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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
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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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