A physical explanation for the depth to the top of
overpressure in the Gulf Coast basin (Poster Section)
Ulisses T. Mello (1) , Garry D. Karner (2), and Roger N. Anderson (2)
(1) Petrobrás Research Center, Cidade Universitária, Qd 7, Ilha do Fundão, Rio de Janeiro, RJ, CEP 21910,
Current address: IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598.
(2) Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, 10964, U.S.A.
A one-dimensional model of fluid pressure and porosity evolution is used to investigate the physical
processes that control the development and maintenance of overpressure in a compacting sedimentary
basin. We show that for shale-dominated sequences the variation of the hydraulic diffusivity in both space
and time is such that it produces a minimum between 2 to 4 km, consistent with observations from
the Gulf Coast basin. This minimum inhibits the upward flow of fluid by acting as a "bottleneck"
and thus determines the shallowest position of the depth to the top of overpressure. Above this region of
bottleneck, overpressure does not develop because the porosity is sufficiently large to produce high values
of hydraulic diffusivity that are conducive to the rapid dissipation of excess fluid pressure. Within the
overpressured shales, compaction propagates downwards through the section, releasing fluids from the
upper part of the section while continuing to restrain the upward flow of fluids from deeper within the
section. As such, overpressures may be maintained within the deeper regions of a basin for 10's-100's of
millions of years. Further, fluid viscosity plays an important role in defining the depth behavior of
hydraulic diffusivity as a function of time. Assuming a temperature-dependent fluid viscosity guarantees
that the hydraulic diffusivity minimum will always exist during the development of the basin. On the basis
of our results, we find that the depth at which the porosity equals 14+-4% correlates with the depth to the
local hydraulic diffusivity minimum and thus the depth to the top of overpressure. Moreover, we interpret
that the 14+-4% represents the threshold porosity for which a shale actually begins to act as a seal. Within
the Gulf Coast basin, the gross sediment facies consists of lower massive shales across which deltaic
systems have prograded allowing the deposition of firstly an alternating series of sandstones and shales
that grade vertically into massive sandstones. The massive sandstones are highly permeable and are
connected hydrologically to the surface. We conclude that these sandstones play little role in the
development of overpressure because of their high permeability except to the extent that the base of the
massive sandstones marks the minimum depth possible for the top of overpressure. In contrast,
overpressuring is observed to develop within either the shale-dominated sequence or the region of
interspersed/interfingering sands and clays. The clay-encompassed sands play only a passive role in the
development and maintenance of overpressure because it is the low-permeability clays that controls the
movement of fluids into and out of the sands.