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Diagenetic pathways linked to labile Mg-clays in lacustrine carbonate reservoirs: a model for the origin of secondary porosity in the Cretaceous pre-salt Barra Velha Formation, offshore Brazil

By
Nicholas J. Tosca
Nicholas J. Tosca
Department of Earth & Environmental Sciences, University of St Andrews, St Andrews KY16 9AL, UKPresent address: Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK
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V. Paul Wright
V. Paul Wright
PW Carbonate Geoscience Ltd and Natural Sciences (Geology), National Museum of Wales, Cathays Park, Cardiff CF10 3NP, UK
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Published:
January 01, 2018

Abstract:

The lacustrine carbonate reservoirs of the South Atlantic host significant accumulations of chemically reactive and Al-free Mg-silicate minerals (e.g. stevensite, kerolite and talc). Petrographic data from units such as the Cretaceous Barra Velha Formation in the Santos Basin suggest that Mg-silicate minerals strongly influenced, and perhaps created, much of the observed secondary porosity. The diagenetic interactions between reactive Mg-silicate minerals and carbonate sediments are, however, poorly known. Here we develop a conceptual model for the origin of secondary porosity in the Barra Velha Formation guided by considerations of the chemistry that triggers Mg-silicate crystallization, as well as the geochemical and mineralogical factors that act as prerequisites for rapid Mg-silicate dissolution during early and late diagenesis. We conclude that sub-littoral zones of volcanically influenced rift lakes would have acted as the locus for widespread Mg-silicate accumulation and preservation. Organic-rich profundal sediments, however, would be especially prone to Mg-silicate dissolution and secondary porosity development. Here, organic matter diagenesis (especially methanogenesis) plays a major role in modifying the dissolved inorganic carbon budget and the pH of sediment porewaters, which preferentially destabilizes and then dissolves Mg-silicates. Together, the sedimentological, stratigraphic and geochemical predictions of the model explain many enigmatic features of the Barra Velha Formation, providing a novel framework for understanding how Mg-silicate–carbonate interactions might generate secondary porosity more broadly in other lacustrine carbonate reservoirs across the South Atlantic.

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Geological Society, London, Special Publications

Reservoir Quality of Clastic and Carbonate Rocks: Analysis, Modelling and Prediction

P. J. Armitage
P. J. Armitage
BP Upstream Technology, UK
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A. R. Butcher
A. R. Butcher
Geological Survey of Finland, Finland
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J.M. Churchill
J.M. Churchill
Shell UK Ltd, UK
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A.E. Csoma
A.E. Csoma
MOL Group Exploration, Hungary
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C. Hollis
C. Hollis
University of Manchester, UK
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R. H. Lander
R. H. Lander
Geocosm, USA
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J. E. Omma
J. E. Omma
Rocktype, UK
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R. H. Worden
R. H. Worden
University of Liverpool, UK
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The Geological Society of London
Volume
435
ISBN electronic:
9781786202901
Publication date:
January 01, 2018

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