EXTENSIVE ALBITE DISSOLUTION IN TRIASSIC RESERVOIR SANDSTONES FROM THE GANNET FIELD, UK NORTH-SEA

A petrographic study has been performed on reservoir sandstones of the Skagerrak Formation (Middle-Upper Triassic) from the Gannet oilfield, central North Sea. Samples show evidence of extensive dissolution of detrital albites, with no concomitant albitization of detrital K-felds pars. Micro-textural relationships suggest that albite dissolution occ urred during burial diagenesis, probably during or after overpressurin g of the reservoir. It is proposed that the most likely cause of albit e dissolution was organic acids associated with the maturation of orga nic matter in the Kimmeridge Clay Formation. Reservoir quality has not been significantly improved owing to the precipitation of a suite of permeability-reducing chloritic clays. The lack of albitization observ ed in these sandstones suggests that the albitization of detrital feld spars in deeply buried sandstones is not a global phenomenon. It is pr oposed that on further burial albite re-precipitation would be unlikel y to occur, as most of the unstable grains probably reacted, and the r eleased ions are probably already incorporated into stable authigenic clays. Consequently, the use of the degree of albitization as a method for timing hydrocarbon emplacement may be limited.