Diagenetic models indicate that rates of porosity loss due to quartz cementation in sandstones can approach or even surpass rates of porosity loss due to compaction. These results help to explain the occurrence of fluid overpressures in sequences that are pre-Tertiary in age and have important implications for seal integrity and hydrocarbon migration.

Fluid overpressure models in the North Sea region that depend primarily on compaction disequilibrium would indicate that fluid overpressures have persisted for at least 50 my and that present-day effective stresses represent maximum values. By contrast, quartz cementation models suggest that fluid overpressures could have developed much later in the burial history of these rocks and in fact are being generated at the present day in response to the active diagenetic alteration of sandstones. Such a scenario is consistent with sandstone intergranular volumes in highly overpressured wells that indicate the sandstones were exposed to maximum effective stresses as much as two times greater than present-day values. Moreover, whereas models dependent primarily on compaction disequilibrium require shale permeabilities that are orders of magnitude lower than measurements, models incorporating diagenetic effects are consistent with larger values.

An implication of flow models incorporating diagenetic effects is that hydraulic fracturing could potentially occur at burial depths in excess of 3000 m in the Central North Sea region for example. This contrasts with previously assumed maximum estimates of approximately 1220 m (Osborne and Swarbrick, 1997). The deeper occurrence of hydraulic fracturing is possible because, unlike mechanical compaction, quartz cementation is comparatively insensitive to effective stress within the range normally encountered in sedimentary basins (Bjohrkum, 1994, Walderhaug, 1996).

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah