DEVELOPMENT OF SEDIMENT OVERPRESSURING AND ITS
EFFECT ON THERMAL MATURATION: APPLICATION TO THE
GULF OF MEXICO BASIN
Ulisses T. Mello (1) , Garry D. Karner (2)
(1) Petrobrás Research Center, Cidade Universitária, Qd 7, Ilha do Fundão,
Rio de Janeiro, RJ, CEP 21910, Brazil.
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.
Abstract
High sedimentation rates can potentially lead to overpressuring and sediment
undercompaction within basins. Sediments with anomalously high porosity, in turn,
induce low thermal conductivity and so tend to act as thermal insulators to the flow of
heat. In the Gulf of Mexico basin (Gulf basin), the generation of overpressure is caused
mainly by the inability of pore pressure fluids to escape at a rate commensurate with
sedimentation. We modeled the generation and dissipation of abnormal sediment pore
pressures due to variations in sedimentation rate, facies, formation porosity and
permeability within Gulf basin using finite-element techniques to solve the differential
equations of both heat and fluid transport within compacting sediments. We assume that
the porosity-effective stress relationship within the sediment follows a negative
exponential steady-state form when the pore-pressure is hydrostatic. An important
feature of our modeling approach is the assumption that sediments are incapable of
significant expansion in response to increasing pore pressure. Sediments are assumed to
hydrofracture when the pore pressure approaches the lithostatic pressure rather than the
usual assumption of porosity expansion even in lithified sediments. From our modeling,
we conclude that significant overpressures have been created (and dissipated) at various
times within the Gulf basin and track, in general, the West to east migration of sediment
loads deposited since the Cretaceous. Although predicted overpressures > 0.75
kpsi (i.e., an excess hydraulic head of 500 m) of Campanian to
Maastrichtian age remain to the present day, the main phase of pressure development in
the Gulf basin is predicted to occur in the Miocene to present. Maximum overpressures
(~13.6 kpsi; excess hydraulic head of 9.4 km) are predicted for the
present day. This Miocene-Quaternary overpressuring, a consequence of rapid sediment
deposition associated with the Mississippi delta system, is also predicted to be
undercompacted. This undercompaction led to increased temperature gradients during
the Miocene and Quaternary despite the fact that the anomalous basal heat flow
engendered by extension had practically been dissipated. We further predict that by the
end of the Neogene, temperatures would be approaching steady-state over broad regions
of the Gulf basin implying that the highest temperatures occur in the deepest parts of the
basin. In contrast, during the Quaternary, the rapid progradation of overpressured and
undercompacted resulted in a thick section that has yet to reach thermal equilibrium and
thus is anomalously cold with respect to its present depth. The predicted vitrinite
reflectance indicates that for most of the Gulf basin history, the depth to the top of the
oil window remained at approximately 2.5+-0.5 km bsf. Similarly, the depth to
the base of the oil window ranged from 3.5 to 6.5 km bsf. This relatively
constant position of the top of the oil window defines a maturation "front" that
propagates from the offshore into the onshore regions of the northern Gulf basin as a
function of time. As such, continuous hydrocarbon generation is predicted to have
occurred continuously within the Jurassic and Cretaceous sections of the onshore region
during the entire Cenozoic. Prior to this, maturation fronts within each of the onshore
basins resulted in early Cretaceous maturation of upper Jurassic source rocks. In the
offshore Gulf Coast area, pre-Tertiary source rocks are predicted to be overmature for
liquid hydrocarbons at the present. In the offshore regions affected by Quaternary
sedimentation, the depth to the top of the oil window has been dramatically depressed in
response to sediment loading and subsidence. These Quaternary sediments have yet to
reach thermal equilibrium.