Rulison Field is a basin-centered gas accumulation located in the Piceance Basin of northwest Colorado. The reservoir consists of lenticular fluvial sands, shales, and coals of the Upper Cretaceous Williams Fork Formation. Typical sand body thicknesses are 10-15 ft, an order of magnitude lower than the seismic resolution which is 105 ft. The sandstone reservoirs are the primary target and are typically low porosities (6-12%) and very low permeabilities (1-10 µD). The best production is dependent on the fracture permeability. The major objectives of the reservoir characterization in Rulison are (1) the imaging of the lenticular sand bodies, (2) the identification of the high quality overpressured productive sandstones, and (3) the determination of the fractured intervals. In order to achieve these goals, a quantification of anisotropy in the reservoir is essential in understanding the seismic amplitude responses. Elastic anisotropic AVO modeling and anisotropic velocity model building are the tools required for (1) the inversion of converted shear-wave surface seismic data and (2) depth imaging of the sand bodies based on non-converted shear-wave VSP data.


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