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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Pre-Stack
Inversion
: An Extension of AVO for Lithology and
Hydrocarbon Fluid Quantification
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By
1Ulterra Geoscience Ltd., Calgary, Canada
2Union Texas Petroleum, Houston
Over the past two decades post-stack
seismic inversion
, the process of
deriving rock properties from seismic
measurements, has evolved significantly.
Recent advances in amplitude versus
offset (AVO) technology have demonstrated
that significant information is also
contained in the pre-stack seismic
data
with regard to fluids and lithology.
Our pre-stack
inversion
methodology,
augments the qualities of AVO and
inversion
to accurately quantify sand/shale
lithology and hydrocarbon fluid
properties directly from pre-stack seismic
data
. The method is demonstrated on
models and Canadian and international
seismic
data
.
Past Methods
of
Inversion
Relied
on
Modeling
Early methods
of recursive
inversion
converted
seismic traces to well log braces,
providing a measurement of the "pseudo
acoustic impedance." The acoustic
impedance could also be expressed as
"pseudo-acoustic velocity" by assuming a
simple relationship between velocity, density
and acoustic impedance. In any event
though, the inverted property was still
acoustic impedance. While the property of
acoustic impedance is more of a geophysical
measurement than a geologic rock
property, it did yield some indication of actual rock types. Most importantly,
it demonstrated that valuable physical information
was present in seismic
data
that
was being overlooked by conventional
wiggle traces.
The resolution of recursive inversion
was
limited to the bandwidth of the seismic
data
(hence the name band-limited
inversion
).
By using spike detection algorithms
to convert the seismic trace to a high frequency sparse reflectivity series prior to
inversion
, sparse-spike
inversion
algorithms
could achieve high resolution. The
"blocky" lithologic boundaries created by
sparse-spike
methods
most accurately
modeled actual geologic conditions
although the output physical quantity was
still "pseudo-acoustic impedance."
Recently, model-based inversion
schemes
have evolved, which essentially relies on
the fact that the forward model of a
"good"
inversion
should very closely
match the actual seismic
data
. Using iterative
forward
modeling
schemes, these
methods
perturb an initial acoustic impedance
model until its forward model
matches the seismic traces. These
methods
have the advantage of allowing some
degree of control over the starting point
and hence the resulting
inversion
. Once
again though, model-based inversions still
derive acoustic impedance.
New Technique Using AVO Gives Better Results
AVO techniques have demonstrated that,
with pre-stack seismic data
, the measurement
of the conversion of compressional
energy to shear energy at interfaces can
yield information about the fluids and
lithology present. More recently, advances
in pre-stack imaging and analysis
have resulted in significantly improved
post-stack signal quality with better
preservation of lithologic information.
This pre-stack inversion
technique
combines
inversion
and AVO technology
with anisotropic petrophysics. This technique
uses pre-stack seismic
data
as well
as sonic, density and gamma ray logs to
directly derive elastic rock properties
including sand/shale content, gas saturation,
water volume, and effective porosity.
More recently, we extended the technique
to detect oil versus gas using absorption
information.
Inverting the P-and S-wave stacks, with low-frequency constraints from sonic,
density and gamma ray logs, yields P-impedance (IP) and S-impedance (IS).
Petrophysical well log analysis, based on
volume averaging, allows inversion
of the
inverse P- and S-impedance to yield mineral
volumes.
Calculating Sand, Clay and Hydrocarbons
Where, Vss and Vclay are the fraction of sand and clay (respectively) in the matrix, the remaining factors are the physical properties corresponding to the impedances of pore water, sandstone, and shale. The constants for water and sandstone remain relatively constant while the impedances of shale may vary slightly with the geologic setting and are usually adjusted as part of the calibration.
This inversion
is applied to the entire pre-stack
seismic
data
set after careful preprocessing
and migration to preserve
AVO effects. The resulting
data
set gives
sand, clay, fluid, and gas volumes for the
entire seismic section. The net/gross sand
volume can be represented by a ratio and
indicates the quantity of sand present out
of the total mineral content.
The method has been successful on
Canadian and international seismic data
.
The input gathers were pre-stack migrated
with a Kirchhoff migration algorithm and
processed to retain AVO effects. A crossplot
was used to calibrate the
inversion
.
The
inversion
indicates the gas saturation
in red (at the top of the sand member under
the well location) and the sand/shale content
in shades from yellow (pure sand) to
green (pure shale). The prospect, on the
downthrown side of the fault, indicates
good gas saturation and highly porous
sand that pinches out becoming tighter and
forming the trap. This prospect has not yet
been drilled.
Conclusions
Pre-stack inversion
demonstrates that
significantly more information is contained
in the seismic wavefield than
End_Page 8---------------
simply acoustic impedance and that we can reliably quantify
rock and fluid properties from seismic data
. The method has
been successfully applied to numerous 2-D and 3-D
data
sets
from Canada, the U.S., and international targets.
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