Triaxial Simulation of Compaction Processes in Mudrocks

Mudstone compaction is generally viewed as a combination of mechanical and chemical processes, in which porosity is lost first as a result of increasing effective stress and then due to mineralogical changes. This study sought to test this concept by subjecting natural mudstone to a series of triaxial experiments. These were carried out under a range of effective stresses up to 50MPa, at temperatures up to 150oC and in a range of fluid chemistries designed to investigate the relative roles of different components of the burial environment. Two longer duration triaxial tests ran for 57 days at a maximum effective stress of 50MPa. One used K-rich pore fluids and a temperature of 150oC which produced an increase in grain density with a decrease in volume, pore size and porosity. These changes occurred without mineralogical transformation. However, the other was carried out at 25oC using Na-rich fluid and did not significantly, permanently compact the mudstone. It remained at a porosity retained from its natural consolidation at <2km despite the application of 50MPa effective stress. The effects of cation exchange with potassium and temperature appear important factors to understand when attempting to model natural diagenetic processes in the laboratory.