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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: 3-D
Seismic
Lithology Analysis Utilizing Extended AVO: A Case Study
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By
Geophysical Development Corporation
In this Gulf of Mexico case study, extended
Amplitude Versus Offset (AVO) analysis
enables the generation of sand and gas reservoir
maps not obtainable from conventional
3-D data. This seismic
lithology interpretation
is the result of calibrating the
3-D/AVO response to lithology and pore
fluid variations.
- Analysis of well data provides lithology, velocity, and density control necessary to estimate Poisson's ratio, as well as rock property variations from wet to gas-filled conditions.
- AVO
modeling
based on these variations yields the anticipated
seismic
lithologic response, thereby linking the log properties to the 3-D
seismic
interpretation.
- Processing to recover the "extended" AVO response from data with source-to- receiver offsets out to two times the target depth enables lithologic mapping when supported by the above calibration.
The reservoir sands range from 6,000 to
13,000 feet in depth, lying above and below geopressure. Conventional seismic
analysis associates the amplitude of the reflections
to the contrast in acoustic impedance,
the product of velocity and density.
On the log data, the target sands appear to
have little contrast in acoustic impedance
with their encasing shales. Because of this
low contrast, the amplitudes on the
seismic
stack sections show little to no discrimination
between sand-shale lithologic
variations or pore fluid variations.
A two-term model for the seismic
amplitude
provides the basis for unraveling the
complex AVO responses of lithologic variations
and gas sands. The first term, the
Normal Incidence reflectivity (NI) responds
to changes in acoustic impedance. The second
term, defined as the Poisson reflectivity
(PR), relates to changes in Poisson's ratio. Crossplots show that even when the sands
have the same acoustic impedance as the
encasing shales, Poisson's ratio discriminates
between them. To obtain a robust estimate
of PR (called model-based PR or
MBPR), the AVO processing incorporates
corrections for anisotropy, which extends
the AVO analysis out to very far offset
traces. Finally, a view of the
lithostratigraphic properties develops by
displaying the NI and MBPR estimates
from the 3-D
seismic
data with a novel color
crossplotting method. Unlike the stack
amplitude, distinct reservoir features appear
in the
seismic
crossplot volume. After calibration,
these 3-D crossplot sections provide
maps of reservoir quality sands and
potential pay intervals, as well as the stratigraphic
setting.
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