Analysis of shear‐wave polarizations from a nine‐component 3‐D dataset, Cedar Hill, New Mexico

Abstract

A multicomponent 3-D seismic survey was conducted by the Colorado School of Mines’ Reservoir Characterization Project over a coal bed methane reservoir near Cedar Hill, New Mexico. The purpose of the survey was to characterize fractures in the coal beds which are of importance to the production of natural gas. Primary objectives were to determine the dominant direction of fracturing, the variations in fracture direction, and the amount of fracturing in the reservoir. Polarization analysis was carried out on the pre-stack data to determine the dominant direction of anisotropy, from which the direction of fracturing may be infered. Statistical methods were applied to aid in the automated analysis of these data. The dominant direction was established as northwest, which agrees with that determined from a VSP. The variance of the polarizations noted on the pre-stack data may be attributed to spatial variations of the polarization directions as determined by analysis of the poststack data. Comparison of the post-stack polarization variations with fault locations mapped from the P-wave data indicates that the major faults separate zones of variable polarization and the polarizations are aligned perpendicular to dip where dip is significant. These findings are consistent with local stress variations caused by a wrench fault system. Significant variations are noted in the degree of anisotropy which are believed to be related to variable fracturing of the coals.