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Caractérisation et interprétation de la variabilité chimique et minéralogique des grès réservoirs.
Implications pour la modélisation numérique de la diagenèse minérale des grès du Ravenscar Group (Yorkshire, Royaume-Uni)

Benoît Maréchal 1, 2, 3
1 LMV-ENSMSE - Laboratoire Magmas et Volcans
Mines Saint-Étienne MSE - École des Mines de Saint-Étienne, SPIN, CNRS - Centre National de la Recherche Scientifique : UMR6524, LMV - Laboratoire Magmas et Volcans : UMR 6524
Abstract : Sandstones are generally high-quality reservoirs for hydrocarbon migration, storage and recovery thanks to their petrophysical properties. Therefore, sandstone characterization is an important field of research in petroleum geology. So, great series of sedimentary environments have been studied by sedimentologists to describe the 3D architecture of sandstone bodies and of permeability barriers and to characterize internal fades distribution.
Petrographic, mineralogic and geochemical properties, that allow to link primary (sedimentation) or secondary (diagenesis) phenomena to porosity and permeability variations, are not taken into account in sedimentological descriptions. Nevertheless, a modelling approach of mineral diagenesis based on physical and chemical processes has been developed for many years. Powerful softwares and computers allow to perform simulations and to propose mass balance calculations that have to be validated by field data.
Then, geochemical and mineralogical characterization of sandstone reservoirs could be very useful for sedimentologic and diagenetic studies. Testing the potential of geochemistry is the aim of this work.
Two geological formations have been studied : Ravenscar Group fluviodeltaic sandstones (Middle Jurassic, Yorkshire coast. United Kingdom) and turbiditic Annot Sandstones (Eocene-Oligocene, South-East Basin, France). Sampling strategy has been defined to estimate vertical and lateral variability at different scales between sedimentary fades as well as within a single bed.
Geochemical study shows that fades are characterized by their chemical composition and by their elementary variability. It also allows to better understand distribution of minerals and elements in the rock.
Thus, entrainment sorting between small heavy minerals and the other particles generates great variations for elements hosted in the different classes of detrital particles (Zr, Cr, Th, etc. for heavy minerals ; Al, Na, K, etc. for clastic particles). Chemical characteristics, valid vertically over several meters, can be defined for each class. Some of them, relating to non-mobile elements, allow to identify variations in sediment source. Segregation has also been observed in the clastic fraction between coarse and non-coarse particles during sedimentation.
Two major diagenetic transformations, i.e. kaolinitization and feldspar albitization, have also generated a particular distribution of the chemical composition of aluminous fraction in Ravenscar Group sandstones.
This result has been used to validate the numerical modelling of the diagenesis of these sandstones with the DIAPHORE code developed by Institut Francais du Petrole. Neoformation of kaolinite is thus realized in the temperature interval 25-80°C, whatever the composition of surface water. Clay precipitation, associated with successive dissolution of feldspars, is induced by circulation of water in disequilibrium with sandstone. Albitization of plagioclase feldspar occurs at low temperature and is poorly dependent on the water composition. Finally, albitization of K-feldspar is effective for temperatures greater than 50°C only if marine or brine waters flow in a reservoir that is subjected to a temperature gradient. Simulations confirm the great influence of permeability and fluid flow for aluminosilicate diagenesis.
Variations of the chemical composition of aluminous fraction in Ravenscar Group sandstones have been reproduced by the simulations. Conditions that allow chemical reactions are consistent with the geological setting of the sandstones. Thus, the major and independent diagenetic transformations require several episodes of water flow.
The results of this work suggest that the geochemical approach could be integrated more systematically within sandstone reservoir studies.
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Submitted on : Friday, August 11, 2006 - 4:14:10 PM
Last modification on : Thursday, February 25, 2021 - 11:28:10 AM
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Benoît Maréchal. Caractérisation et interprétation de la variabilité chimique et minéralogique des grès réservoirs.
Implications pour la modélisation numérique de la diagenèse minérale des grès du Ravenscar Group (Yorkshire, Royaume-Uni). Sciences de la Terre. Ecole Nationale Supérieure des Mines de Saint-Etienne, 2000. Français. ⟨tel-00089197⟩

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