Reservoir Quality of Clastic and Carbonate Rocks: Analysis, Modelling and Prediction
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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
Correspondence: [email protected]
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Published:January 01, 2018
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CiteCitation
Nicholas J. Tosca, V. Paul Wright, 2018. "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", Reservoir Quality of Clastic and Carbonate Rocks: Analysis, Modelling and Prediction, P. J. Armitage, A. R. Butcher, J.M. Churchill, A.E. Csoma, C. Hollis, R. H. Lander, J. E. Omma, R. H. Worden
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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.
- Africa
- aliphatic hydrocarbons
- alkanes
- Atlantic Ocean
- Brazil
- carbonate rocks
- carbonate sediments
- cerolite
- chemical composition
- clay mineralogy
- clay minerals
- colloidal materials
- concentration
- Cretaceous
- crystal growth
- crystal structure
- depositional environment
- diagenesis
- East Africa
- East African Lakes
- gels
- genesis
- hydrocarbons
- lacustrine environment
- lake sediments
- Lake Turkana
- magnesian silicates
- Mesozoic
- methane
- nucleation
- organic compounds
- pH
- pore water
- porosity
- provenance
- reservoir properties
- reservoir rocks
- Santos Basin
- secondary porosity
- sedimentary rocks
- sediments
- sepiolite
- sheet silicates
- silicates
- solubility
- solution
- South America
- South Atlantic
- stevensite
- talc
- theoretical models
- thermodynamic properties
- Barra Velha Formation