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Thesis (M.Eng.)(Metallurgical)--University of Pretoria, 2006. / Includes summary. Includes bibliographical references. Available on the Internet via the World Wide Web.

Thesis (M.S.)--Pennsylvania State University, 1972. / Includes bibliographical references (leaves 150-152).

Thesis--Novi Sad. / At head of title: Živojin S. Živanov. On cover: Univerzitet u Novom Sadu. Bibliography: p. 50-51.

Carbonate reservoirs hold more than 50 % of Earth’s remaining conventional hydrocarbon. However, recovery from these reservoirs is notoriously difficult due to the complex and multiple scales of porosity. This heterogeneity is a function of both the depositional environment and of subsequent diagenetic processes. This thesis examines the processes that have controlled the reservoir quality of three Early Cretaceous carbonate oil fields (A, B, and C), in particular the role of deposition, diagenesis and the timing of oil charge in controlling final properties. Results are then used to help provide a theoretical basis for the modelling and prediction of reservoir quality and to improve the calibration of basin models. Field A and B are stacked and highly compartmentalised giant oil fields in the U.A.E. that are dominated by muddy fabrics and have a highly variable porosity (0- 35 %) and permeability (0.01-830 mD). Although the depositional environment strongly determines the location of reservoirs extensive diagenesis, through cementation and dissolution, has greatly modified the porosity and permeability of the reservoirs. Bulk δ13C values obtained from the main pore occluding calcite and dolomite cements are similar to the δ13C values of bulk micrite for the reservoir interval in which they are now present. This suggests that the cements that are occluding the pore space in each stacked reservoir are locally sourced and implies that each reservoir behaves as a relatively closed system during cement precipitation. In-situ (SIMS) δ18OVPDB values were obtained for the complete calcite cementation history of multiple reservoirs in Field A and B. The δ18OVPDB values for the first (oldest) calcite cement zone in each reservoir can be related to the global δ18OVPDB marine curve during the Hauterivian-Aptian and to million-year scale major climatic cooling events. The δ18OVPDB values for successive cement zones then progressively decrease, which is related to successive precipitation as a result of increasing temperature during burial in a relatively closed system. In-situ (SIMS) δ18OVPDB data together with oil inclusion occurrence suggest that initial oil charge (from the Dukhan Formation), at ~ 55-45 Million years ago (Mya) in Field A, reduced the cementation rate in the oil reservoir and preserved porosity. Whereas in the coeval aquifer a large volume of cement precipitated, after oil entered the oil reservoir, that greatly reduced porosity. Furthermore, the most reduced δ18OVPDB and mMg/mCa values are obtained from the cements in the shallowest (youngest) reservoirs, suggesting that cementation ceased in the deepest reservoirs first. This can be related to hydrocarbon stopping cementation or to the complete occlusion of effective porosity in the older reservoirs prior to the younger. After calcite and dolomite cementation ceased in the reservoirs of Field A and B a large scale dissolution event has been identified which significantly enhanced porosity. This dissolution event is then followed by the precipitation of authigenic kaolinite. Basin modelling reveals that this dissolution event is likely to be related to the thermal maturation of sedimentary organic matter that is present within local intraformational seals and to the migration of organic acids prior to a second hydrocarbon charging event (at ~ 45 Mya). The aluminium, that is required for the formation of kaolinite, would then have been brought into the system by complexing with the organic compounds derived from this maturation event. Field C is an oil field located in offshore Brazil. The field is dominated by high energy facies that have porosities which range from 5 % to 39 %, and permeabilities from 0.1 mD to 8.1 D. The depositional poro-perm properties of the oil reservoir have undergone little diagenetic alteration; however, the aquifer is extensively cemented and the porosity is much reduced. All the cements identified, by both petrography and stable isotopic analyses, in the oil reservoir are early and are thought to have formed from a pore fluid similar to, or slightly evolved from, Early Cretaceous seawater. Basin modelling suggests that oil may have entered the field slightly after deposition (at ~105 Mya) and led to the preservation of high porosities and permeabilities in the oil reservoir by stopping cementation.

