Using LMR for dual attribute lithology identification

Abstract

Summary Amplitude versus offset (AVO) analysis of pre-stack seismic data has made several advances since Shuey’s (1985) approximation to the Knott’s-Zoepprittz equation made it practical. By combining AVO attributes and geology, petrophysical properties of the rocks and fluids that make up the reservoirs that interpreters are interested in are better predicted than with AVO analysis alone. Lambda-Mu-Rho (LMR) analysis is one example of how interpreters are using advanced AVO analysis to identify hydrocarbons and reservoir rocks (Goodway et. al., 1997; Gray and Andersen, 2000). LMR analysis, even in the most classic of circumstances, requires cross plotting or the interpretation of multiple volumes of data to correctly interpret lithology and fluids. By using petrophysics to scale the Lambda-Rho (λρ) and Mu-Rho (µρ) volumes, lithology volumes can be based upon rock properties and AVO. Typically this is done through interactive cross-plot analysis of the LMR volumes which are often difficult to reproduce and save for future work. Theory Goodway demonstrated how LMR analysis can be used to identify gas sands. This comes from the separation in responses of both the λρ and µρ sections to gas sands versus shales. In some reservoirs, it is possible to separate lithologies at an even finer scale so as to identify wet sands from shales and carbonates. This can become particularly important in steam flood and injector planning in order to identify the optimum zones to inject fluids. By using petrophysical parameters to scale the results of LMR analysis, 3D seismic volumes can be converted into lithology cubes. Many different lithologies can be identified by cross-plots of λρ versus µρ. Each lithology has a different rock properties response subject to fluid content and mineral properties (including grain shape). The combination of the fluid compressibility along with the mineral properties and grain shapes yielding different LMR results. For example in a gas sand, the high compressibility (or low incompressibility) of gas combined with the high rigidity of the spherical sand grains, result in a low λρ value (~ 20 GPa.). With an understanding of these properties for the lithologies and fluids present, typically from petrophysics, a relatively high degree of precision in lithologies and fluids can be obtained.