A PETROGRAPHIC AND COMPUTATIONAL INVESTIGATION OF QUARTZ CEMENTATION AND POROSITY REDUCTION IN NORTH-SEA SANDSTONES

Petrographic data indicate that the bulk of silica cement in the Juras sic quartzose sandstones of the North Sea basin originated from quartz dissolution at mica and illitic clay interfaces which border stylolit es. The distribution of cement is increasingly controlled by the dista nce from stylolites with increasing temperature, Dissolution of quartz at mica/quartz interfaces does not alter the mica grains chemically a nd can apparently proceed without mechanical deformation of the mica, No evidence is seen for quartz cement sourced from quartz/quartz inter faces, Taking account of these observations, a quartz redistribution m echanism was developed that includes dissolution of quartz at stylolit e interfaces catalyzed by the interaction of quartz grains and mica/il litic clay surfaces, diffusional transport of dissolved silica into th e interstylolite regions, and its subsequent precipitation on quartz g rains by kinetically controlled crystallization reactions, The variati ons with th of the abundance and distribution of silica cement are con trolled the temperature dependence of quartz dissolution/precipitation rates and aqueous diffusion coefficients, Chemical compaction proceed s with decreasing interstylolite distance as quartz cementation fills pore voids expelling fluid, Steady-state results were obtained for the combined kinetic, transport, and mass conservation equations to chara cterize the rate and extent of chemical compaction and quartz redistri bution in sedimentary sandstones in response to quartz/mica interactio ns at stylolite interfaces, A close correspondence between computed re sults and the petrographic observations demonstrates the consistency o f the proposed mechanism with the natural process.