Rock-physics Foundation for AVO Analysis
A direct correlation of lithology to stacked and migrated seismic data, although an attractive goal, is usually an elusive one. In extreme cases, such as hard limestone formations encased in clastics, lithologic information may be obvious in the seismic amplitudes. For subsurface formations characterized by small velocity changes between different lithologies, however, such a correlation may not be possible. Similarly, acoustic logs themselves are poor indicators or differentiators of lithology, unless they are combined with other logs such as density or porosity logs. One main reason for this is that reservoir rocks such as sandstone, limestone, and shale each exhibit large acoustic-velocity ranges that may overlap significantly. In addition, this limited information is available only at the location of a well, and seismic data are looked upon to provide it elsewhere. Although the goal of direct correlation from seismic to lithology seems simple, serious thought suggests that it could be a complicated exercise. The seismic response of subsurface rocks depends on the contrasts in compressional- and shear-wave velocities and densities. Those contrasts in turn depend on the rock’s lithology, porosity, pore-fluid content, and pressure, all of which affect seismic-wave propagation (e.g., Gregory, 1977; Castagna et al., 1993). That dependence requires knowledge about variations in the elastic properties of rock frames, their mineral constituents, and pore fluids, as well as a model for the interactions among them. Rock physics provides the link between the physical properties of rocks and their seismic response, and that link establishes the P-wave velocity (VP), S-wave velocity (VS), and density (ρ) of the subsurface rocks, along with their relationships to the rocks’ elastic moduli (bulk modulus κ and shear modulus μ), porosity, pore fluid, temperature, pressure, and the like.