On the depth, time and mechanism of petroleum generation

Abstract

The subsurface petroleum generation process has been studied. In the Los Angeles and Ventura basins, California, hydrocarbon content and hydrocarbon/noncarbonate carbon ratio increase slowly with depth and age in upper and lower Pliocene shales and more rapidly in the deeper and warmer upper Miocene shales. During the oil generation process, and simultaneous with the increase of the hydrocarbon/noncarbonate carbon ratio, the composition of the shale normal paraffins and naphthenes boiling above 325°C gradually changes and eventually becomes very similar to that of the waxy oils of the basin. In the Los Angeles basin as well as in the Ventura basin the bulk of petroleum is generated at depths where the subsurface temperature is above 115°C and where, therefore, the shales are sterile. The conclusion that petroleum is formed from sediment organic matter essentially by thermal, nonbiological processes is inescapable. Because petroleum generation is a chemical process, it is strongly temperature-dependent. This explains the strong increase of the hydrocarbon/noncarbonate carbon ratio at greater depths, where the temperature is higher. It also explains the greater depth of oil generation and of equality of shale and oil hydrocarbon composition in the Ventura basin compared with the Los Angeles basin. This is caused by the smaller temperature gradient in the Ventura basin. The importance of the temperature history of potential oil source rocks is discussed. Appreciable amounts of oil are formed only if the temperature history of the source sediments is sufficiently intense and if the nature and the amount of the organic source material are appropriate. In the Los Angeles and Ventura basins most producing horizons are much shallower than the oil source beds, and vertical migration of petroleum is very common. Evidence is presented that vertical migration of petroleum takes place through faults and fractures, not through the bulk of the shales.