Mineral substitution: Separating the effects of fluids, minerals, and microstructure on P- and S-wave velocities

Abstract

We have developed a new technique of mineral substitution that accurately predicts the change in rock stiffness upon changes in the elastic properties of the rock mineral frame, assuming no changes in the microstructure or mineral volume fraction. The method is rigorous, the results are realizable, and the predictions are always within bounds. We have applied mineral substitution to separate the effects of composition (mineralogy and pore fill) on the rock stiffness from the effects of microstructure, i.e., porosity and pore shape. Application of mineral substitution on laboratory measured data sets of effective moduli (sandstones, limestones, and dolomites) revealed that if the original minerals making up the frame of sandstones (predominantly quartz) were replaced (or substituted) with calcite mineral properties, the newly predicted P- ([Formula: see text]) and S-wave velocity ([Formula: see text]) trends for the modified sandstones were very similar to the previously observed quadratic [Formula: see text]-[Formula: see text] empirical trends for limestones. Similarly, the [Formula: see text]-[Formula: see text] trends for modified limestones, with original minerals replaced by quartz mineral properties, were very similar to the observed linear [Formula: see text]-[Formula: see text] empirical trends for sandstones. The remaining minor differences between the modified and previously observed [Formula: see text]-[Formula: see text] trends might be attributed to the differences in rock microstructure. These findings are striking and suggest that [Formula: see text]-[Formula: see text] trends of natural rocks are dominated by mineralogy and the effects of microstructure are minor. We also found new [Formula: see text]-[Formula: see text] trends for rocks composed of various minerals, including zeolite, feldspar, pyrite, alpha-cristobalite, and halite.