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GCAdvances in Spectral Decomposition and Reflectivity Modeling in the Frio Formation of the Gulf Coast*
By
Michael D. Burnett1 and John P. Castagna2
Search and Discovery Article #40113 (2004)
*Adapted for online presentation from the Geophysical Corner column in AAPG Explorer, January, 2003, entitled “ISA Method Spotted Gas Anomalies” and prepared by the authors. Appreciation is expressed to the authors, to R. Randy Ray, Chairman of the AAPG Geophysical Integration Committee, and to Larry Nation, AAPG Communications Director, for their support of this online version.
1Fusion Geophysical, Dallas ([email protected])
2University of Oklahoma, Norman, Oklahoma ([email protected])
The seismic interpreter now has
the use of a simple tool to aid in the search for hydrocarbons by allowing
individual target reflection events to be spectrally analyzed and compared to
gas response modeled from well
logs with and without pay. The seismic reflection
is then decomposed to spot a positive gas response.
Until now, spectral decomposition techniques have used windowing methods to decompose the seismic trace into its constituent frequencies -- but these methods:
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Mix reflection events and introduce unwanted artifacts into the data.
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Restrict the usefulness of spectral decomposition to the inspection of single-frequency maps to try to relate amplitude maxima to geologic events.
Recent advances in techniques have
yielded a method that does not use windowing to decompose the trace: The
Instantaneous Spectral Analysis
(ISA) method uses a wavelet transform technique
to produce single-frequency reflection events that are accurately localized in
time. Each full-spectrum reflection can be visualized and analyzed at its
uncontaminated single-frequency equivalents.
uGeneral statementuFigure captionsuFrio bright spotu3-D map displays
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Frio Seismic Bright Spot
Figure 1
shows a high amplitude reflection characterizing a
Frio reservoir in which gas is trapped
stratigraphically due to a sand pinchout. The
Frio sand, which is about 68 feet thick, is shown in
Figure 2, with the What cannot be seen is the behavior of the individual seismic frequencies; i.e., what effect does the hydrocarbon charge make on the amplitudes of each discrete frequency. Because the ISA technique allows uncombined reflectivity to be examined, as no windowing is used during the calculation, the pay reflectivity can be isolated and studied. This new approach allows one to show the reflection's response to the hydrocarbon charge at various frequencies via a "frequency gather," as shown in Figure 3a.The display shows increasing frequency to the right with the strongest amplitudes in warm colors. This is very similar to the familiar AVO gather -- except where adjacent traces represent the reflection's response to changing offset in the AVO gather. Here each trace represents the reflection's amplitude at a single frequency, or amplitude versus frequency (AVF). The anomalous response caused by the pay clearly can be seen as a very high amplitude with a peak frequency that is shifted toward the high end of the useable bandwidth. When the process is run on the entire seismic line, single-frequency panels are produced, as shown in Figure 3b and 3c. Note that at 10 Hz, the pay does not exhibit high amplitude, while at 36 Hz, it is one of the brightest events on the section. The Frio bright spot on the 36 Hz seismic line in Figure 3c agrees with the frequency gather shown in Figure 3a. The pay has relatively little energy at 10 Hz, but at 36 Hz, it is one of the few remaining events to have high reflection strength. This is in contrast to the strong events centered between 2.0 and 2.1 seconds at the wellbore. They have visually lower frequency and their strongest reflection amplitudes are closer to 10 Hz. When viewed as a frequency panel movie, the changing contrast becomes very striking. 3-Dimensional Map DisplaysWhen the ISA process is applied to cubes of seismic data, the results are a series of single-frequency cubes that are loaded onto the workstation and interpreted. Figure 4 shows four slices from the frequency cubes on the pay horizon:
As Figure 4 shows, the pay is acting completely different than the surrounding sand when viewed at discrete frequencies. This is even more apparent when all the frequency maps are viewed as a movie. The pay has a distinctly different dynamic frequency response than the background because the hydrocarbons have changed the reflectivity of the reservoir.
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