A compaction model was developed and applied to five sites drilled as part as Ocean Drilling Program Leg 161, during which overpressured sediments were cored. The long-term compaction coefficient for the porosity variation is very high (1.3-3.4 x 10-7 Pa-1). Assuming that the fluid overpressures result from disequilibrium compaction, the fluid overpressures can be estimated from the difference between the hydrostatic porosity (i.e., the porosity distribution that would have resulted at equilibrium compaction with hydrostatic pore-fluid pressures) and the porosity deduced from downhole measurement analysis. Fluid overpressuring starts at very shallow depths (120-150 m below seafloor) and, in some cases, very quickly reaches the reduced lithostatic pressure. The reduced lithostatic pressure corresponds to the upper limit of pore-fluid overpressure before which natural fracturing occurs in unconsolidated sediments. Fluid overpressure is commonly correlated with the presence of gas (mostly methane). The ingredients for capillary sealing, two fluid phases in a layered sequence of fine and coarse sediments, exist in all the sedimentary sections described in this paper, and layers filled with free gas are clearly revealed as spikes in the porosity derived from the density log. Capillary sealing is shown to be quantitatively capable of retaining the overpressures observed.
1Examples
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2Cornell University, Department of Geological Sciences, 14853 Ithaca, NY, U.S.A.
revil@lmf-aix.gulliver.fr
3CEREGE, CNRS, Dept. Geophysics, BP 80, 13545 Aix-en-Provence, Cedex 4, France.
4Laboratoire de Mesures en Forage, ODP-NEB, BP 72, 13545 Aix-en-Provence, Cedex 4, France.
5GAÏA, 16 Bd. Notre-Dame, 13006, Marseille, France.
Date of initial
receipt: 23 May 1997
Date of acceptance: 19 December 1997
Reproduced
online: 12 February 2004
Ms 161SR-274