Diagenesis of lower Cretaceous presalt continental carbonates from the West African margin: simulations and analogues

Direction : Gilbert Camoin, Eric Gaucher et Christophe Durlet

Soutenue le 29 mai 2017

Abstract

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.

Jury

Bruno Hamelin – president
Eric Verrecchia – examiner
Aurélien Virgone- examiner
Jacques Pironon – reviewer
Catherine Lerouge – reviewer

titre:

d

date_de_soutenance_these:

29 mai 2017

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20170529

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Teboul

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Diagenesis of lower Cretaceous presalt continental carbonates from the West African margin: simulations and analogues

Direction : Gilbert Camoin, Eric Gaucher et Christophe Durlet

Soutenue le 29 mai 2017

 

Abstract

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.

 

Jury

Bruno Hamelin - president
Eric Verrecchia - examiner
Aurélien Virgone- examiner
Jacques Pironon - reviewer
Catherine Lerouge - reviewer