The efficiency of a sandstone reservoir is a function of the initial depositional parameters (grain-size, sorting and composition) and of the post-depositional evolution. The depositional parameters are related to the sedimentary environment and the post-depositional evolution depends on the paleo-dynamic geological setting. The reservoir quality of sandstone is currently appraised in Eni with two methodologies. The first is based on the Reservoir Efficiency Index© (REI), a quantitative indicator of the quality of a clastic reservoir calculated on the petrographic data as obtained from thin sections. The values of REI at calibration wells are mapped through a dedicated geostatistical tool (Softkrig). The second methodology is based on a reservoir quality modelling software (Touchstone™), able to predict the mechanical compaction, the quartz cement precipitation and the illite formation. Both methodologies have been applied to an African reservoir. The reservoir is of Cretaceous age and the main reservoir quality reducing factor is the cementation by quartz, linked to the burial/thermal histories in the different zones of the basin. The case-study shows the pros and cons of the methodologies. The REI mapping is recommended in the play fairway analysis, while Touchstone is suitable for the determination of porosity and permeability on prospect/leads.


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