Statistical AVO intercept-gradient analysis of direct S-waves: A methodology for quantitative fracture characterization

Abstract

Summary This study focuses on the effects of fractures on the elastic properties of a rock and illustrates which P-wave and Swave Amplitude Variation with Offset (AVO) attributes might be helpful for quantitative interpretation of fracture properties like fracture-orientation, fracture-density and fracture-fill. A statistical rock physics model is constructed for Ordovician age carbonates of the Arbuckle Formation, encountered in wells drilled in the Wellington Field, Kansas. Our model incorporates a priori information about matrix porosity distribution and pore shapes obtained from well logs and analysis done on drill cores. Estimations of Pwave and S-wave seismic velocities and anisotropy parameters obtained from rock physics models, are used to study AVO behaviors of P-waves and fast and slow Swaves in directions parallel to, and perpendicular to fractures. AVO modeling shows that azimuthal variations in S-S mode AVO gradient can be used to determine the average fracture-orientation at a subsurface location; whereas, azimuthal variations in P-P mode AVO attributes are less than the uncertainty of lithology and fracturedensity, which limits the ability of P-waves to determine fracture-orientation. Results of this study demonstrate that in angle and azimuth space, the AVO intercept attribute of slow-S-wave in the plane parallel to fractures, quantitatively estimates the (crack density) parameter of a rock formation. In contrast, P-P azimuthal AVO attributes do not correlate well with crack density. This study also demonstrates that the AVO gradient of slow S-wave in direction normal to fracture-orientation is a seismic attribute for discriminating fluid fill in fractures. Conclusively, S-wave AVO attributes are less affected by subsurface uncertainty compared to P-wave attributes and hence can significantly reduce ambiguity in quantitative fracture characterization.