Page 204 - Origin and Prediction of Abnormal Formation Pressures

SEISMIC METHODS OF PRESSURE PREDICTION 179

well impedances. Extensive testing proved that this scalar, which varies to some extent
with space and time, can be effectively used to generate absolute acoustic impedances
from consistently processed seismic data. Velocity information is then extracted from
the impedances using the velocity-density transformation.

Pore pressure and seismic amplitude versus offset (AVO)

As discussed previously, the amplitude of the seismic reflection is influenced by
the reservoir pressure. Moreover, reservoir fluids also affect the seismic velocities. The
shear and compressional waves respond differently to reservoir fluids (and lithology), as
well as to the reservoir pressure. These facts offer the opportunity to predict pressure and
fluid content using seismic velocities. Other challenges include distinguishing between
the presence of overpressure and gas saturation from seismic response. Some laboratory
tests have been helpful in this regard (for example, see Fig. 7-6).
Lindsay and Towner (2001) demonstrated how to improve predictions. Rock proper-
ties and amplitude versus offset modeling help to understand the frequently ambiguous
amplitude and AVO signatures found in seismic data. The aim is to understand the
elastic reservoir properties and their dependence upon pore fluids. Inasmuch as the
seismic reflectivity data are a measurement of changes in the elastic rock properties
across interfaces, the elastic properties of the sealing caprock are as important to the
reflectivity solution as those of the reservoir.
Pore pressure has a greater influence on the elastic properties of shale than it has on
the properties of sands and sandstones because of the influence of adsorbed water on
the clay particles. Inasmuch as the pore pressure could be related to shale dewatering,
at least in Tertiary sand-shale sequences, the amount of adsorded water correlates with
pressure. Pore pressure, therefore, becomes a critical parameter in the rock property
and reflectivity models because of its disproportionate influence on the shale caprock.
Fig. 7-7 shows that essentially identical reservoir sands with similar fluids may have
dramatically different amplitudes and AVO signatures simply because of their pore
pressure.
In sand-shale sequences, the elastic properties of rocks vary as a function of the
pore pressure. The properties of shales vary more as a function of pressure than
do the sands. Consequently, in order to generate high-precision rock property and
reflectivity models, the influence of pore pressure on the reservoir rock and shale seal
(caprock) must be included. An independent estimate of pore pressure is required when
models are made for prospects away from the well control. Fortunately, there is a
strong correlation between the pore pressure and seismically derived interval velocity
(Fig. 7-8). Additionally, this velocity is presented in three dimensions.

Pore pressure estimation from seismic velocities

The pore pressure calculated from 3-D prestack depth migration velocities was used
successfully in the Gulf of Mexico subsalt trend. Various papers have been published
on this topic showing the usefulness of implementing pore pressure prediction in a
3-D volume. In all cases, pore pressure is estimated by measuring the deviation of