We have measured the acoustic properties and mineralogic composition of
48 rock specimens from mixed carbonate-siliciclastic outcrops of the
Permian upper San Andres formation in Last Chance Canyon, New Mexico.
The goals were: (1) identify and model the parameters controlling the
sonic velocities; (2) assess the influence of postburial diagenesis on
the acoustic velocities.
The variation in sonic velocity in the 0 to 25% porosity range is
primarily controlled by porosity, and secondly by the ratio of
carbonate-siliciclastic material. Linear multivariate fitting resulted
in a velocity-porosity-carbonate content transform that accurately
predicts sonic velocity at different effective stresses. The slope of
the velocity-porosity transform steepens with increasing carbonate
content, which may be explained by the higher velocity of carbonate
minerals. Another reason may be the property of carbonate minerals to
form more perfect intercrystalline boundaries that improve the
transmission properties of acoustic waves and are less sensitive to
changes in effective stress.
The velocity ratio V-p/V-s is an excellent tool to discriminate between
predominantly calcitic lithologies (ratio between 1.8 and 1.95) and
predominantly dolomitic and quartz-rich lithologies (ratio between 1.65
and 1.8). Gardner's experimental curve overestimates, and the
velocity-porosity transforms by Wyllie and Raymer underestimate, the
observed sonic velocities, probably because they do not account for
variations in texture, carbonate mineralogy, and pore geometry.
Petrographic observations show that postburial diagenesis is minor and
does not seem to significantly affect porosity. Therefore, the outcrop
data set can be regarded as a proxy for the subsurface analog.
These findings underline the significantly more complex acoustic
behavior in mixed carbonate-siliciclastic sedimentary rocks than in pure
siliciclastics where mineralogic composition explains most of the
observed relationships between porosity and sonic velocity.