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doi:10.2204/iodp.proc.310.203.2009 Data report: petrophysical properties of “young” carbonate rocks (Tahiti Reef Tract, French Polynesia)1Klaas Verwer2, 3 and Hendrik Braaksma4, 5AbstractWe measured the acoustic properties and mineralogic compositions of 79 rock specimens from mixed carbonate-volcaniclastic minicores drilled from cores of the Holocene and Pleistocene reef tract in Tahiti, French Polynesia (Integrated Ocean Drilling Program Expedition 310). Three significant sediment groups with distinct physical properties were distinguished: coralgal-microbialite framework, coralgal-microbialite framework with dispersed volcaniclastic grains throughout the micrite matrix, and cemented volcaniclastic-skeletal sand and coral framework. Variations in acoustic velocity (porosity range = 1%–35%) are primarily controlled by porosity, secondly by the ratio of carbonate-volcaniclastic material, and thirdly by the introduction of fringing cement. Linear regression fitting resulted in a velocity-porosity-carbonate content transform that predicts acoustic velocity at different levels of volcaniclastic contamination. The intersection point of the velocity-porosity transform with the zero-porosity axis decreases with decreasing carbonate content, which may be explained by the presence of clay minerals and organic content. Another observation is the property of magmatic minerals with high matrix velocity to positively affect the transmission properties of acoustic waves at high porosity. The Tahiti reefs are significant in the fact that the framework is formed by microbialite in association with coralgal reef systems. Microbialite is often volumetrically the most important component of the reef rock and forms a dense mass that is capable of withstanding extraneous particles being dispersed within the matrix. Carbonate-lithoclastic sand from an older underlying reef sequence displays different petrophysical behavior. The slope of the velocity-porosity transform is steep, with large data scatter that is controlled by the presence of fringing cement lining the grains. Cement-rich samples show positive deviations from the velocity-porosity transform, whereas cement-poor samples have relatively low velocity for a given porosity. For all sediment groups, Gardner’s experimental curve underestimates the observed acoustic velocities, probably because it does not account for variations in texture, mineralogy, and pore geometry. The velocity-porosity transform by Wyllie describes observed acoustic velocity trends for the clean carbonate samples but significantly underestimates velocity when magmatic minerals are introduced into the matrix. These findings underline the significantly more complex acoustic behavior in mixed carbonate-volcaniclastic sedimentary rocks than in pure siliciclastics and pure carbonates where mineralogic composition and rock texture, respectively, explain most of the observed relationships between porosity and acoustic velocity. |
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