It is now firmly established that crude oil and most natural gas, collectively termed petroleum, are generated from kerogen in sedimentary source rocks. The organic origin of crude oil is beyond doubt based on optical activity (Oakwood et al. 1952; Hills and Whitehead 1966) and isotopie composition (Silverman 1964). The chemical structure of biological markers in ancient sediments and crude oils compared to that of living cell constituents (Calvin 1969; Albrecht and Ourisson 1971; Tegelaar et al. 1989a), and regularities in crude oil composition according to sedimentary environments (Tissot and Welte 1984) further confirm an organic origin. However, it was uncertain for a long time at what depth petroleum forms in the earth. The discovery of hydrocarbons in Recent sediments by Smith (1952) gave support to a shallow origin for oil. Baker (1960) and Meinschein (1961) noted that the amount of hydrocarbons in Recent sediments could account for known oil reserves. However, Stevens (1956) found only a few simple aromatic hydrocarbons in Recent sediments as compared to the numerous complex aromatic hydrocarbons in ancient sediments and crude oils. Other authors (Emery and Hoggan 1958; Dunton and Hunt 1962; Hunt 1975) noted the abundance of light hydrocarbons (C4–C13) in petroleums and their absence in young sediments. It was thus argued that petroleum must form at greater burial depths.