sand is roughly 9 km long, 1.5 km wide, and up to 100m thick. On conventional P-wave seismic data one observes a weak, inconsistent reflector at the top of the reservoir and intra-reservoir shales, but a strong oil-water contact response. Te reason that the top of the Alba reservoir is almost seismically invisible is that there is little or no contrast in P-wave reflectivity between the reservoir sands and the shale cap rock. However, wireline sonic logs show an increase in shear-wave impedance at the shale-sand interface that is significant; hence it was expected that PS converted reflection data would properly image the Alba reservoir (see Chapter 1, Figure 1.48). In early 1998, Chevron acquired a 67 km2


3D-4C survey at Alba. Te PS data provided a clear and high-quality image of the reservoir body. By comparing the OBS data with streamer data, reservoir fluid changes after four years of production and water injection can be directly imaged. Chevron’s study improved the reservoir characteristics of Alba, allowing better placing of new development wells, and it furthermore documented the usefulness of PS converted data to image low P-wave impedance contrast reservoirs displaying large shear-wave impedance contrast. Even though reservoirs with low P-wave


reflectivity due to the small contrast in P-wave impedance are well imaged by PS waves, due to the large contrast in S-wave impedance, large-angle stacks of conventional P-wave towed-streamer data can potentially provide a satisfactory image. P-wave AVO predicts that shale-sand interfaces with small P-wave impedance contrast, but high S-wave impedance contrast should have significant P-wave reflectivity for large angles of incidence. Terefore, before deciding on 4C-OBS acquisition for imaging of low P-wave impedance contrast reservoirs, the petroleum seismologist should evaluate whether towed-streamer AVO sections can solve the problem at hand. In some cases, however, the reservoir may be


an obstructed area limiting the access for towed- streamer operations. In this case, 4C-OBS is definitely the best solution.


Quantification of amplitude anomalies: Marine 4C seismic – the use of concurrent, combined pressure and shear wave seismic data – unfortunately has not gained true acceptance in the E&P industry as a tool that can reduce risk by providing information about subsurface rocks and distributions of pore fluids. Tis is partly a cost issue. Nevertheless, 4C seismic technology can solve many seismic


and geological challenges which cannot reliably be solved by the use of P-wave data information alone. When interpreting P-wave data from towed-streamer


surveys, as is common industry practice, it is extremely difficult to find out whether P-wave amplitude anomalies such


Figure 2.24: (a) Conventional streamer P-wave image of the Alba channel. Note the weak top sand event in the mid of the section at around 2s travel time. The converted wave PS image shows dramatically improved imaging of the sand channel due to the high PS reflectivity between shale and sand. (b) The dipole sonic log through the Alba reservoir sand shows a large contrast in shear-wave velocity (left) and a small contrast in P-wave velocity (right) with the surrounding shales. The green curves represent velocities in sand. The red and blue curves represent velocities in shales above and below the sand channel, respectively.


as ‘bright spots’ and ‘flat spots’ (discordant events) are related to hydrocarbons or related to lithology. P-waves are not only influenced by rock types but also by fluids and it is difficult to discriminate between these effects. By including information on S-waves from ocean-bottom seismic surveys as well, it is possible to say whether bright spots and flat spots are most likely to be related to lithology or fluid effects since, unlike their P-wave counterparts, S-waves are almost insensitive to a rock’s fluid content, seeing mainly the rock’s matrix. Seismic flat spots are caused by the interface between two


71


a


b


Courtesy Chevron


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