The History of Sediment Overpressuring in the Gulf of
Mexico Basin and its effect on Thermal Maturation
(AAPG Poster Section presentation)
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,
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.
One of the most dramatic geological events in the development of the
Gulf of Mexico basin has been the rapid, late-stage deposition of Tertiary
and Quaternary sediments of the Mississippi delta system. Locally
important Mesozoic depocenters also exist and relate primarily to
the distribution of extension responsible for the formation of the
Gulf of Mexico. Significant
geopressures have been created (and dissipated) at various times within
the Gulf and in general, track the west to east migration of sediment
loads deposited during and after the Cretaceous. The purpose of this
poster is to explore the implications of sediment overpressuring in
an inelastic compaction-driven fluid flow framework in terms
of the temporal behavior of pore pressure and steady-state |and|
non steady-state temperatures.
PREDICTING OVERPRESSURE: CRITICAL PARAMETERS
We modeled the generation and destruction of abnormal sediment
pore pressures due to variations in sedimentation rate, facies type,
and formation porosity and permeability using a finite-element analysis
to solve the coupled differential equations of both heat and fluid
transport in a "fully compacted" sediment matrix system. Critical
parameters in this analysis are the sediment specific storage, hydraulic
conductivity and hydraulic diffusivity. Many of these parameters have
equivalents in the equations describing the conductive-advective flow of heat.
We will discuss each of these "sediment" parameters in turn:
APPLICATION TO THE GULF OF MEXICO
The generation of overpressure is caused mainly by the rapid deposition
of sediment. The inability for pore pressure to escape at a rate commensurate
with sedimentation leads to sediment overpressuring. In the ensuing
figures, we compare and contrast the development of sediment overpressures
and porosities induced by rapid, late-stage deposition of Quaternary
sediments of the Mississippi delta system along a series of north-south
cross-sections of the Gulf of Mexico (at 264*, 268* and
272* long.) and a theoretical "well" at 28* lat. and 268* long.
TIME-DEPTH SECTIONS: A THEORETICAL WELL
The selected well represents a typical geological situation in
the Gulf Coast region where massive progradational sands overlie
sands and intercalated shales which in turn overlie massive shales. The
accompanying figures are time-depth diagrams generated by forward
modeling the 1-D development of the Gulf (as opposed to time-depth plots
obtained by backstripping). As can be seen from the isopach maps, the
highest sediment deposition rates in this region occur during the Miocene
and Quaternary. Our figures summarize the effect of varying sedimentation on
the hydraulic head, porosity, temperature and vitrinite reflectance
as functions of time.
Overpressures >0.75 kpsi (i.e., a hydraulic head of 500 m) began to
develop in the Campanian (83 Ma) at depths of approximately 2.5 km.
However, significant pore pressures were developed in the Miocene
(22 Ma) with a maximum hydraulic head of 9.4 km (-> 13.6 kpsi)
predicted for the present day. The thermal effect of this
large overpressuring associated with sediment undercompaction
increased the temperature gradients from the Miocene to the Present
despite the fact that the basal heat flow engendered by extension
continued to decrease through time.
A rapid increase in temperature gradient can also be induced by
processes other than overpressuring. For example, the relatively rapid
increase of temperature gradient at shallow depths during the early
Cretaceous is a consequence of the different compaction characteristics
of shales, sandstones and carbonates. In this case, the temperature
gradient variations are associated with the change in facies from
sands to shales across the early Cretaceous shelf (i.e., these shales
represent the "classic" thermal blanket). The predicted vitrinite
reflectance follows closely the behavior of the isotherms with the
onset of the oil window (0.6 %Ro) being close to 110+-10 *C
isotherm whereas the peak of oil generation (1.0 %Ro), the
end of oil generation (1.35 %Ro) and the limit of wet gas preservation
(2.2 %Ro) all following approximately the 150+-10 *C,
170+-10 *C, and 210+-10 *C isotherms, respectively.
In the Gulf of Mexico, the generation of overpressure is intimately
linked to the sedimentation rate. For example, at the end of the Neogene,
rapid sedimentation related to sediment input from the Mississippi
delta system generated a high hydraulic head (~ 9 km -> 13.4 kpsi).
As the sediments continued to prograde south into the Gulf Coast area,
maximum overpressures shifted towards the Sigsbee Plain area (values
of ~13.5 km| -> 20.3 kpsi) with the top of overpressure
being approximately at a depth of 3 km.
On the other hand, the dissipation of overpressure depends on the
hydrological properties of the sediment (i.e., porosity and permeability).
The relationship between the development of overpressure and sediment
load can be readily appreciated in the modeled sections, especially
close to the Paleogene shelf break. Since subsequent Neogene and
Quaternary sediments have bypassed this part of the margin, no
significative load has been emplaced on the margin and therefore
no overpressuring is generated.
Overpressures are predicted in upper Jurassic and lower Cretaceous
rocks in the western part of the Gulf basin beginning in the Paleogene
(see prediction of hydraulic head figure for the Paleogene; 264*
long). In general, overpressuring is an important characteristic of the
Rio Grande and West Texas areas since the Maastrichtian and is not
restricted to Quaternary sedimentation in the Gulf Coast basin.
The maximum values of overpressure are always developed close to
the basement with the top of overpressure rarely being shallower
than |2 km.
Undercompaction is associated mostly with Quaternary, Neogene and
Paleogene overpressured sediments. Although maximum overpressuring
to occur in the deepest part of the basin, the sediments are in fact
normally compacted (to the point of even approaching completion).
For example, the overpressuring predicted within the normally
compacted late Jurassic sediments is due to: (1) the sealing effect of
the overlying early Cretaceous shales and carbonates compounded by (2)
the amplification of pore pressures by the Miocene-Quaternary sediment
load. This is because compaction is assumed to be geologically
unrecoverable (i.e., it is an inelastic process). In contrast,
both overpressuring and undercompaction are predicted within the
Quaternary-Miocene section of the Gulf Coast basin.
Our modeling predicts that by the end of the Neogene, temperatures have
almost reached steady-state (i.e. equilibrium) over broad regions of the
Gulf Coast basin. Consequently, the highest temperatures occur in the deepest
parts of the basin. However, during the Quaternary, rapid progradation of
cold, high porosity sediments loaded the pre-existing sections and induced
additional subsidence in the region of the Sigsbee Plain. The high
porosity of this sediment creates an anomalously low thermal conductivity
and thus acts as a thermal insulator to the conductive flow of heat. This
Quaternary section has yet to reach thermal equilibrium and so will be
anomalously cold with respect to its depth. That is, even though the
Neogene section may be deeper than its counterparts in the northern Gulf
Coast basin, it will tend to be colder.
The calculated vitrinite reflectance indicates that for most of the basin
history, the top of the oil window remained at approximately 3 km.
Similarly, the base of the oil window ranged from 4 to 6.5 km. At
the present day, the depth to the top of the oil window is strongly
affected by the rapidly deposited Quaternary sediments.
FACTORS CONTROLLING THE TOP OF GEOPRESSURE
A physical explanation for the Top of Geopressure (overpressure) in
the Gulf Coast area is the based on the behavior of hydraulic diffusivity
with depth that shows a minimum at depths ranging from 2 to 4km due
to high porosity and permeability a shallow depths and very low bulk
compressibility (or equivalently specific storage) at deeper sediments.