We have analyzed two laboratory data sets obtained on high-porosity ro
ck samples from the North Sea. The velocities observed are unusual in
that they seem to disagree with some simple models based on porosity.
On the other hand, the rocks are unusually poorly-cemented (for labora
tory studies, at least), and we investigate the likelihood that this i
s the cause of the disagreement. One set of rocks, from the Oseberg Fi
eld, is made of slightly cemented quartz sands. We find that we can mo
del their dry-rock velocities using a cementation theory where the gra
ins mechanically interact through cement at the grain boundaries. This
model does not allow for pressure dependence. The other set of rocks,
from the Troll Field, is almost completely uncemented. The grains are
held together by the applied confining pressure. In this case, a lowe
r bound for the velocities can be found by using the Hertz-Mindlin con
tact theory (interaction of uncemented spheres) to predict velocities
at a critical porosity, combined with the modified Hashin-Strikman low
er bound for other porosities. This model, which allows for pressure-d
ependence, also predicts fairly large Poisson's ratios for saturated r
ocks, such as those observed in the measurements. The usefulness of th
ese theories may be in estimating the nature of cement in rocks from m
easurements such as sonic logs. The theories could help indicate sand
strength in poorly consolidated formations and predict the likelihood
of sand production. Both theoretical methods have analytical expressio
ns and are ready for practical use.