Organic detritus deposited in sediments is composed principally of carbon, hydrogen, oxygen, and nitrogen, and has the potential for the generation of hydrocarbons. At the time of deposition, only small amounts of hydrocarbons are present. The type and quantity of hydrocarbons eventually generated depend largely on the hydrogen content of the kerogen.

With increased burial, disseminated sedimentary organic matter undergoes carbonization by processes very similar to the thermochemical reactions causing coalification. Carbonization is a thermal process marked by the generation of volatiles relatively rich in oxygen and hydrogen and by the formation of a kerogen residue increasingly enriched in carbon. The most significant oxygen-rich volatile is carbon dioxide, and the most significant hydrogen-rich volatiles are hydrocarbons.

By measuring changes in the elemental composition of the kerogen as a function of depth, the principal volatile products of the carbonization reactions can be determined. Data from Oligocene and younger Cenozoic samples from the Louisiana Gulf Coast indicate that carbon dioxide is the principal volatile product of the early stage of carbonization and that hydrocarbons are not significant products until the later stages. The amounts of hydrocarbons generated during kerogen carbonization are vast compared to those formed by any other natural source or process.

The data indicate that carbonization or, more specifically, hydrocarbon generation is a rate-controlled process which follows the principles of chemical kinetics. That is, the younger the sediment, the higher the temperature required to reach the level of carbonization associated with hydrocarbon generation. For example, significant hydrocarbon generation occurs in the Oligocene at a temperature of 170°F (77°C), in the lower Miocene at a temperature of 186°F (86°C), and in the upper Miocene at a temperature of 205°F (96°C). Higher temperatures are required for significant hydrocarbon generation in post-Miocene sediment.

These results support the conclusions of earlier research geochemists that kerogen with relatively low hydrogen contents, e.g., similar to percentages found in coals, is restricted principally to gas generation during carbonization. Thus, the relatively low hydrogen content in kerogen from selected Louisiana wells suggests that the sections penetrated are better sources for gas than for oil.