The Chlorite-Bearing Reservoirs: Effects of the Main Petrographic Parameters from Reservoir Quality

Andrea Ortenzi, Monica Arduini, and Francesca Golfetto
ENI S.p.A. E&P Division, San Donato Milanese, Italy

Fe-chlorite bearing sandstones are present in many basins of different age; owing to the presence of pore-lining/pore-filling Fe (and Mg)-chlorite they are a reservoir even in high depth/high temperature regimes. On the other side, the abundance and thickness of the coatings is a limiting factor for permeability; in order to predict the reservoir quality, it is thus important to understand the factors that influence the chlorite coatings growth.

As the chlorite-bearing reservoirs are deposited in specific settings, and namely in the transitional environments, where mixing between Fe-rich fresh waters and marine waters occurs, it is mandatory to sort out the effect of textural parameters (grain-size and sorting) and of grain composition on chlorite abundance. The grain composition is also affecting the type (Mg vs Fe-Chlorite) and crystalline structure (platelets vs filaments) of the chlorite that will form on grain surfaces.

Another parameter is the texture of the chlorite itself in the pore-space; the reservoir quality strongly changes with the proportion of pore-filling vs pore-lining chlorite; moreover, the efficiency also depends on the relationship between grain-size and the thickness/type of chlorite coatings.

Moreover, as in many cases the burial and thermal histories seem to affect the chlorite distribution and thickness, we have also considered the effect of these two additional variables.

In order to answer these questions, a huge data-set, comprising many basins (North Africa, Santos, North Sea, Middle Indus, Nile delta) and stratigraphic age (Silurian, Jurassic, Cretaceous and Miocene) was used. On all samples quantitative petrography, pore network characterisation and X-Ray diffraction analyses were carried out.

Finally, an attempt to model the reservoir quality considering the concurrence of all the above mentioned parameters was performed.

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