The Devonian-Carboniferous reservoir of the Clair Field contains a complex and variably abundant clay mineral assemblage. An abrupt vertical change in clay mineralogy has been observed in both wells studied (UKCS 206/8-7 and 206/8-8) from being rich in Mg-chlorite, chlorite-smectite (including dioctahedral corrensite), illite, illite-smectite and Fe-chlorite, to being smectite-rich. This change broadly coincides with the unconformable boundary between the Lower Clair Group and Upper Clair Group of the reservoir which possibly defines the Devonian-Carboniferous boundary. The clay minerals present reflect interaction between: (1) tectonic stability and climate that controlled the detrital composition of the sediment; (2) the pore-waters that developed; and (3) the result of diagenetic modification.
Matrix porosity and permeability is severely restricted where the pore-lining clays are abundant (>15%) and where the clays bridge or fill pores. The type of clay mineral species appears to have a minor influence on reservoir quality, although they may be extremely important during production. The best reservoir quality occurs in mature aeolian sediments with a thin illite and illite-smectite clay coating.