LANDER, R. H., H. M. HELSET, L. M. BONNELL, and J. C. MATTHEWS
Geologica AS, Stavanger, Norway

Abstract: Implications of Sandstone Diagenesis for Fluid Overpressure Development and Seal Failure

Sandbodies can undergo large changes in pore volume that have the potential to induce fluid overpressures. For example, diagenetic modeling suggests that a medium-grained quartz arenite buried to 5 km over 60 m.y. could develop fluid overpressures in response to quartz diagenesis if it is exposed to thermal gradients >30°C/km. Results indicate that such a sandstone will lose ~37% of its initial pore volume from mechanical compaction within the first 2 km of burial. The sandstone will then lose ~35% of its original pore volume at greater depths due to quartz diagenesis. The later phase of pore volume loss is particularly important because unlike compaction derived overpressures that would need to be retained for ~40 m.y., the diagenetic driving force would be active at the present day. Moreover, the ability to bleed off displaced pore fluids at depth is much lower than near the depositional surface.

Modeling suggests that quartz diagenesis plays a secondary role in the overall extent of fluid overpressure development in shale-rich basins that have rapid burial rates. By contrast, results indicate that quartz diagenesis has the potential to be a major factor contributing to the generation of fluid overpressures in basins that have relatively slow burial rates. In such cases, fluid overpressures may form in response to high thermal gradients when sandstones are encased by low permeability materials.

Quartz diagenesis may be an important control on seal failure at depths greater than 3000 m in all overpressured basins. Unlike other mechanisms for fluid pressure generation such as compaction disequilibrium or oil-to-gas cracking, the quartz cementation process is apparently insensitive to fluid pressure. Consequently, tile process does not slow down as fluid pressures increase toward the fracture pressure.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas