A petrographic and computational investigation of quartz cementation and porosity reduction in North Sea sandstones

Abstract

Petrographic data indicate that the bulk of silica cement in the Jurassic quartzose sandstones of the North Sea basin originated from quartz dissolution at mica and illitic clay interfaces which border stylolites. The distribution of cement is increasingly controlled by the distance from stylolites with increasing temperature. Dissolution of quartz at mica/quartz interfaces does not alter the mica grains chemically and can apparently proceed without mechanical deformation of the mica. No evidence is seen for quartz cement sourced from quartz/ quartz interfaces. Taking account of these observations, a quartz redistribution mechanism was developed that includes dissolution of quartz at stylolite interfaces catalyzed by the interaction of quartz grains and mica/illitic clay surfaces, diffusional transport of dissolved silica into the interstylolite regions, and its subsequent precipitation on quartz grains by kinetically controlled crystallization reactions. The variations with depth of the abundance and distribution of silica cement are controlled by the temperature dependence of quartz dissolution/ precipitation rates and aqueous diffusion coefficients. Chemical compaction proceeds 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 characterize the rate and extent of chemical compaction and quartz redistribution in sedimentary sandstones in response to quartz/mica interactions at stylolite interfaces. A close correspondence between computed results and the petrographic observations demonstrates the consistency of the proposed mechanism with the natural process.