Generally speaking, most petroleum reservoir rocks are sandstones. Sandstones are most often made by the sands that are deposited near the mouths of rivers where the water velocity is no longer sufficient to move them. The sands deposit along the terminus of the sand bars found at the river mouth, often along a slope of 25 or so, as depicted in Figure 14.
Although the sands are not laid down in flat layers, the process may build a horizontal layer.
Clays are more fine-grained materials (dirt) that are carried out to deeper water before they settle to make shale. Shale deposits tend to be layered somewhat more horizontally than sandstones. Specific locations of sand deposition change with the passing of storms and seasons, leaving a wood-grain-like appearance in the rock.
The river delta itself is a complicated, ever-changing arrangement of channels and bars, constantly moving along the coastline. At any one time the delta seems to be moving seaward as deposits are left, but later settling, compression, or elevation of sea level can cause it to move landward.
Sand is important because its porosity enables oil to accumulate and its permeability enables the oil to move to a well. Shale is important because it contains the products of former life on earth, and their hydrocarbons. 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. Geophysicists on the surface see with seismic wavelengths ( 30 meters) the final interbedded three-dimensional mixture of sands and shales.
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. Many carbonates (and clastics) contain oil that cannot be extracted because of lack of permeability. Permeability in carbonate rocks arises through several obscure processes. The seismologist knows carbonates as rocks with greater velocity than clastic rocks. Typically, a carbonate has a 20-50% greater velocity than a nearby clastic rock. Clastic rock sometimes contains limestone, in which case it is called marl.