Page 76 - Geology and Geochemistry of Oil and Gas

52 TEMPERATURE AND PRESSURE IN THE SUBSURFACE

Fig. 3.11. System of abnormally-high formation pressure (AHFP).

study. Once the temperature correction is applied, a normal compaction trend for
shales is established for the well. The overpressured zones are identified by the
deviation from the normal trend of pore-pressure gradient. The same method can be
applied for determining the overpressure from sonic or neutron logs (see Chilingar
et al., 2002).
If different wells exhibit different and distinct pressure gradient patterns, each well
represents a separate pressure compartment. Layering and variations in the
overpressure are sometimes due to the fact that pressure builds up in the shales
and then bleeds off into the associated permeable sands and sandstones. An excellent
prediction criteria and sensitivity analysis of formation pressure in sealed layers were
proposed by Dobrynin and Serebryakov (1978, 1989) and Khilyuk et al. (1994).

3.2.2. Seismic data

Pore pressure and fracturing pressure can be determined using the seismic velocity
and empirical relationships among seismic velocity, rock density, and Poisson’s
ratio. A typical methodology can be summarized as follows:
(1) the stacking velocity from seismic data is calibrated using well velocities;
(2) the normal pressure gradient curve is calculated for the sonic velocity curves;
(3) the pore pressure is determined from the seismic velocity data using the normal-
trend curve; thus, the pore pressure distribution in a section is established; and
(4) the fracture-pressure gradient is determined from the (a) pore-pressure gradient,
(b) interval velocities, and (c) empirical relationships among velocity, density,
and Poisson’s ratio.