The storm-dominated paleotropical carbonate succession exposed on Anticosti Island in Eastern Canada represents one of the most complete, thickest, and well-preserved successions in the world spanning the O/S Boundary. This study develops a new high resolution integrated lithostratigraphic, cyclostratigraphic, and chemostratigraphic framework for the upper Hirnantian to lower Telychian (Upper Ordovician to lower Silurian) succession on Anticosti, by examining ~450 m of strata from a recent stratigraphic drill core (Martin La Mer), supplemented by ~120 m of outcrop, all from the south-central part of the island. Four facies assemblages and three time-specific facies were identified in this succession and can be organized into three orders of superimposed transgressive-regressive cycles. New high resolution isotopic curves were produced by sampling well-preserved bulk micrite at a resolution of 0.5-1.0 m per sample; in total 443 samples were taken from core and 168 from outcrop, corresponding to the Ellis Bay, Becscie, Merrimack, Gun River, Menier, Jupiter and Chicotte formations. Four distinct positive carbon isotope excursions are recognized in the study interval; the upper Hirnantian (+5‰), Lower Aeronian (+2‰), Upper Aeronian (+6‰), and Valgu (+3.5‰) excursions. These δ13C excursions accompany lithology, and sea level changes and are likely driven by transitions between humid and arid climate states coupled with ocean changes. Multi-ordered δ18O trends are recognized to occur in association with δ13C trends; long-term and intermediate scale δ18O fluctuations are likely controlled by glacio-eustasy and Silurian climate fluctuations, while high-frequency fluctuations may record an astronomical forcing signal.

