Influence of Clay, Calcareous Microfossils and Organic Matter on the Nature and Diagenetic Evolution of Pore Systems in Mudstones

Abstract

Mudstones exert a fundamental control on the flow of both aqueous and non‐aqueous fluids in sedimentary basins. Predicting their flow and storage properties requires an understanding of pore size and connectivity, yet there are very few quantitative descriptions of pore systems of mineralogically and texturally well‐characterised mudstones. We use a combination of electron microscopy, mercury injection capillary pressure porosimetry, and CO2 sorption methods to generate a quantitative description of the size distribution, connectivity, and evolution of pore systems in a sequence of Posidonia Shale mudstones buried to 100‐180 °C. We place the pore data into a detailed mineralogical, petrographical and textural context to show that the nature and evolution of porosity and pore systems can be described in terms of associations with clay‐rich, microfossil‐rich and organic matter‐rich domains, common to many mudstones. Pore size distributions are described by power laws and pore systems are well connected across the full nanometer‐micrometer spectrum of pore sizes. However, connected networks occur primarily through pores < 10 nm radius, with typically 20‐40% of total porosity associated with pores with radii < ca. 3 nm, within both organic matter and the clay matrix. Clay‐rich, microfossil‐rich and organic matter‐rich domains have distinct pore size distributions which evolve in very different ways with increasing thermal maturity. We suggest that the flow of aqueous and non‐aqueous fluids depends not only on the overall connectivity of pores but also the larger‐scale connectivity and wetting state of clay‐rich, microfossil‐rich and organic matter‐rich domains.