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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Litho-Stratigraphy of the Miocene in the Texas and Louisiana
Transitional Zone: How to
Recognize
Sequence
Boundaries and Hydrocarbons from Seismic Data
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By
1Geophysical Development Corporation
2ORYX
3Colorado School of Mines
Throughout the Tertiary basins in the Gulf Coast, there are areas where the acoustic-impedance values of shales, wet sands and gas sands are approximately equal. Hydrocarbon zones do not appear as bright spots and are difficult to detect with conventional 3-D seismic data. In some areas, even AVO has not been successful. This difficulty normally occurs when the rock properties are not calibrated to the various local AVO attributes.
A 3-D AVO study in the Texas and
Louisiana transitional zone has been completed
using well
log
suites, core analyses,
and field production histories.
Results from this study are
illustrated in Figure 1, where a
conventional 3-D section is
shown in the upper portion.
This section is a mixture of both
chrono- and litho-stratigraphic
reflections. In the bottom portion
of the figure only the
litho-stratigraphic reflections
are retained.
Correlation of the well
log
curves and the field production
histories to the lower section
indicate that all the high-amplitude
events are associated with
proven hydrocarbon zones. It is
obvious that the reflection
amplitudes in the upper conventional 3-D seismic section
do
not identify lithology if the
high-amplitude events in the
lower litho-stratigraphic section
are truly hydrocarbon events.
The basis for unraveling complex AVO
responses to various rock types and fluid
content is a two-term seismic reflection
model. The first term, the Normal
Incidence reflectivity (NI), which has historically
been related to chrono-stratigraphic
reflections, responds to changes
in acoustic impedance. The second term,
defined as the Poisson Reflectivity (PR),
relates to changes in Poisson's Ratio.
Unlike NI, PR remains sensitive to lithologic
variations within the geologic environment
and is thus associated with lithostratigraphic
reflections. The rock property
contrasts, which generate the NI and
PR response, become evident by crossplotting
well
log
values of the natural
log
of acoustic impedance versus Poisson's
Ratio. The 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
from seismic data, the AVO processing
incorporates corrections for anisotropy,
which extends the AVO
analysis
out to
very-far offset traces. After calibration to
log
data, the resulting PR sections depict
reservoir quality sands and potential pay
intervals as litho-stratigraphic sections.
Figure 1. Conventional 3-D section (upper) contains both lithostratigraphic and chronostratigraphic reflections and shows little indication of hydrocarbons while the lithostratigraphic section (lower) depicts eight known gas reservoirs.
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