Crosswell traveltime tomography and reflection imaging assisted a reservoir characterization effort in an area of poor-quality surface seismic data. Both the tomogram and the reflection image proved useful in the description of the fractured reservoir interval.The velocity tomogram shows that: (1) the vertical resolution was sufficient to identify and characterize a 50-ft (15 m) thick lithological unit of brittle rocks, which was the most important interval for the characterization of this fractured reservoir; (2) different lithological units present sufficient velocity contrast to be identifiable on the tomogram; and (3) the tomogram velocity is higher than the sonic velocity implying that the rocks in the interwell area may be anisotropic. Correlation of the lithologies with the tomogram implies that the major controlling factor of the anisotropy is the shale content in the formation.The crosswell reflection image, generated by a VSP-CDP mapping technique, defines the fractured reservoir interval in terms of high-frequency reflections. The lateral resolution of this reflection image is difficult to define because the survey coverage is nonuniform as a result of the receiver spacing being much larger than the source spacing. The dips of the reflections do not quite agree with the dips that are inferred from well log ties. We believe this disagreement is a result of the anisotropy of the medium and the use of an isotropic imaging algorithm. Improved data acquisition (finer spatial sampling) that would allow better wavefield separation techniques to be used would probably have produced higher quality crosswell reflection images.

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