Detection and analysis of naturally fractured gas reservoirs: Multiazimuthseismic surveys in the Wind River basin, Wyoming

Multiazimuth binning of 3-D P-wave reflection data is a relatively simple b ut robust way of characterizing the spatial distribution of gas-producing n atural fractures. In our survey, data were divided into two volumes by ray azimuth (approximately perpendicular and parallel (+/-45 degrees) to the do minant fracture strike) and separately processed. Azimuthal differences or ratios of attributes provided a rough measure of anisotropy. Improved imagi ng was also attained in the more coherent fracture-parallel volume. A neura l network using azimuthally dependent velocity, reflectivity, and frequency attributes identified commercial gas wells with greater than 85% success. Furthermore, we were able to interpret the physical mechanisms of most of t hese correlations and so better generalize the approach. The apparent veloc ity anisotropy was compared to that derived from other P- and S-wave method s in an inset three-component survey. Prestack determination of the azimuth al moveout ellipse will best quantify velocity anisotropy, but simple two- or four-azimuth poststack analysis can adequately identify regions of high fracture density and gas yield.