The radionuclides may become mobile or be deposited by migration of water or oil. Some organic complexes, notable humic acids, create mobile complexes of uranium. Uranium and other minor and trace elements have an affinity for crude oil. They are probably residues of consolidated organic and marine deposits. Petroleum is often assumed to have migrated to a position of minimum hydraulic potential in a reservoir rock, which may or may not be derived from the same source deposits as the petroleum.
Shales can adsorb the series radionuclides.
The radionuclides may also be present bound to organic matter in minor minerals or as precipitates or coprecipitates in the cementing material that binds the rock. Sandstones Sandstones are usually made of grains that are primarily quartz but may contain some potassium-containing feldspar. On the whole, sandstones are low in both the series and non-series radionuclides. However, many deposits of uranium are found at the boundary of different layers of sandstones. Carbonate Rocks Carbonate rocks are limestones or dolomites derived by chemical precipitation from water or by the buildup of shells, bones, and teeth of organisms.