Viabilities of seismic ray impedance and elastic impedance for hydrocarbon-sand discrimination

Abstract

Theoretical evaluation of the elastic impedance (EI) and the ray impedance (RI) reveals that RI has a more reliable value range and is less sensitive to noise than EI. We devised a new measurement [Formula: see text] to estimate the ray impedance from elastic impedance derived by existing techniques. The recovered [Formula: see text] was expressed in the form of a normalization of EI. It solved the range variability problem of EI and had the same interpretation capability as RI. In addition, reflection coefficients represented by [Formula: see text] showed good agreement with the Zeoppritz equation even at postcritical angle of incidence. Tests of these three attributes (RI, [Formula: see text], and EI) were performed on the log data of three different types of reservoir: a typical Class III marine gas-sand, a Class I tight gas-sand, and a Class II oil-bearing sand in thin sand-mud interbedded layers. Although the crossplots of EI against acoustic impedance (AI) showed visually similar characteristics for a gas-sand as that of RI, based on the linear/quadratic discrimination analysis, RI appeared to be more applicable than EI for characterizing gas sands, especially tight gas sands. [Formula: see text], estimated from EI, had a comparable value range to the AI, and retained the interpretation ability of the original RI. Application on real seismic data showed that existing EI inversion results could be improved straightforwardly by means of the introduced transformation.