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from:
Chapter 18Velocity
Estimation for Pore-Pressure Prediction
David W. Bell
Conoco Inc.
Ponca City, Oklahoma
ABSTRACT
The speed of propagation of compressional-wave energy in the subsurface, known simply as “formation
velocity
,” is strongly influenced by compaction, particularly in young clastic basins. Because pore pressures
affect compaction, changes in
velocity
can be calibrated to changes in pore pressure. Velocities
derived from surface seismic data provide indirect pressure measurements at undrilled locations. The
accuracy depends on the validity of the relationship between pressure and
velocity
, the quality of the
velocity
measurements at enough points to perform the calibration and prediction, and the reliability
of average velocities to correctly convert from seismic time to depth.
A key step is construction of a velocity
profile with depth that simultaneously defines both the
compaction characteristics and a valid time-depth curve. A linear fit to the logarithm of the sonic transit
time with depth is commonly assumed to represent the normal compaction trend. Such a
velocity
-depth
trend, however, does not produce a time-depth relationship that accurately converts seismic measurements
in time to depth. A linear fit of
velocity
with time provides a consistent fit to both time-depth
and
velocity
-depth data and is a better empirical representation of the normal compaction trend. The
linear
velocity
-time model can be used to smooth through inaccuracies in seismic stacking-
velocity
picks
where applied to geologically consistent units.
This chapter illustrates relationships between velocity
and the geologic setting and establishes an
empirical model for the normal compaction trend. It then reviews various assumptions and techniques
for converting seismic stacking velocities into representative formation velocities. It concludes with a
step-by-step recommendation for estimation and calibration of
velocity
from seismic data.
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