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Thesis (M.S)--University of Wisconsin--Madison, 1982. / Typescript. 4 diagrams on folded leaves in pocket. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 161-165).

Les découvertes d’hydrocarbures dans les séries carbonatées très enfouies présel du Crétacé inférieur des marges brésiliennes ont relancé l’exploration dans la marge conjuguée ouest-africaine. Ces carbonates continentaux contiennent des faciès atypiques (ex. : boundstone à shrubs, grainstone à sphérulites), diversement interprétés comme hydrothermaux et/ou lacustres. Comprendre leur diagenèse et leur distribution est un enjeu crucial pour l’exploration. En tant que contribution à cet objectif, ce travail établit des liens entre la géochimie des carbonates continentaux et leur substratum. L’association de différents critères géochimiques permet de tracer l’origine lithologique des éléments chimiques incorporés dans les travertins et tufs aragonitiques et calcitiques (CATT). Le δ18O et le δ13C sont discriminants pour des CATT issus de roches ultramafiques. Les teneurs en Ba, Sr, Cr et Be permettent d’identifier les contributions volcaniques à granitiques. Des modèles numériques démontrent l’influence de la T°C et pCO2 sur l’altération d’un exemple de substrat enfoui. Cette altération produit des fluides aux pH alcalins à neutres, riches en Fe, HCO3+CO3 et en alcalins. Les concentrations en Ca, Mg, et Si, varient en fonction de la température. De tels apports peuvent favoriser des cimentations carbonatées à siliceuses. L’étude de l’habitus de tels ciments précoces de microquartz fibreux suggère une précipitation à partir (1) de fluides alcalins très évaporés, sous probable influence mantellique ou (2) de fluide neutres à acides, non à peu évaporés, sous possible influence felsique. L’ensemble des résultats démontre l’hétérogénéité spatiotemporelle des fluides diagénétiques. / The discoveries offshore Brazil of deeply buried oil-prone Early Cretaceous carbonate reservoirs revitalized the exploration of their coeval counterparts offshore West Africa. These continental carbonates exhibit atypical facies (e.g. shrubby boundstone, spherulitic grainstone), which are interpreted as lacustrine and/or hydrothermal. A predictive understanding of the distribution and diagenesis of these deposits is required for exploration. As part of this long-term project, the present thesis shows the link between the geochemical signature of continental carbonates and their substratum. The use of several geochemical tracers allows the characterization of the source of elements of calcitic and aragonitic travertine and tufa (CATT). δ18O and δ13C values are particularly discriminant when CATT originate from ultramafic rocks, whereas Ba, Sr, Cr and Be concentrations can be used to identify a contribution from volcanic rocks and granites. Numerical models show the influence of T°C and pCO2 on the alteration of an example of buried substrate. This alteration produces mid-alkaline to near-neutral pH, with high Fe, HCO3+CO3, and alkali concentrations. Ca, Mg, and Si concentrations are temperature dependant. Such input can lead to carbonate and silica cementations. The study of the habitus of such early silica cements, characterized as fibrous microquartz suggest a precipitation from (1) alkaline, highly evaporated fluids, under a mantellic influence and (2) neutral to acid, moderately to non-evaporated fluids, under the influence of felsic rocks. These conclusions highlight the heterogeneity in space and time of the diagenetic fluids influencing the presalt carbonates.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1991. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

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Late diagenetic, fault-controlled dolomitisation has received much interest as it is an important host for MVT-mineralisation and hydrocarbons, and an excellent proxy for fluid flow and reaction in carbonate systems. The source of fluids of sufficient volume and the correct chemistry to explain the volume of dolostone is much debated. Recent work has shown how seawater convection along deep-seated crustal lineaments is focused in zones of structural complexity. Since dolomitisation is favoured where there is a precursor high magnesium calcite or dolostone, it is possible that such a process is a critical precursor to the formation of these late diagenetic dolostones from evolved brines during extension and transpression. In the Pennine Basin and North Wales, UK, late diagenetic fault/fracture controlled dolostones developed on the margins of Mississippian carbonate platforms that grew on the rotated footwalls of normal faults and a basement of Lower Palaeozoic metasediments. Conceptual models for their formation focus on expulsion of fluids from Serphukovian-Bashkirian sediments within adjacent hanging wall basins, by compactional dewatering or rupture of overpressured compartments and seismic pumping. This project aims to determine the source, composition and drive mechanism of fluids that formed a large (~60km2), non-stratabound dolostone body exposed within the Viséan sediments on the southern margin of the Derbyshire Platform, through a combined regional sedimentological, diagenetic and structural framework using multiscale, interdisciplinary techniques. Techniques include field observation, transmitted light and CL analysis, bulk major and trace element analysis including rare earth elements, stable isotope (oxygen/carbon), and strontium isotope analysis. The Derbyshire Platform underwent burial, several episodes of fluid-flow, and multiple phases of diagenetic overprinting. The products of fluid circulation in this area consist of dolomitisation and Mississippi Valley-type (MVT) mineralisation, affecting the carbonates of the Lower Carboniferous (Viséan) succession. Dolomitisation on the Derbyshire Platform is aligned to deep-seated basement faults and extrusive, intraformational volcanic beds, and five dolostone phases have been identified. These are present as matrix replacive and cement phases that are spatially and temporally related to deep seated structural lineaments. It is proposed that stratabound, early post-rift dolomitisation resulted from the geothermal convection of a mixed meteoric-seawater that interacted with the Viséan extrusive and intrusive volcanics on the Derbyshire Platform, providing additional magnesium for dolomitisation. This previously undescribed model of dolomitisation is key to explaining the anomalously large quantity of dolomitisation observed on the Derbyshire Platform and has implications to other carbonate platforms where dolomitisation is interpreted as fault-controlled. Subsequent phases of dolomitisation are fault-controlled, with each phase becoming increasingly confined to fractures. Timing of dolomitisation is interpreted to be a Carboniferous event, with later mineralisation also being of late Carboniferous in age, with basin de-watering on to the platform via faults/fracture systems and the development of pockets of overpressuring. Illite-smectite clay transformations within Viséan basinal sediments provided the necessary magnesium required within select fault/fracture systems. Consequently, burial calcite cements and MVT mineralisation was precipitated within fractures and dissolution-enhanced secondary porosity, with fluids derived from the overlying Namurian succession which also acted as the seal. This project provides a step-change in our ability to predict the location of late diagenetic fault/fracture controlled dolomitisation in rift basins by demonstrating the importance of dolomitisation by mixed meteoric-seawater on platform margins to the localisation of late diagenetic dolostone bodies. It also highlights the complex interplay between basin kinematics, host rock permeability and timing of fluid supply through episodic fault reactivation, connecting platforms to basin compartments, which ultimately controlled the positioning of dolostone geobodies on platform margins. This has implications to the exploration of both minerals and hydrocarbon within dolostone hosts, and will inform studies of fluid transfer and reaction in carbonate systems.

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