Offshore sediment overpressures of passive margins: Mechanisms, measurement, and models

Abstract

[1] Fluid pressure in excess of hydrostatic equilibrium, or overpressure, in offshore environments is a widespread phenomenon that contributes to the migration and storage of fluids, solutes, and energy and to the potential mechanical instability of these sediments. Overpressure exists in deep and shallow systems and is most likely to be found where low-permeability (<10 À16 m 2 ) layers have inhibited pore fluid escape or there have been large forcing mechanisms (e.g., rapid sedimentation, tectonic stressing, heating, and volume-creating reactions). The genesis and magnitude of overpressure can be controlled by physical processes (e.g., rapid sedimentation (>mm/yr), tectonic loading, and lateral fluid transfer) and thermal and chemical processes (e.g., aquathermal expansion, hydrocarbon generation, mineral diagenesis, and organic maturation). In systems where nearlithostatic overpressures are generated, potentially unstable sediments are created. Failures of these sediments can create large-scale natural disasters, generate fractures, and damage seafloor and subseafloor infrastructure. Detailed characterization of overpressured systems has been accomplished through geological and geotechnical analyses, including investigation of physical-mechanical properties (mainly porosity, consolidation state, and shear strength), inversion of geophysical data (e.g., compressional and/or shear velocities), measurement of in situ properties, and postevent analyses. Process-based models have been developed to explain the origin of overpressure in terms of rate of overpressure genesis. This allows identification of potentially unstable zones and assessment of the potential for failure. Future development in measurements and in coupling of models will lead to more accurate analysis and prediction of fluid pressure in offshore sediments, which in turn will facilitate better hazard analyses and will enable safer and more cost-effective offshore drilling practices and other offshore infrastructure development.Citation: Dugan, B., and T. C. Sheahan (2012), Offshore sediment overpressures of passive margins: Mechanisms, measurement, and models, Rev. Geophys., 50, RG3001,

Glacially generated overpressure on the New England continental shelf: Integration of full‐waveform inversion and overpressure modeling

“…Localized zones of relatively high overpressure could develop from a number of mechanisms, including (1) in situ fluid expansion; (2) rapid sedimentation; or (3) lateral pore pressure transfer [Swarbrick and Osborne, 1998;Bowers, 2002;Dugan and Sheahan, 2012]. Our study region has been repeatedly glaciated throughout the late Pleistocene, which can lead to sustained overpressures long after glacial retreat [Bense and Person, 2008].…”

“…This is commonly referred to as flow focusing [Yardley and Swarbrick, 2000;Flemings et al, 2002;Dugan and Flemings, 2002;Dugan and Sheahan, 2012]. Based on the orientation of the bright spots along strike of the dipping Miocene layers, we infer they are all related to the same clinoform, which could have high permeability (light gray clinoform within the Oligocene/Miocene unit, Figure 11).…”

“…The overpressures predicted by our fullwaveform inversion and numerical modeling could be the mechanism driving fluid flow linked to this SGD. Overpressure is a primary driving force for offshore groundwater flow [Dugan and Sheahan, 2012] and thus has a strong control on SGD. Understanding and characterizing SGD is important as it is an integral part of the total nutrient flux to the oceans, which affects marine biological production [Johannes, 1980;Church, 1996;Slomp and Cappellen, 2004].…”

“…These overpressures are often formed by a combination of rapid sedimentation, fluid expansion and migration, and/or phase changes [Bredehoeft and Hanshaw, 1968;Bethke and Corbet, 1998;Hart et al, 1995;Gordon and Flemings, 1998;Swarbrick and Osborne, 1998;Dugan et al, 2003]. Understanding and locating overpressure is important as it can affect subsurface fluid flow, submarine groundwater discharge (SGD), compaction and deformation of sediments, slope stability, offshore drilling strategies, fracture formation, and fluid migration pathways [e.g., Roberts and Nunn, 1995;Gordon and Flemings, 1998;Dugan and Flemings, 2002;Dugan and Sheahan, 2012]. Thus, an accurate understanding of overpressure is central to our understanding of offshore fluid flow regimes and their influence on some continental shelf processes.…”

Sedimentological characteristics and seafloor failure offshore SW Taiwan

“…Moreover, the high deposition rates also led to mass slope failure at the Gulf of Mexico (Solheim et al 2007;Dugan and Sheahan 2012). Based on the slope stability calculation from the Ursa region in Gulf of Mexico, the overpressures were generated when deposition rates reach 1.5 cm yr -1 .…”