Quartz content and its impact on acoustic velocity of microbialites: examples from Hamelin Pool, Western Australia

Abstract Carbonates exhibit complex pore structures that cause significant velocity variation at a given porosity despite their typical monomineralogic composition. This study investigates how quartz admixture affects sonic velocity in microbialites -stromatolite heads and sheet-like, tabular microbial buildups (pavement)- in Hamelin Pool in Shark Bay, Western Australia to elucidate the relationship between pore structure, mineral composition and acoustic properties. Eighty core plugs were analyzed for porosity, pore structure, quartz content, and ultrasonic velocities. Compared to other carbonates, these microbialites have relatively simple pore strucutre, but display large pore size variations. Their velocities range from 3903 m/s to 5504 m/s in stromatolites and 3369 m/s to 5424 m/s in pavements. Although quartz (with lower intrinsic velocity than carbonate minerals) should theoretically reduce sonic velocity, samples with differing quartz content often exhibit similar velocities. For instance, samples with high quartz content but large pores can match velocities of samples with significantly lower quartz content but smaller, complex pores. A clear inverse correlation between quartz content and velocities emerges only when samples share comparable pore geometries and porosity. These findings demonstrate that pore geometry can override mineralogical effects of admixed quartz on carbonate acoustic properties. This underscores the critical control of microstructural characteristics and highlights the necessity of integrating detailed pore structure analyses alongside mineralogy for accurate petrophysical interpretation and seismic response prediction in mixed carbonate-siliciclastic systems.