3-D structural inversion applied to seismic data from the Southern North Sea
The first 3-D structural inversion case study is from the Southern Gas Basin of the North Sea. It involves a complex overburden associated with salt tectonism. The survey area is approximately 90 km2. The surface area is roughly a square with dimensions of 10 km in the crossline direction and 9 km in the inline direction. Survey statistics include more than 9 million traces recorded and nearly 300 000 stacked traces. The nominal fold of coverage is 32, 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.
|Layer Velocities||Reflector Geometries|
|Dix conversion of rms velocities||vertical-ray time-to-depth conversion (vertical stretch)|
|stacking velocity inversion||image-ray time-to-depth conversion (map migration)|
|coherency inversion||poststack depth migration|
|image-gather analysis||prestack depth migration|
To estimate a 3-D velocity-depth model, the following procedure composed from the list of inversion methods in Table 9-1 is used:
- Coherency inversion to estimate layer velocities and 3-D poststack depth migration to delineate reflector geometries within the overburden, and
- constant half-space velocity analysis of image gathers from prestack depth migration to estimate the substratum velocity with stacking of image gathers to delineate the reflector geometry of the base-Zechstein (top-Rotliegendes) target horizon.
The procedure outlined above is applied layer-by-layer starting from the surface to resolve layer velocity and reflector geometry of one layer before moving onto the next. This minimizes the influence of any errors made in one layer on the parameter estimates for the next layer.
Figure 10.6-1 shows selected crosslines from the unmigrated 3-D volume of DMO-stacked data. Superimposed on the displays in Figure 10.6-2 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 zones are the Rotliegendes sands just beneath the base Zechstein, and the underlying Carboniferous Westfalien sequence. Note the traveltime distortions along the base-Zechstein event (the red horizon) — a classic case of the deleterious effect of a complex overburden with strong lateral velocity variations. The complex overburden includes the collapsed structure above the apex and to the left of the salt diapir, and the complex geometry of the top-salt boundary (the brown horizon) which is where the most severe ray bending takes place.
Figure 10.6-1 Selected crosslines from the 3-D volume of unmigrated DMO-stacked data. The crossline direction is northwest-southeast, with the northwest being on the right-hand side of the displays. The crosslines are from southwest to northeast over the survey area with increasing line numbers. (Data courtesy Schlumberger Geco-Prakla and Amoco UK Exploration and Production Company.)
Figure 10.6-3 shows the time surfaces derived from the interpretation of the 3-D volume of unmigrated stack data. These time horizons are considered equivalent to zero-offset time horizons, and as such, are used in coherency inversion to estimate layer velocities. Horizons TH1-TH6, in ascending order, correspond to base Upper and Lower Tertiary, base Cretaceous chalk, base Upper and Lower Triassic, and base Zechstein. The water depth in the survey area is shallow and the Upper Tertiary velocities near the water bottom are very close to the water velocity. Therefore, the water layer was considered as part of the Upper Tertiary layer. In fact, we observe that there is no appreciable velocity contrast at the base Upper Tertiary, and hence, the whole Tertiary section can be considered as one single layer in earth modeling.
- Estimation of the overburden model
- Model representation by tessellation
- 3-D coherency inversion
- 3-D poststack depth migration
- Estimation of the substratum model