A PHYSICAL EXPLANATION FOR THE POSITIONING OF
THE DEPTH TO THE TOP OF OVERPRESSURE IN SHALE-
DOMINATED SEQUENCES IN THE GULF COAST BASIN, U.S.A.
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.
An 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 depth, 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 maintain high values of hydraulic diffusivity that are
conducive to the rapid dissipation of excess fluid pressure. Within the overpressured
shales, compaction propagates downward 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 are predicted to 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 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 control the movement of fluids into and out of the sands.