Viabilities of PP‐ wave ray and elastic impedance for hydrocarbon‐sand discrimination

Abstract

Elastic impedance (EI) only represents the physical property of a subsurface medium within a limited interval, due to the assumption of a constant angle of incidence. Theoretical evaluation of elastic impedance (EI) and ray impedance (RI) reveals that RI has a more reliable dimension and is less sensitive to errors than EI. In this paper, a new measurement (I) is proposed to estimate ray impedance from the elastic impedance derived by existing techniques. Recovered I is expressed in the form of a normalization of the elastic impedance (EI), and it has the same dimension and interpretation capability as RI. Tests of these three measurements are performed on the log data of three different types of reservoirs: a typical Class III marine gas sand, a Class I tight gas‐sand and an oil‐bearing sand in thin sand‐mud inter‐beded layers. We show that RI is capable of characterizing all three types of reservoirs. In particular, it appears to be more applicable than EI for characterizing tight gas sands. The cross‐plots of EI against acoustic impedance (AI) show good characterization for gas sand, but approximately linear trends are found for all three cases. RI estimated from EI has a comparable dimension to the acoustic impedance (AI), and maintains the interpretation ability of the original RI. We also show the estimation of RI for real inverted sections of the tight gas sand.