Evolution of porewater chemistry in Neogene sedimentary formation by one-dimensional numerical modeling considering the effect of burial diagenesis

Abstract

Deep in thick marine sedimentary formations, groundwater may exist that is thought to have been altered by the burial diagenesis process from seawater taken up during stratigraphic deposition. In such a field, groundwater flow is judged to be slow because it is not affected by freshwater infiltration even after the uplift and erosion of the formation. During diagenesis, dehydration from silicates and other processes are thought to cause changes such as a decrease in porewater salinity. However, dehydration from minerals alone cannot explain water quality changes quantitatively, and the water quality evolution process is unclear. In this study, we constructed an analytical model that considers dehydration from silicates and upward migration of pore water due to compaction during the burial process and examined the possible evolution of pore water quality during the burial process. As a result, two trends of water quality were identified: the influence of dehydration reaction from opal-A to quartz and the strong effect of dehydration from clay minerals, which were close to the observations from borehole investigations. These results indicate that low-salinity water with heavier oxygen isotope ratios than seawater can be formed during the burial diagenesis without mixing with freshwater.