Overpressure dissipation mechanisms in sedimentary sections consisting of alternating mud-sand layers

Abstract

In stable subaqueous environments, sedimentary deposition often results in thick mudstone-rich successions, with mainly argillaceous sediments, containing some intercalated sandstone beds and siltstones laminae. Such depositional sequences are commonly hosting abnormally pressured fluids in some part of their structure. Abnormally pressured compartments – generally overpressured ones – are indicative of effective sealing conditions along their boundaries in order to prevent the dissipation of hydraulic pressure. This work focuses on the various mechanisms of overpressure dissipation in such overpressured mudstone-dominated systems. The study is based on the mathematical and numerical simulation of hydraulic phenomena occurring in various geometries of sandstone-mudstone series. Using 1D models, it is shown that, once overpressure is developed in such a compartment, it can be kept inside for a very long time even if no other pressure process is acting. A basic mechanism is that the mudstone-sandstone succession reacts actively to pressure dissipation: when overpressure is released outward of the compartment, subsequent compaction tends to occur in the neighbouring sediments; this in turn results in a continued pressure generation and tends to maintain the overpressure in the compartment at geological time scale. Natural hydraulic fracturing tends to dissipate overpressure but it does not release the entire overpressure because fracturing is a threshold mechanism which keeps the stress level in the seals at their hydraulic fracturing limit. The opening of faults may modify the pressure pattern through the rapid generation of efficient fluid flow paths; however, its effect is a quasi-instantaneous release of overpressures in sandstones but not in mudstones. 2D numerical simulation suggests that, once the fault is closed, the overpressures remaining in mudstones are rapidly transmitted into adjacent sandstones which then tend to recover their previous high pressure. The most efficient mechanisms of overpressure dissipation in mudstone overpressured compartments are large-scale and long-term tectonic uplift and resulting erosion of overburdens. Studies on the conditions and mechanisms of overpressure dissipation in mudstone-rich successions provide insight into processes of hydrocarbon migration and accumulation processes and the efficiency of cap rock in basins that have experienced multiple episodes of tectonic disturbance.