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This study investigates two cases of velocity predictions for North Sea chalk: fluid substitution of compressional velocity (vP) and estimation of shear velocity (vS). Both high frequency laboratory and intermediate frequency logging data are included. We test five rock physics methods and two empirical relations that have been established or tested for other lithologies. The predicted velocities are tested against measured velocities or velocities estimated from Gassmann''s relations, which are confirmed to give good velocity estimates for water-saturated chalk from data on dry samples. In one case, we test predicted fluid effects on compressional velocity without knowing shear velocity. The fluid substitution effect on the compressional velocity is well estimated from all the four methods: the bound averaging method (BAM), the isoframe method, the method of replacing bulk modulus in the Gassmann''s relations with P-wave modulus and the method of decomposition of measured P-wave modulus into shear and bulk modulus. In the other case, we test shear velocity estimated from compressional velocity. We recommend the isoframe method for shear wave estimation for saturated chalk. All methods tend to underestimate shear velocity in dry chalk. For water-saturated chalk, also the empirical second-order polynomial relation between compressional and shear velocity of water-saturated limestone of Castagna et al. [Castagna, J.P., Batzle, M.L., Kan, T.K., 1993. Rock physics—the link between rock properties and AVO response. In: Castagna, J.P., Backus, M.M. (eds.), Offset-Dependent Reflectivity-Theory and Practice of AVO Analysis, Investigations in Geophysics, no. 8. Society of Exploration Geophysicists, 135–171.] gives good shear velocity estimates. The empirical constant ratio between compressional and shear velocity of water-saturated limestone of Pickett [J. Pet. Technol. 15 (1963) 650.] overestimate low shear velocities for high porosity chalk and the shear velocity is systematically underestimated by the method of decomposing P-wave modulus. [Copyright &y& Elsevier]

Abstract: Chalk develops as a result of diagenesis of pelagic calcareous ooze. In a newly deposited ooze sediment, porosity ranges from 60% to 80% but porosity reduces with burial. We studied how different porosity reduction mechanisms change the strength of these deep sea carbonate-rich sediments and effect Biot''s coefficient, β. In calcareous ooze, β is one. Mechanical compaction reduces porosity, but only leads to a minor decrease in β. Recrystallization renders particles smoother, but does not lead to reduction in β unless it gives rise to pore stiffening cementation. Pore stiffening cementation causes β to fall, even when porosity remains constant. Biot''s coefficient correlates with strength-indicating properties: compressional and shear modulus, oedometer modulus, yield strength, strain from direct loading and creep strain. Our data indicate that β may be used for predicting the diagenetic process involved in porosity reduction and strengthening of chalk during burial diagenesis. [Copyright &y& Elsevier]