EARLY DISCORDANT DIAGENESIS IN THE BRENT-GROUP, MURCHISON FIELD, UK NORTH-SEA, DETECTED IN HIGH VALUES OF SEISMIC-DERIVED ACOUSTIC-IMPEDANCE

Acoustic impedance inversion of seismic data in the Murchison Field, U K North Sea has revealed a zone of high acoustic impedance within the Rannoch Formation of the Brent Group which is discordant to stratigrap hy. The high values of acoustic impedance are caused by the diagenetic growth of siderite and pyrite early in the burial history of the rock s, probably while semi-consolidated at a depth of less than 1000 ft. T he zone is approximately parallel to the overlying Base Cretaceous unc onformity and its formation may have been controlled by a temperature gradient and/or groundwater chemical boundary that was parallel to gro und surface at that time.