Geophysical Prospecting, 2005, 53, 481–496 Ultrasonic velocities of North Sea chalk samples: influence of porosity, fluid content and texture Birte Røgen, 1 Ida L. Fabricius, 1∗ Peter Japsen, 2 Christian Høier, 2 † Gary Mavko 3 and Jacob M. Pedersen 4 1 Environment & Resources DTU, Bygningstorvet Building 115, Technical University of Denmark, 2800 Kongens Lyngby, Denmark, 2 Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350 Copenhagen K, Denmark, 3 Department of Geophysics, School of Earth Sciences, Stanford University, Stanford, CA 94305-2215, USA, and 4 Ødegaard A/S, Titangade 15, 2200 Copenhagen N, Denmark Received November 2002, revision accepted August 2004 ABSTRACT We have studied 56 unfractured chalk samples of the Upper Cretaceous Tor Forma- tion of the Dan, South Arne and Gorm Fields, Danish North Sea. The samples have porosities of between 14% and 45% and calcite content of over 95%. The ultrasonic compressional- and shear-wave velocities (V P and V S ) for dry and water-saturated samples were measured at up to 75 bar confining hydrostatic pressure corresponding to effective stress in the reservoir. The porosity is the main control of the ultrasonic velocities and therefore of the elastic moduli. The elastic moduli are slightly higher for samples from the South Arne Field than from the Dan Field for identical porosities. This difference may be due to textural differences between the chalk at the two loca- tions because we observe that large grains (i.e. filled microfossils and fossil fragments) that occur more frequently in samples from the Dan Field have a porosity-reducing effect and that samples rich in large grains have a relatively low porosity for a given P-wave modulus. The clay content in the samples is low and is mainly represented by either kaolinite or smectite; samples with smectite have a lower P-wave modulus than samples with kaolinite at equal porosity. We find that ultrasonic V P and V S of dry chalk samples can be satisfactorily estimated with Gassmann’s relationships from data for water-saturated samples. A pronounced difference between the V P /V S ra- tios for dry and water-saturated chalk samples indicates promising results for seismic amplitude-versus-offset analyses. INTRODUCTION Knowledge of the relationships between the petrophysical properties of reservoir rocks and the parameters that can be observed with geophysical methods has increased during the last decades, but knowledge of the acoustic properties of chalk is still limited, even though several studies have been pub- lished. Some of the earlier studies are as follows. For unspec- ∗ E-mail: il@er.dtu.dk †Present address: DONG/A/S, Agern All´ e 24–26, 2970 Hørsholm, Denmark. ified limestone data, Castagna, Batzle and Kan (1993) pro- vided a second-order polynomial fit to the V P /V S relationship. Lind (1997) found that Wyllie’s time-average model (Wyllie, Gregory and Gardner 1958) can be modified to give good estimates of the porosity of chalk from velocity data by appli- cation of the concept of critical porosity. Raiga-Clemenceau, Martin and Nicoletis (1988) suggested an acoustic formation factor as an alternative to Wyllie’s time-average equation and provided empirical constants for calcite, among other miner- als. Walls, Dvorkin and Smith (1998) fitted a modified up- per Hashin–Shtrikman (MUHS, Nur et al. 1998) bound to log data on chalk from the Ekofisk Field, resulting in an C  2005 European Association of Geoscientists & Engineers 481