A TWO-PARAMETER MODEL OF ELASTIC WAVE VELOCITIES IN ROCKS AND NUMERICAL AVO MODELING

Abstract

Acoustic wave propagation is sensitive to many rock properties including fluid content, porosity, and pore structure, among others. Pore structure is one of the important parameters in controlling both seismic wave velocity and permeability in sandstones and carbonate rocks. For a given porosity of two similar rocks with different pore structure, their acoustic wave speeds can differ 2 km/s and permeability can span nearly six orders of magnitude from 0.01 mD to 20 D in both sandstones and limestones. In this paper, we introduce a two-parameter velocity model defined by porosity and a new pore structure parameter called as frame flexibility factor. Using this model, we define three pore structure types (PST) to quantify the pore structure effects on elastic properties of sedimentary rocks. These three PSTs have their distinct characteristics on synthetic shot gathers and common midpoint gathers. This study indicates that it may be feasible to use this new concept and method to detect pore structure variations in reservoir rocks from field seismic data. This study also helps explain why analysis of amplitude variation with offset (AVO) in some cases fails for fluid detection: pore structure effect on seismic waves can mask all the fluid effects, especially in carbonate rocks.