Arctan Aa I Ro: a different seismic AVO attribute for fluid-lithology identification.

A different AVO attribute, obtained from calculating the arctan of flo / Ro, two commonly used byproducts of AVO inversion (Ro = normal incidence reflectivity; flo = pseudo Poisson reflectivity), has been used to generate seismic sections which clearly indicate the presence of hydrocarbons. This technique is based on the existence of a preferred "background fluid-angle" in a Ro versus flo crossplot. In such crossplot, hydrocarbon bearing reservoirs show as anomalous angular values away from this trend. Well log and seismic data were analyzed for two actual cases: (1) a typical bright spot, gas reservoir (class 3 AVO response); and (2) a low impedance contrast, condensate reservoir (class 2 AVO response), and an interpretative-display scheme developed for the generation of the "arctan flo I Ro sections". For both cases, the final "arctan flo I Ro sections" directly indicate those events related to the hydrocarbon bearing reservoirs. This technique provides a new attribute to be examine when discriminating seismic data in the search for such reservoirs, primarily those hard-to-find low impedance contrast sandstones.