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GCSeismic Depth Interpretation in Thrustbelts*
By
Nancy House1
Search and Discovery Article #40131 (2004)
*Adapted from the Geophysical Corner column in AAPG Explorer, May, 2004, entitled “Depth Reckoning speaks Volumes” and prepared by the author. Appreciation is expressed to the author, to Alistar R. Brown, editor of Geophysical Corner, and to Larry Nation, AAPG Communications Director, for their support of this online version.
1Geophysicist, EnCana Oil & Gas USA, Denver CO ([email protected])
A
big challenge for modern seismic is the ability to image complicated structures.
Fold and thrustbelts are characterized by rapid velocity
variations due to
juxtaposed rock types.
Generally, if you can see a structural image on seismic, the next step is to determine where that structure is actually located in depth. Once the interpretation is correctly depth positioned, cross-section balancing can be used to help create a geologically viable three-dimensional model. The correct depth model results in better volumetric estimates of reserves.
uGeneral statementuFigure captionsuMigrationuThrustbelt exampleuCross-section balancing, reservoir modelinguConclusions
uGeneral statementuFigure captionsuMigrationuThrustbelt exampleuCross-section balancing, reservoir modelinguConclusions
uGeneral statementuFigure captionsuMigrationuThrustbelt exampleuCross-section balancing, reservoir modelinguConclusions
uGeneral statementuFigure captionsuMigrationuThrustbelt exampleuCross-section balancing, reservoir modelinguConclusions |
Time vs. Depth Migration
One of the
first lessons geophysicists learn about seismic
Generally
an interpretation is done using time-migrated For cases where beds are dipping, the energy is refracted at high contrast interfaces, similar to the effect on the image of a straight pole inserted at an angle into a smooth pool of water; the pole appears bent at the air-water interface. In severe cases there may be no seismic image below high contrast boundaries.
Both "pre"
and "post" stack depth migration were developed to address ray bending
in areas of high
Because
time and money are always limited, where there is an adequate image to
start with, a simplified depth migration technique can be used. Image
rays are the theoretical ray paths taken by time-migrated seismic
events. The time-migrated
Figure 2
illustrates a depth-migrated interpretation of the same model shown in
Figure 1, accounting for the refraction and
ray bending at the interfaces. The model exhibits a compaction The time migration (Figure 1) adequately corrects for the shallowest interface, but it incorrectly positions the deeper events. The depth-migrated model (Figure 2) correctly positions the steepened flanks of the anticline with the horizontal position also changed along the dipping flanks compared to the inaccurate time-migrated structure. Thrustbelt Example
An example
from South America (Figure 3) is used to
illustrate typical thrustbelt interpretation challenges. This seismic
cross-section has a geometry similar to the models with a younger
formation above the main detachment fault. It has a strong compaction
gradient in the
Image ray
depth migrating the interpretation results in the image produced in
Figure 4, where the depth-migrated result is
based on the interpreted
The
time-migrated interpretation and Seismic for Cross-Section Balancing, Reservoir ModelingBalancing geologic cross-sections is an important geologic tool for working in thrustbelts. By using a grid of 2-D seismic profiles in which each profile is image ray depth-migrated prior to cross-section balancing, the interpreter can produce a 3-D structurally balanced interpretation based on 2-D seismic. This in turn produces less error in drilling prognosis and tying wells in structurally complex areas -- and it also improves the ultimate volume calculations of trapped hydrocarbons. In the complex overthrust model example here, the output of the image ray depth-migrated interpretations was used as input to a balanced geologic cross-section. The resulting depth-migrated interpretation required little or no correction of the basic shape of the formations or the faults to produce a geologically feasible balanced cross-section (Figure 5). With a more accurate depth representation of the structural geometry of a reservoir, the resulting volume calculations are more accurate. This is commonly the largest variable in the reserve calculations.
Three-D
visualization, attribute Conclusions
Today's
seismic processing produces not only zero offset
In complex
areas, accurate well ties are important to help define a proper |