Pressure seals exist at depth that have impressively low permeability and are robust, despite evidence of tectonic or other fracturing. We suggest that there likely is some mechanism whereby the quality of a seal improves with increasing pressure gradient to which the seal is subjected. Thus, through diagenesis pressure seals self-enhance and heal, if breached.
A generic mechanism for the development of quality seals is proposed that involves a positive feedback as follows. The pressure dependence of diagenetic reactions can lead to precipitation from a fluid as it moves down a pressure gradient. Precipitation in a given zone will decrease permeability and tend to focus the overall pressure gradient in the zone. However, the augmented local pressure gradient promotes an even greater rate of precipitation in the zone of the original precipitation. Thus there is the tendency toward increased local pressure gradient development.
In the context of compartments, there is an even greater tendency toward seal self-enhancement. As the seal develops about an overpressuring compartment interior, the overall pressure drop across the developing seal increases—i.e., fluid escape is increasingly retarded as the seal develops. Thus there is a greater pressure head localized to the region where the pressure gradient-induced precipitation is taking place.
It is shown that one way this may occur is via the pressure dependence of equilibria of aqueous and mineral reactions. As an illustrative example, we consider the carbonate-quartz system, as layers of such cements constitute an important contribution to the banded seals in sandstone observed in a number of basins. We find that thissystem allows for the rather efficient development or healing of seals. This phenomenon is evaluated by using the quantitative, reaction-transport code CIRF.A. Numerical simulations show the time scales for the development and healing processes and their dependence on the salinity, temperature, depth, applied pressure head, and fluid chemistry to which the seal is subjected. The existence of such self-enhancing sealing mechanisms gives us new insights into the nature of seals and compartments. Including the processes underlying this phenomenon should aidin the prediction of the location and properties of compartments.
