Hydrothermal Fluid-Rock Experiments and Geochemical Modeling of Illite Forming Processes in a Paleozoic Sandstone, Middle East

Abstract

Summary The formation of illite in hydrocarbon-bearing clastic sedimentary formations can have diverse effects on reservoir quality, as illite cement may preserve porosity by reducing quartz cementation or can degrade permeability by blocking pore space or pore throats. In the present study, hydrothermal experiments and geochemical modeling were performed on core material and formation water from Late Carboniferous to Early Permian sandstones from the Middle East to reconstruct the formation of secondary minerals under various burial scenarios. Geochemical modeling with SOLMINEQ.88 suggests oversaturated conditions for muscovite (as illite proxy) for shallow and deep field scenarios, whereas hydrothermal experiments with present-day formation water and core material, and SEM micrographs and spectra encountered incipient illite crystallization on kaolinite grains at 150°C and 450 bar. Temperature has been proven to be a major controlling parameter on illite precipitation, but grain mineralogy (such as the presence of feldspar, early diagenetic clay assemblage, kaolinite and smectite) or grain-surface substrate may control grain-coating illite cement distribution on a local scale. This study suggests that moderate grain coverage of smectitic infiltrated clays may not always be improved by the later formation of diagenetic illite coats during deep burial if there is insufficient potassium for illitization.