CROSSWELL SEISMIC STUDY IN A SEISMICALLY POOR DATA AREA

Crosswell traveltime tomography and reflection imaging assisted a rese rvoir characterization effort in an area of poor-quality surface seism ic data. Both the tomogram and the reflection image proved useful in t he description of the fractured reservoir interval. The velocity tomog ram shows that: (1) the vertical resolution was sufficient to identify and characterize a 50-ft (15 m) thick lithological unit of brittle ro cks, which was the most important interval for the characterization of this fractured reservoir; (2) different lithological units present su fficient 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 th e lithologies with the tomogram implies that the major controlling fac tor of the anisotropy is the shale content in the formation. The cross well reflection image, generated by a VSP-CDP mapping technique, defin es the fractured reservoir interval in terms of high-frequency reflect ions, The lateral resolution of this reflection image is difficult to define because the survey coverage is nonuniform as a result of the re ceiver spacing being much larger than the source spacing. The dips of the reflections do not quite agree with the dips that are inferred fro m well log ties. We believe this disagreement is a result of the aniso tropy of the medium and the use of an isotropic imaging algorithm. Imp roved data acquisition (finer spatial sampling) that would allow bette r wavefield separation techniques to be used would probably have produ ced higher quality crosswell reflection images.