved, in spite of the potential for armoring of reactive surfaces and reduction of permeability, as demonstrated by the formation listvenite. These natural processes of hydrothermal alteration and weathering could potentially be accelerated to provide a permanent storage solution for the disposal of CO2 via the in situ formation of solid carbonate minerals in peridotite.

The explosivity of volcanic eruptions is governed in part by the rate at which magma ascends and degasses. Because the timescales of eruptive processes can be exceedingly fast relative to standard geochronometers, magma ascent rate remains difficult to quantify. As an exception to this principle, magmatic volatiles can re-equilibrate on timescales relevant to explosive eruptions, producing evidence for diffusion that can be assessed by various micro-beam techniques. Because the solubility of water and other magmatic volatiles decreases substantially at lower pressures, magmas erupt with a minute fraction of that which was initially dissolved. Melt inclusions, melt embayments, and trace amounts of H2O incorporated into the structure of nominally anhydrous minerals have the potential to retain information about the initial concentrations of magmatic volatiles prior to degassing. In this thesis, I present an assessment of the viability of these hydrous inclusions and mineral phases in preserving initial magmatic conditions in light of post-eruptive cooling effects. In addition, I also present an investigation of the potential of utilizing this volatile loss to estimate time scales of magma ascent during the 1974 sub-plinian eruption of Volcán de Fuego in Guatemala. To test the possibility of systematic H2O re-equilibration in olivine-hosted melt inclusions, I designed a natural experiment using ash, lapilli, and bomb samples that cooled at different rates owing to their different sizes. Ion microprobe, laser ablation-ICPMS, and electron probe analyses show that melt inclusions from ash and lapilli record the highest H2O contents, up to 4.4 wt%. On the other hand, MIs from bombs indicate up to 30% lower H2O contents (loss of ~ 1 wt% H2O) and 10% post-entrapment crystallization of olivine. This evidence is consistent with the longer cooling time available for a bomb-sized clast, up to 10 minutes for a 3-4 cm radius bomb, assuming conductive cooling and the fastest H+ diffusivities measured in olivine (D ~ 10-9 to 10-10 m2/s). On the other hand, several lines of evidence point to some water loss prior to eruption, possibly during magma ascent and degassing in the conduit. The duration of magma ascent that could account for the measured H2O loss was calculated to range from 10 to 30 minutes for the fast mechanism of H+ diffusion and 3.7 to 12.3 hours for the slow mechanism of H+ diffusion. Thus, results point to both slower post-eruptive cooling and slower magma ascent affecting MIs from bombs, leading to H2O loss over the timescale of minutes to hours. Utilizing an established method for assessing magma ascent rates, concentration gradients of volatile species along open melt embayments within olivine crystals were measured for use as a chronometer. Continuous degassing of the external melt during magma ascent results in diffusion of volatile species from embayment interiors to the bubble located at their outlets. The wide range in diffusivity and solubility of these different volatiles provides multiple constraints on ascent timescales over a range of depths. We focused on four 100-200 micron, olivine-hosted embayments which exhibit decreases in H2O, CO2, and S towards the embayment outlet bubble. Compared to the extensive melt inclusion suite also presented in this thesis, the embayments have lost both H2O and CO2 throughout the entire length of the embayment. We fit the profiles with a 1-D numerical diffusion model that allows varying diffusivities and external melt concentration as a function of pressure. Assuming a constant decompression rate from the magma storage region at approximately 220 MPa to the surface, H2O, CO2 and S profiles for all embayments can be fit with a relatively narrow range in decompression rates of 0.3-0.5 MPa/s, equivalent to 11-17 m/s ascent velocity and an 8 to 12 minute duration of magma ascent from ~10 km depth. A two-stage decompression model takes advantage of the different depth ranges over which CO2 and H2O degas, and produces good fits given an initial stage of slow decompression (0.05 - 0.3 MPa/s) at high pressure ( > 145 MPa), with similar decompression rates to the single-stage model for the shallower stage. The magma ascent rates reported here are among the first for explosive basaltic eruptions and demonstrate the potential of the embayment method for quantifying magmatic timescales associated with eruptions of different vigor. I investigated the utility of clinopyroxene as a recorder of the initial water and magma ascent rate using natural phenocrysts erupted during the 1974 eruption of Volcán de Fuego and the 1977 eruption on Seguam Island. The partitioning of water between clinopyroxene and melt was determined by analyzing melt inclusions and the adjacent clinopyroxene host by ion microprobe. For 10 Cpx-hosted MIs from Seguam, the partition coefficient is best predicted by the temperature-dependent parameterization by O'Leary et al. (2010). The diffusivity of H2O in clinopyroxene exhibits a four order of magnitude range in previous experimental studies that prevents a direct interpretation of concentration profiles as a chronometer. To constrain the diffusivity in magmatic phenocrysts, H2O concentration profiles were measured in Cpx from Fuego by ion microprobe and exhibit characteristics that are consistent with diffusive re-equilibration during magma ascent. Using the duration of ascent calculated from the melt inclusions and embayments (10 to 30 minutes), a range of H+ diffusivity was determined that would satisfy these timescales (10-9.20 to 10-10.45 m2/s). The calculated DH+ values are on the same order as the highest diffusivities for H+ in Cpx measured in the laboratory. A comparison of H2O concentrations measured in Cpx from lava and tephra samples from the Seguam eruption demonstrated that Cpx from lava retains less H2O in comparison to the H2O measured in the tephra. Using the DH+ values obtained from the Fuego Cpx, I showed that the difference in H2O between the lava and tephra Cpx can be attributed to post-eruption H2O loss during the estimated ~ 13 minute emplacement of the lava flow. The results from this work indicate that iron-rich clinopyroxene from slowly-cooled basaltic lavas should not be used to reconstruct initial magmatic water contents. The novel findings reported in this thesis are two-fold. Based on evidence from olivine-hosted melt inclusions in volcanic bombs and clinopyroxene in a pahoehoe lava flow, it is unlikely that the initial concentration of water can be preserved if a volcanic product undergoes slow post-eruptive cooling. This fact implies that a portion of the published data on H2O concentrations in olivine-hosted melt inclusions and clinopyroxene may reflect unrecognized H2O loss via diffusion and highlights the importance of reporting the type of volcanic deposit or the clast size from which a sample is extracted. The second novel finding of this thesis concerns the convergence in magma ascent rate estimates from three independent chronometers. In one of the first studies of this magma type, I report relatively fast time scales for magma ascent (~10 minutes from mid-crustal depths) for a basaltic, sub-plinian eruption. Furthermore, the similarity of the estimated timescales from melt inclusions, embayments, and clinopyroxene indicate the validity of any of these chronometers in tracking magma ascent rate. This further expansion of the methods for assessing time scales of volcanic eruptions enables researchers to pursue the complicated relationship between magmatic volatiles, ascent rate, and volcanic explosivity.

Thesis (M.S.)--Ohio State University, 1974. / Includes bibliographical references (leaves 151-160). Available online via OhioLINK's ETD Center.

Inaug.-diss.--Basel. / Curriculum vitae: p. [207]. "Separatabdruck aus Eclogae geologicae Helvetiae, bd. XVIII, hft. 1." Literaturverzeichnis: p. 205-206.