Improved AVO fluid detection and lithology discrimination using Lamé petrophysical parameters; “λρ”,“μρ”,&“λ/μ fluid stack”, from P and S inversions

Abstract

However the underlying physics in the wave equation; does not involve seismic velocities, but instead the ratio of density (p) to modulus (M). So converting velocity measurements to Lame’s modulii parameters of rigidity and incompressibility (h) offers new insight into the original governing rock property factor It will be shown that an improved identification of reservoir zones is possible by the enhanced sensitivity to pore fluids from pure compressibility, as well as lithologic variations represented by fundamental changes in rigidity, incompressibility, and density parameters as opposed to mixed parameters of seismic velocities. Theory, method and log analysis motivation Standard analysis methods given above, though appearing different, rely fundamentally on Vp, Vs and density variations, thereby masking the original modulus parameterization as mentioned. Some authors point out the need for a more physical insight afforded by rigidity (Wright 1984,Thomsen 1990,Castagna et al. 1993b) in the above equations. Castagna also indicates that the link between velocity and rock properties for pore fluid detection, is through the bulk modulus that is embedded in Vp. However both and more so Vp have the most sensitive pore fluid indicator diluted by varying factors of the rock matrix indicator (ie. non-pore fluid). This can be seen in the following relationships; and Recent AVO inversion schemes incorporate an explicit density term (Stewart 1995, Smith 1996) to potentially extract modulii, but as the number of unknowns increase and exceed the measured quantities (intercept and offset gradient amplitude) so these complex equations are less robust and the extracted values more inaccurate. From these observations the standard approaches may be considered either too insensitive or unnecessarily complex as rock property indicators. The proposal here is to use modulii/density relationships to velocities V or impedances I, given as; + and These relationships enable extraction of the orthogonal Lame parameters and from logs with measured density or and from seismic without known density. The simple derivations are; and = 2 = Note, Poisson’s ratio analysis being related to (Vp/Vs) 2 , comes closes to measuring the most “rock property sensitive”