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GCSeismic Previous HitDepthNext Hit Interpretation in Thrustbelts*

By

Nancy House1

 

Search and Discovery Article #40131 (2004)

 

*Adapted from the Geophysical Corner column in AAPG Explorer, May, 2004, entitled “Previous HitDepthNext Hit 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])

 

General Statement 

A big challenge for modern seismic is the ability to image complicated structures. Fold and thrustbelts are characterized by rapid Previous HitvelocityNext Hit 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 Previous HitdepthNext Hit. Once the interpretation is correctly Previous HitdepthNext Hit positioned, cross-section balancing can be used to help create a geologically viable three-dimensional model. The correct Previous HitdepthNext Hit model results in better volumetric estimates of reserves.

 

 

uGeneral statement

uFigure captions

uMigration

uThrustbelt example

uCross-section balancing, reservoir modeling

uConclusions

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uGeneral statement

uFigure captions

uMigration

uThrustbelt example

uCross-section balancing, reservoir modeling

uConclusions

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uGeneral statement

uFigure captions

uMigration

uThrustbelt example

uCross-section balancing, reservoir modeling

uConclusions

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uGeneral statement

uFigure captions

uMigration

uThrustbelt example

uCross-section balancing, reservoir modeling

uConclusions

Figures Captions

Figure 1. Time-migrated interpretation of simple four-layer Previous HitvelocityNext Hit model. Shallowest layer exhibits a compaction gradient causing Previous HitvelocityNext Hit to increase linearly with Previous HitdepthNext Hit. Deeper layers have constant velocities with higher velocities in the deeper layers.

Figure 2. Image ray Previous HitdepthNext Hit-migrated cross section of three layers of the interpretation shown in Figure 1. Several of the image rays have been highlighted to demonstrate the effects of the refraction and bending of rays across the Previous HitvelocityNext Hit boundaries and in the shallow layer.

Figure 3. Complex overthrust interpretation of time-migrated seismic data from a South American thrustbelt. Note the major discontinuity between shallow thrust sheets and deep imbricate thrusts. This is where Previous HitdepthNext Hit conversion is challenging because of ray bending and large Previous HitvelocityNext Hit contrasts.

Figure 4. Previous HitDepthNext Hit-migrated interpretation of complex overthrust model of Figure 3. Note the gaps in the deeper blue layers where further iteration is needed to reconcile interpreted events that are not reasonable with the current Previous HitvelocityNext Hit model. Arriving at a final model is an iterative process.

Figure 5. Structurally balanced Previous HitdepthNext Hit-migrated interpretation of imbricate thrusts below a geologically younger layer with its own complex internal structure. Compare to Figure 3.

Figure 6. Better reservoir volumetrics and drilling prognosis result from an accurate Previous HitdepthNext Hit-migrated interpretation.

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Time vs. Previous HitDepthNext Hit Migration 

One of the first lessons geophysicists learn about seismic data interpretation is that the seismic image is not located where it appears. It gets "migrated" to compensate for reflections not emanating from directly beneath the surface recording point, or zero offset trace location. Traditional time migration methods Previous HitusingNext Hit smoothed stacking velocities are considered good when diffractions are collapsed to a point and the image appears focused -- but this may not correctly position the images in Previous HitdepthNext Hit. Time migration appropriately locates most events for simple cases where there is not a significant lateral Previous HitvelocityNext Hit contrast across layers or steep Previous HitdipNext Hit in the overlying Previous HitvelocityNext Hit boundaries

Generally an interpretation is done Previous HitusingNext Hit time-migrated data that is converted to Previous HitdepthNext Hit by vertically stretching the observed travel times. Known depths from well ties are used to adjust the final map to fit the structure depths. Previous HitDepthNext Hit converting by vertically stretching the interpretation in Figure 1 would result in the same structural shape, with each layer scaled in Previous HitdepthNext Hit based on the velocities used for the migration. 

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 Previous HitdepthNext Hit migration were developed to address ray bending in areas of high Previous HitvelocityNext Hit contrasts and dipping interfaces. However, pre-stack Previous HitdepthNext Hit migration is expensive and time-consuming, and it requires a detailed prior understanding of the Previous HitvelocityNext Hit Previous HitdepthNext Hit model to achieve a solution.  

Because time and money are always limited, where there is an adequate image to start with, a simplified Previous HitdepthNext Hit migration technique can be used. Image rays are the theoretical ray paths taken by time-migrated seismic events. The time-migrated data can be Previous HitdepthNext Hit-migrated by image ray migrating the interpreted interfaces. 

Figure 2 illustrates a Previous HitdepthNext Hit-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 Previous HitvelocityNext Hit in the shallowest layer and constant, highly contrasted velocities in the two deeper layers. 

The time migration (Figure 1) adequately corrects for the shallowest interface, but it incorrectly positions the deeper events. The Previous HitdepthNext Hit-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 Previous HitvelocityNext Hit field combined with steeply dipping beds. This geometry causes the apparent location of the points below this interface to be affected by the gradual bending of the rays through the Previous HitvelocityNext Hit gradient and refraction at the interfaces.  

Image ray Previous HitdepthNext Hit migrating the interpretation results in the image produced in Figure 4, where the Previous HitdepthNext Hit-migrated result is based on the interpreted Previous HitvelocityNext Hit field. Deeper events that appear chaotic in this figure indicate areas where the interpreted events are not resolved by the Previous HitvelocityNext Hit model. 

The time-migrated interpretation and Previous HitvelocityNext Hit model can be iteratively modified until the resulting Previous HitdepthNext Hit-migrated model is geologically reasonable. Iterating the model interactively -- so one can see the changes -- allows the interpreter to gain insight into the raypaths that produced the images on the time-migrated seismic section.

 

Seismic for Cross-Section Balancing, Reservoir Modeling 

Balancing geologic cross-sections is an important geologic tool for working in thrustbelts. By Previous HitusingNext Hit a grid of 2-D seismic profiles in which each profile is image ray Previous HitdepthNext Hit-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 Previous HitcalculationsNext Hit of trapped hydrocarbons. 

In the complex overthrust model example here, the output of the image ray Previous HitdepthNext Hit-migrated interpretations was used as input to a balanced geologic cross-section. The resulting Previous HitdepthNext Hit-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 Previous HitdepthNext Hit representation of the structural geometry of a reservoir, the resulting volume Previous HitcalculationsNext Hit are more accurate. This is commonly the largest variable in the reserve Previous HitcalculationsNext Hit

Three-D visualization, attribute analysis and interpretation with accurate well ties, and reservoir model building for simulation are significantly improved by creating more accurate Previous HitdepthNext Hit representation of surfaces and faults (Figure 6).

 

Conclusions 

Today's seismic processing produces not only zero offset data (un-migrated) and time-migrated data sets, but with the increase in computer capabilities, Previous HitdepthNext Hit-migrated volumes are becoming readily available to the interpreter. 

In complex areas, accurate well ties are important to help define a proper Previous HitvelocityNext Hit field for creating a Previous HitdepthNext Hit-migrated image. In these cases, it is also important to understand the raypaths and to use the best estimate of travel time Previous HitvelocityNext Hit fields before proceeding with well design, Previous HitdepthTop prognosis, and volumetric estimates of the reserves.

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