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 VP/VS 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.
ASJC Scopus subject areas
- Geochemistry and Petrology