Silicification and illitization due to diagenesis result in the modification of porosity and permeability in many North Sea reservoirs, including Greater Alwyn. A good estimation of the mineral composition is required as input/output data for geochemical modelling modes intended to reproduce these effects. Using sedimentological studies to determine laterally equivalent facies, three facies were chosen. "Initial states" for modelling were thus defined as the present-day composition of less-evolved laterally-equivalent facies, "final states" as the present-day composition in the most evolved part of the basin. Precise actual mineral compositions were calculated using a reconciliation program, DATREC. Average compositions calculated for each facies in each field provided data useful in constraining input/output values used in modelling, and gave insights into depositional variation. Arithmetic tests assuming isochemical transformations, constraining the chemical compositions of "initial states" to fit actual mineralogical compositions of "final states", were run using this software. They show that, from an arithmetical point of view, if the assumption of equivalent facies holds, isochemical reactions can occur in some, but not in all facies, thereby demonstrating the importance of checking equivalence of facies and diagenesis hypotheses by quantitative calculations.