3-D structural inversion applied to seismic data from the Central North Sea
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| Series | Investigations in Geophysics |
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| Author | Öz Yilmaz |
| DOI | http://dx.doi.org/10.1190/1.9781560801580 |
| ISBN | ISBN 978-1-56080-094-1 |
| Store | SEG Online Store |
The second 3-D structural inversion case study is from the Central North Sea; it involves a 3-D survey that covers an area of nearly 1800 km2. The surface area is roughly a rectangle with dimensions of approximately 70 km in the northwest-southeast direction, which is coincident with the crossline direction, and 25 km in the inline direction. Survey statistics include 456 million traces recorded, 228 million prestack traces processed, 5.7 million stacked traces, and 11.4 million traces input to 3-D poststack time migration after crossline trace interpolation. The nominal fold of coverage is 40, the inline trace spacing is 12.5 m, and the crossline trace spacing is 25 m before trace interpolation and 12.5 m after trace interpolation.
We want to conduct a layer-by-layer earth model estimation, and use the following combinations of inversion methods:
- Coherency inversion to estimate layer velocities and 3-D poststack depth migration to delineate reflector geometries.
- Stacking velocity inversion to estimate layer velocities and image-ray depth conversion of time horizons interpreted from the time-migrated volume of data to delineate reflector geometries.
We then want to compare the depth structure maps for the target horizon base Zechstein (top Rotliegendes) obtained from the two procedures outlined above.
Figure 10.7-1 shows selected inlines from the unmigrated 3-D volume of DMO-stacked data. Superimposed on these displays are the time horizons that correspond to layer boundaries with significant velocity contrast. These are, starting from the top, base Upper and Lower Tertiary, base Cretaceous chalk, base Upper and Lower Triassic, and base Zechstein (the red horizon). The target zone is the Rotliegendes sands just beneath base Zechstein, and the underlying Carboniferous Westfalien sequence.
The Zechstein formation comprises two units of anhydrite-dolomite embedded in the halite unit. One of the anhydrite units is very close to the top boundary of Zechstein (the green horizon) and is concordant with it. The deeper, second anhydrite unit has a very complex geometry as a result of the intensive salt tectonics in the area. The abundance of diffractions within the Zechstein formation is associated with this anhydrite-dolomite unit. One of the objectives of this case study is to investigate whether the complex geometry of the second anhyrite-dolomite unit has any subtle, but deleterious effect on the geometry of the target horizon — base Zechstein (top Rotliegendes).
Figure 10.7-2 shows selected inlines as in Figure 10.7-1 from the 3-D poststack time-migrated volume of data. The velocity field used for 3-D poststack time migration is a smoothed version of the 3-D stacking velocity field. Note the complex geometry of the second anhydrite-dolomite unit within Zechstein. The faults along the base Zechstein act as seals on potential reservoir margins. Accurate positioning of these faults, and determination of their displacements in depth, are therefore, central to exploration and development objectives in the area.
Figure 10.7-1 Part 1: Selected inlines from the 3-D volume of unmigrated DMO-stacked data. The inline direction is northeast-southwest, with the northeast being on the right-hand side of the displays. The inlines are labeled from northwest to southeast over the survey area with decreasing line numbers. The color-coded horizons correspond to layer boundaries with significant velocity contrast. (Data courtesy Schlumberger Geco-Prakla, project courtesy Amoco UK Exploration and Production Company.)
Figure 10.7-1 Part 2: Selected inlines from the 3-D volume of unmigrated DMO-stacked data. The inline direction is northeast-southwest, with the northeast being on the right-hand side of the displays. The inlines are labeled from northwest to southeast over the survey area with decreasing line numbers. The color-coded horizons correspond to layer boundaries with significant velocity contrast. (Data courtesy Schlumberger Geco-Prakla, project courtesy Amoco UK Exploration and Production Company.)
Figure 10.7-2 Part 1: Selected inlines as in Figure 10.7-1 from the 3-D poststack time-migrated volume of data. The inline direction is northeast-southwest, with the northeast being on the right-hand side of the displays. The inlines are labeled from northwest to southeast over the survey area with decreasing line numbers. The color-coded horizons correspond to layer boundaries with significant velocity contrast.
Figure 10.7-2 Part 2: Selected inlines as in Figure 10.7-1 from the 3-D poststack time-migrated volume of data. The inline direction is northeast-southwest, with the northeast being on the right-hand side of the displays. The inlines are labeled from northwest to southeast over the survey area with decreasing line numbers. The color-coded horizons correspond to layer boundaries with significant velocity contrast.
Figure 10.7-3 Time horizons interpreted from the volumes of unmigrated DMO-stacked and 3-D poststack time-migrated data as in Figures 10.7-1 and 10.7-2, respectively. Horizon 1, which is not shown here, corresponds to the water-bottom reflection. Horizons 2-7, in the ascending order, correspond to base Upper and Lower Tertiary, base Cretaceous chalk, base Upper and Lower Triassic, and base Zechstein.
Figure 10.7-4 Horizon-consistent stacking velocities that correspond to unmigrated time horizons shown in Figure 10.7-3. Horizons 2-7, in the ascending order, correspond to base Upper and Lower Tertiary, base Cretaceous chalk, base Upper and Lower Triassic, and base Zechstein.
| Horizon | Layer Boundary | Velocity Range (m/s) |
| 2 | Base Upper Tertiary | 1450 – 1950 |
| 3 | Base Lower Tertiary | 1650 – 2350 |
| 4 | Base Cretaceous | 2150 – 2500 |
| 5 | Base Upper Triassic | 2550 – 2800 |
| 6 | Base Lower Triassic | 2850 – 3100 |
| 7 | Base Zechstein | 3200 – 3550 |
Figure 10.7-3 shows the time surfaces derived from the interpretation of the 3-D volumes of unmigrated and time-migrated stack data. Horizons 2-7, in ascending order, correspond to base Upper and Lower Tertiary, base Cretaceous chalk, base Upper and Lower Triassic, and base Zechstein. The time horizons interpreted from the unmigrated DMO-stacked volume are considered equivalent to zero-offset time horizons. They are used in Dix conversion of stacking velocities, stacking velocity inversion, and coherency inversion to estimate layer velocities. The time horizons interpreted from the time-migrated volume are used in map migration by way of image-ray depth conversion to obtain the corresponding depth horizons.
Figure 10.7-4 shows the horizon-consistent stacking velocities that were extracted from the 3-D stacking velocity field along time horizons interpreted from the unmigrated volume of stacked data shown in Figure 10.7-3. Table 10-1 shows the range of lateral velocity variations within each layer.
See also
- 3-D coherency inversion combined with 3-D poststack depth migration
- 3-D stacking velocity inversion combined with 3-D image-ray depth conversion
External links
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