Abstract

P -wave velocities measured in Cretaceous skeletal grainstones from Limburg, southeastern Netherlands, are significantly higher in moderately cemented samples than in poorly cemented samples from similar depositional facies. Petrography suggests that this relationship is due to differences in the extent of development of intergranular cementation as influenced by original grain mineralogy, rather than a significant difference in primary porosity. The time-average velocity equation for a calcite matrix fails to predict the cementation effect on sonic velocity at high porosities. Linear and nonlinear time-average regressions show strong correlation between velocity and porosity. Whether the underlying trend is linear or nonlinear is a fundamental question that cannot be determined with this data set. Comparison between carbonate grainstones and siliciclastic sandstones reveals that: (1) the grainstones have gradients in velocity-porosity space similar to those in siliciclastic arenites but have velocities that are up to 0.3 km/s higher than those in clean arenites and up to 0.8 km/s higher than in argillaceous arenites; and (2) grainstones develop a rigid framework that transports elastic waves at significantly higher porosities than arenites, 40-50% compared to 30-40%. Higher sonic velocity presumably results from the higher matrix velocity of calcite versus quartz, and the higher elastic moduli created by the effective cement bridges connecting grains.

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