types of fluids in a reservoir. Tey are recognisable if the reservoirs are more than twice the seismic tuning thickness and also relatively soft. Tis phenomenon is frequently used as a direct hydrocarbon indicator (DHI) in conjunction with seismic amplitudes and AVO techniques in exploring for hydrocarbons. Flat-spot recognition has been particularly successful in the North Sea, where it has been applied to both exploration and reservoir monitoring (e.g., the Gannet-C Field). Sometimes, however, seismic reflections caused by other phenomena, such as remnant multiples and lithology variations, have been misinterpreted as fluid contacts simply because they look flat seismically and show up in the downdip location, which has led to the drilling of unsuccessful prospects. Trough integrating the P- and S-wave data information,
petroleum geoscientists can distinguish between rock and fluid property effects, thus making a significant, predictive contribution to decreasing exploration risk. We give two examples of the potential of integrating P-wave and S-wave information below. In the marine exploration context, one of the first
applications of the joint interpretation of P-wave and S-wave data was to verify the P-wave bright-spot amplitude anomaly seen in the P-wave section in Figure 2.25 as a direct indication of gas saturation. At a reservoir that changes laterally from water-saturated to
gas-saturated, the bright spot represents a strong increase in P-wave reflectivity resulting from a significant drop in P-wave velocity in the presence of the gas. Since the S-wave velocities are not very sensitive to differences in water and gas saturation, there should be no S-wave bright spot associated with gas saturation. When this is observed on the S-wave section, it suggests that the reservoir is gas-saturated. Had there been an S-wave amplitude anomaly coinciding with the P-wave amplitude anomaly, the bright spot would not have been associated with gas saturation. Te second example is
PP-data
related to characterisation of gas hydrates. (See Chapter 8 for an introduction to this topic.) Te bottom simulating
reflection (BSR), as observed on conventional P-wave records, such as towed- streamer data, is the most commonly used indicator of the presence of gas hydrate accumulations below the sea floor. P-wave data alone, however, seem to fail to detect gas hydrates when the BSR is absent. On the other hand, PS data integrated with P-wave data enable a better interpretation of the nature, structure, distribution and
72 Bright spot PS-data time time
quantification of gas hydrates, regardless of the existence of a BSR. In order to test what information PS data can provide about gas hydrate sediments and their characteristics, PGS acquired a 4C line profile over a location in the Norwegian Sea, where a BSR has been identified on conventional streamer P-wave data. Parts of the P- and PS-wave migrated stacks from this multi-component line are shown in Figure 2.26. By comparing the migrated stacks, we observe that events at the BSR area and below are quite different on the two data sections. Te BSR is clearly visible on the P-wave data but is not observable on the PS-wave data. PS reflections are not masked by the gas effects, as PP
reflections are, thus providing better stratigraphic and structural information, while PS reflections seem to follow the sediment layers. In this area, the fact that the BSR is not detected on the PS-wave section suggests that the gas hydrates in the sediments above the BSR have not stiffened the sediment framework, indicating that the hydrate is not cementing the grain contacts. By contrast, if the hydrate had formed at grain contacts, it could have acted as a cementing agent. Te sediment framework would then become stiffer, resulting in increased P-wave and S-wave velocities above the BSR. PS-wave data in this particular situation could potentially show the BSR and could potentially give more detailed information about the stiffness of the sediment and gas hydrate concentration (Andreassen et al., 2003).
Quantitative VP/VS velocity ratio: Te VP/VS velocity ratio (or Poisson’s ratio) is recognised as a key indicator of the presence of hydrocarbons in clastic formations. Tis ratio,
Figure 2.25: Comparison of P-wave and converted PS-wave images through the same geological section. The promising amplitude anomaly (or bright spot) on the P-wave section may be caused either by hydrocarbons or by rock effects (lithology). The disappearance of the bright spot from the PS section strongly indicates that it is caused by hydrocarbons alone (courtesy Statoil. Adapted from Ikelle and Amundsen, 2005).
No bright spot
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