Publisher Summary The origin of oilfield waters is related to many natural processes. Initially meteoric water reacted with weathered rock, soil, and organic matter. The excess waters that did not penetrate the rock or soil caused the rock and soil to erode and channels formed through which the water could move more easily. Forces of gravity caused the water to move from areas of high potential to areas of low potential, and as the waters moved, the concentrations of dissolved solids in them increased. Some of these waters found their way to lakes and the sea. As they entered the lakes or seas, their movement slowed, causing some of the suspended particles in them to deposit. Mixing of the waters with the more saline waters in the sea caused dissolved carbonate and organic compounds to precipitate. Mechanisms that cause the oilfield waters to differ in composition from water originally deposited with the sediments include ion exchange, infiltrating waters, sediment leaching, mineral formation, sulfate reduction, and ultrafiltration through clay-shale membranes.