An integrated field, petrography and sedimentological analysis is coupled with major/traceelements and stable isotope (δ13C, δ18O) compositions in order to decipher the paleoenvironments and the diagenetic conditions in the Neoproterozoic SCIc Formation (> 635-575 Ma) in the Niari/Nyanga and Comba areas (Republic of the Congo). The study recognizes seven major carbonate microfacies and paleoenvironements on the basis of mineralogy, bathymetry, energy, salinity and diagenetic process. It allowed to establish a sedimentological model recording a ramp setting with identified proximal and distal sedimentation. The transition between the shallow- and deeper-water environments is highlighted by the development of an extensive oolitic shoal, about 100 km long, a few km wide and up to 75 m-thick. The evolution of the sedimentation through the 22 studied sections allow to propose a sequence stratigraphy analysis with metre- to pluridecametre- thick depositional sequences from the fifth- to third-orders and highlights the effects of diagenesis on the primary signature of these carbonates. Two categories of fifth-order or elementary parasequences (or cycles) are recognized: (i) subtidal cycles, bounded by marine flooding surfaces across which facies deepen; and (ii) peritidal cycles bounded by subaerial exposure surfaces. These cycles are the result of the interplay of relative sea-level changes due to eustatic variations related to periodic extensional tectonic events affecting the whole basin and leading to uplifted phase in the context of a general block faulting system. The cycles, and the parasequence sets (fourth-order) they are included into a Highstand Systems Tract (HST) of a third-order sequence related to the deposition of the SCI Subgroup. The HST developed above the maximum flooding surface linked to both the deglaciation of the Marinoan Snowball Earth event and extensional tectonic events affecting the whole basin. Giant stromatolitic bioherms (stacked on up to 20 m) are topped by ooid shoals (up to 75 meters thick) with intraclasts and reworked grapestones. These shoals initiated the general regressive prograding phase of the HST that ended with evaporitic and karstic conditions at the top of the SCIc Formation. The equivalent stratigraphic unit of the Bas-Congo Basin (DRC) is compared and a sequence stratigraphy correlation between RC and RDC is attempted, despite no paleontologic data are available, and highlighted the role of the2tectonics affecting both basins. The negative δ18O (-10.3 ‰ to -5.1 ‰) and δ13C (-8.8 ‰ to - 1.1 ‰) values (n=288) in the SCIc Formation record a diagenesis in the meteoric and/or burial realms. Different diagenetic phases have been identified from syn-depositional/early diagenesis to shallow burial. In coarse-grained levels, an isopachous calcite rim cement (initially aragonite as suggested by very high Sr levels in these carbonates) occurs around the components while a blocky calcite cement fills the remaining intergranular pores. Hypersaline conditions are attested by euhedral sulfate minerals now calcitized and silicified. The exposure of these carbonates to meteoric waters was not strong enough or too long to favor large dissolutions, allowing preservation of high Sr values (up to 10000 ppm) in the micritic and in the oolitic facies. These high values suggest that the initial mineralogy was dominantly aragonite. A comparison with the Lower Congo (Democratic Republic of the Congo) and Nyanga (Gabon) subbasins in the West Congolian System shows that the meteoric circulation in the lower part of the Schisto-Calcaire Group was regional and not local, and could be due to climatic changlllles during late Neoproterozoic times. Finally, as revealed by their purity in calcium carbonate the oolitic facies is a prime target for the cement industry. The best facies can be recognized very easily in the field thanks to a very intense stylolitization (very tight stylolites, with amplitudes up to 10 cm). At the regional scale, the Niari / Nyanga and Comba basins in the Republic of Congo present the best sedimentary model that is, it is the most complete showing on the field all facies from the microbial 'MISS' facies with giant stromatolites to lagoons and emersion facies with microbial mats, isolated behind a large oolitic shoal which is sometimes interdigitated vertically and horizontally with the giant stromatolites. It is this very complete and detailed succession which made it possible to highlight a phase of tectonic uplift and also made it possible to differentiate the sequences developing directly on the Archean basement from those which overcome the SCIb siltstones. Facies equivalent to Gabon and the Democratic Republic of Congo do not allow to observe all the geometries of these sedimentary bodies. In any case, correlations are possible between these three regions, thanks to the sequential analysis and despite the fact that the δ18O and δ13C have no stratigraphic value, the primary signal having been quite early alter by the meteoric diagenesis. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Anticosti Island, located in Eastern Canada, displays one of the most complete, best exposed, and most fossiliferous carbonate successions spanning the Ordovician-Silurian (O/S) Boundary in the World. This study develops a new high-resolution framework for the post End-Ordovician extinction strata (~ 260 m thick) exposed in coastal outcrops and recovered from a continuous drill core (La Loutre #1), both located in the western part of the island. In total, eight facies, all associated with a storm-dominated carbonate system, were recognized and organized into a multi-order depositional cycles. A new high resolution isotopic curve with more than 300 data points from well-preserved bulk micrite samples covers the late Hirnantian to Early Aeronian time interval and corresponds to the upper Ellis Bay, Becscie, Merrimack and lower Gun River formations. Two distinct positive carbon isotope excursions are present in the late Hirnantian part of the Ellis Bay Formation (+5‰) and in the lower Aeronian part of the Gun River Formation (+2‰). These positive isotopic carbon excursions provide a distinctive chemostratigraphic signature for regional and global correlations with other O/S sections. Like the Quaternary δ18O marine signal, our δ18O record is largely coupled with multi-order cyclic facies changes. This study demonstrates the importance of glacio-eustasy following the End-Ordovician glacial maxima as one of the primary factors controlling the stratigraphic architecture of paleotropical neritic carbonates during the Early Silurian.

Calcium carbonate minerals (CaCO3) are important for our understanding of past marine conditions as well as tools for constructing paleoclimate. However, very little experimental work has been done to determine the influence of oxygen depletion on the geochemistry of CaCO3. To determine how oxygen depletion affects elemental incorporation and partitioning, aragonite was grown inorganically in artificial seawater at pressures of 1 atm and 5 bars (0.1%CH4-N2 mixture). Solution of Na2CO3 was used to induce aragonite precipitation. N2 was bubbled through solution in order to minimize oxygen content and iron powder was used to trap remaining O2. Experimental products (aragonite and fluid) were analyzed with ICP-MS, and isotope ratio mass spectrometer. Results suggest that Eh affects incorporation of Mn, S, Cu, and V into aragonite. No methane oxidation was observed.

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