These escape to nearby sands, but often not to the earth's surface, because of covering impermeable shales. The acoustic properties of sands and shales often overlap, though there is a slight tendency for shales to have a lower velocity than sands. Mixtures of sands and shales are called clastic rocks. The word clast means break. Clastic rocks are made from broken bits of crystalline rock.
Most sedimentary rocks are clastic rocks.
Most oil is found in clastic rocks. But much oil is also found in association with carbonates such as limestone. Carbonates are formed in shallow marine environments by marine organisms. Permeability in carbonate rocks arises through several obscure processes. The seismologist knows carbonates as rocks with greater velocity than clastic rocks.