Abstract Free liquid and solid hydrocarbons comparable to those in petroleum are constituents of practically all sedimentary rocks. The quantities vary from a few parts per million in continental deposits to several thousand parts per million in bituminous marine shales. Associated with this oil are two other substances—a soluble asphalt, and an insoluble pyrobituminous material called kerogen. There is no systematic change in the hydrocarbon content of sedimentary rocks with depth or age. The biggest variations seem to be between different formations and different facies of the same formation. Metamorphisn appears to decrease the hydrocarbons and to alter the kerogen of a rock. Comparisons of reservoir hydrocarbons with the hydrocarbons from their presumed source rocks show that the latter tend to contain a greater percentage of aromatic hydrocarbons. Commercial accumulations of hydrocarbons in reservoir rock represent only a fraction of the total hydrocarbons estimated to exist in a finely disseminated form in practically all sedimentary rocks of marine origin.