Reservoir Quality Prediction in Sandstones through Rock Microcharacterisation

Andrea, Ortenzi ¹; Arduini, Monica ¹; Golfetto, Francesca ¹; Consonni, Alberto ¹
(1)Geolab, Eni E&P, San Donato Milanese, Italy.

The problem of reservoir quality in sandstones may be afforded through the modelling of the chemical and physical phenomena occurring during diagenesis. Considering the complexity of these phenomena, a strong simplification is needed and only a limited number of dissolution/precipitation mechanisms may be taken into account.

The other way round is to figure out the effects on the reservoir quality of the whole set of compositional and diagenetic variables. This approach is based on the quantification of these variables through image analysis assisted modal point-counting at the optical microscope and quantitative pore network characterisation at the scanning electron microscope, carried out on the same polished thin section. The approach cannot be based on pure statistics but has to be founded on the expected reaction of reservoir quality to the presence of a given amount of a component/cement. After these quantitative analyses a “rock micro-characterisation” spreadsheet is obtained, comprising more than 60 variables for each sample. The variables are lumped into classes that are thought to have a given impact on reservoir quality, for instance all cements that completely occupy a pore are considered as “occlusive” regardless of their mineralogy. Based on these classes three indexes are calculated and their arithmetic mean is called Reservoir Efficiency Index (R.E.I.). The indexes are relevant to the pore dimension, to the amount of grain-authigenesis-matrix and to the texture of authigeneses in the pore space. The R.E.I. also contains corrections for the basic depositional parameters such as grain-size and sorting and it is correlated to a given porosity-permeability range. The R.E.I. is always related to a specific sedimentary facies.

This approach has many advantages and some drawbacks. The main advantage is to provide a comprehensive approach that can be used in all type of reservoirs, from very fine to very coarse grained and from conventional to low permeability and in any phase of E&P activity. The main drawback is that it is very sensitive to the quantitative data and thus to the ability of point-counting in correctly quantify the different petrographic variables.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.