One of the unsolved problems in the origin of petroleum is where do gases and gasoline-range hydrocarbons come from. A number of investigations have shown the heavier hydrocarbons are constituents of living things and are present in Recent as well as ancient sediments. No significant concentrations of low molecular-weight hydrocarbons have been reported in Recent sediments and there are few reports of their occurrence in ancient sediments (excluding petroleum accumulations). This paper reports finding no C 4 -C 8 hydrocarbons in Recent sediments whereas a whole suite of C 4 -C 8 hydrocarbons typical of crude oil were found in ancient sediments from the Precambrian through the Tertiary. This strongly suggests these hydrocarbons are formed with time after deposition of the sediments.