DIAGENESIS, POROSITY AND PERMEABIL ITY IN

a highly variable sequence of sandstones, mudstones and limestones. The more permeable lithologies of the upper part of the sequence constitute an important local aquifer. Diagenetic and post-lithification processes have strongly influenced the porosity, permeability and mineralogy of the aquifer rocks. Calcite cementation reduced the porosity and permeability of the sandstones and limestones in the upper part of the aquifer. Cementation occurred in at least three stages ranging from early diagenesis to po t-compactional diagenesis. Late-stage dissolution of ca cite took place along fractures and bedding discontinuities, restoring the porosity and p rmeability of these sediments and developing secondary porosity and permeability where original clastic carbonate was removed. Invasion of the Corallian porewaters by waters charged with CO 2 is postulated as a mechanism by which calcite was removed. At the base of the aquifer, early diagenetic dissolution of biogenic silica created a high secondary porosity. Silica was reprecipitated in the dissolution voids and matrix as op l-CT. Authigenic smectite and zeolites are also associated with opal CT. These phases are believed to have precipitated from pore-waters rich in ions derived from the alteration of volcanogenic detritus associated with the Corallian sediments