The Porosity-Preserving Effects of Microcrystalline Quartz Coatings in Arenitic Sandstones: Examples from the Norwegian Continental Shelf

Summary

Microcrystalline quartz coatings on detrital quartz grains are not uncommon in reservoir sandstones from the Norwegian continental shelf. The present burial depths for these sandstones range between 4150 and 4419 m. Effective pressures in the reservoirs range between 20 and 40 MPa. Scanning electron microscope investigation reveals various amounts and developments of macrocrystalline quartz overgrowths coexisting with the microcrystalline variety. The microcrystalline quartz crystals are between 0.5 and 5 µm in diameter with an average diameter around 1.5 µm. Authigenic quartz crystals larger than this are always in optical continuity with the detrital quartz. The presence of microcrystalline quartz in the system indicates that the porewater has been oversaturated with respect to macroquartz and microcrystalline quartz. The microcrystalline quartz coatings hindered quartz cementation and slowed down pressure dissolution. The reason for this is that pressure dissolution controlled quartz supersaturation, without effective reprecipitation or removal of silica from the system (system size, 10–100 m), will result in an elevated supersaturation of silica, balancing further pressure dissolution. In such situations porosity will be preserved to greater burial depths than expected.