Solution seams in the Mamuniyat Formation, El-Sharara-A field, SW Libya: Impact of reservoir performance

Solution seams have been identified as a significant component in the Ashgillian (Upper Ordovician) Mamuniyat Formation sandstone reservoir in the El-Sharara-A field (SW Libya). They have been observed in cores with a frequency of up to 25 solution seams per metre, and they have a permeability of 1-6mD in a reservoir whose permeability is otherwise 100-1,000mD. The reservoir sandstones (quartz arenites) were deposited as shallow-marine shoreline sands associated with glacial outwash deposits. The potential of solution seams to act as permeability baffles has not previously been considered in reservoir engineering models of this field. In this study, by measurement of permeability and subsequent reservoir simulation, we show that solution seams are likely to have had a negligible effect on effective horizontal permeability, and a small but positive effect on vertical sweep efficiency (due to reduced vertical permeability). Solution seams are present in many Palaeozoic reservoirs in North Africa, and may be particularly significant (in terms of their effect on production) in relatively poor quality reservoirs.