American Oil and Gas Reporter - July 2020 - 59

SpecialReport: Geophysics & Computing
FIGURE 4
P-Impedance Inversion with Zero Phase Wavelet for
Kirchhoff (Left) versus CRAM (Right)

FIGURE 5
Density and Vp/Vs Ratio Volume Inverted
from CRAM Gathers

Density

Vp/Vs-ratio

resolution. This is evident in the interval
between Austin Chalk and Top Buda
away from the well.
Because the data preconditioning is
the same for both types of gathers, the
differences in the results is attributed to
a lack of reliable large opening angles.
The lateral heterogeneity seen in the
CRAM products could be related to variations in the lithofacies.
While both techniques replicate what
generally is observed in the wells at a
broader scale, the detailed analysis in
Figure 4 shows how the results from the
offset gather do not match the measured
log impedance below the Buda formation.
Above the Buda, the inversion results
match the log results very well, but there
is a shift below it at depth. This can be
attributed to the far offset stretch or to
the fact that the Kirchhoff migration does
not produce a stationary wavelet with
depth, since it uses only a single ray path
arrival. To the contrary, CRAM does not
show a deterioration in results with depth.
If there are more than two ray paths
that pass through caustics, then the phase
rotation from those ray paths will not be
accurately accounted for. This phenomenon may cause the phase to vary with
depth. In contrast, CRAM accounts for
multiple ray path arrivals, as well as all
possible phase shifts caused by caustics.
For that reason, the inversion from CRAM
angle gathers fits very well both at the
Buda formation itself, and above and

below the Buda formation.
After inverting for P-wave impedance,
S-wave impedance and density volume,
one can easily calculate mu-rho, lambda-rho and Vp/Vs volumes.
Figure 5 shows density and Vp/Vs
sections. As expected, CRAM density
and Vp/Vs ratio show higher resolution.
With a reliable density volume, other important petrophysical attributes necessary
for reservoir characterization, such as
lambda-rho and mu-rho, can be computed.
For instance, the product of Young's Modulus and density can be computed confidently. This is a property that can show
areas containing material with a tendency
to fracture.
The results from this South Texas onshore dataset show that performing simultaneous seismic inversion from CRAM
angle domain gathers is optimal and
straightforward. Obtaining such reliable
inversion results will allow interpreters
to confidently move to any next level,
such as geostatistical inversion or machine

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learning probabilistic facies classification,
for example.
Generating CRAM angle domain gathers has several advantages, including the
fact that trustworthy angle traces can be
produced for up to 50 degrees, and these
traces are less affected by migration
stretch. Such high-quality angle traces
enhance confidence in density inversion
and other derived attributes.
r
Editor's Note: The South Texas data
presented in this article is provided courtesy of Seitel Inc.

ELIVE
MENYOLI

Elive Menyoli has more than 15
years of experience in the oil and gas
industry. Prior to joining Emerson,
he worked at Marathon Oil and Total
E&P USA, in deepwater projects.
Menyoli holds an M.S. in physics from
the University of Goettingen, Germany,
and a Ph.D. in geophysics from the
University of Hamburg, Germany.
JULY 2020 59



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