Velocity in high porosity sands strongly depends on the position and amount of intergranular material. Velocity is high if the original grains are cemented at their contacts. It is low if the pore-filling material is placed away from the contacts. The amount and type of intergranular material can be theoretically determined from velocity and porosity data by adjusting an effective-medium theoretical model curve to a trend in the data and then assuming that the microstructure of the sediment is such as used in the model. We apply this diagnostic method to clean sands in the Heimdal formation (the North Sea). The results show that the reservoir-zone sand in one well under examination has slight contact cementation, while it is completely uncemented and friable in another well. This conclusion is supported by microscopic grain images. The rock physics diagnostic is important for establishing a velocity-porosity trend consistent with local geology. Such trends can improve the accuracy of porosity prediction from seismic data. ¿ 2000 American Geophysical Union