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Organic detritus deposited in sediments is composed principally of carbon, hydrogen, oxygen, and nitrogen. At the time of deposition, only small amounts of hydrocarbons are present. However, this organic matter has the potential to generate hydrocarbons in quantities that depend largely on its hydrogen content.
Studies show that organic matter disseminated in sediments, when heated, undergoes carbonization by mechanisms very similar to the thermochemical processes responsible for 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 organic matter as a function of depth, the principal volatile products of the carbonization reactions can be determined. Data from the Gulf Coast Tertiary indicate carbon dioxide is the principal volatile product of early carbonization a d that hydrocarbons are not significant products until the later stages. The amounts of hydrocarbons generated during carbonization are vast compared with amounts from any other naturally occurring source or process.
The data indicate that the rate of carbonization or, more specifically, hydrocarbon generation is a chemical process that follows the general rules of chemical kinetics. That is, as sediment age decreases, the temperature required to reach the level of carbonization associated with hydrocarbon generation increases. For example, significant hydrocarbon generation occurs in the Oligocene at a log temperature of 175°F and above; in lower Miocene at log temperature 186°F; and in upper Miocene at log temperature 205°F. Appreciably higher temperatures are required for significant hydrocarbon generation in post-Miocene sediment.
It is suggested that kerogen with relatively low hydrogen levels (similar to levels found in coals) would generate gas rather than oil. Thus, the relatively low hydrogen level in organic matter from the rocks discussed suggests that the sections penetrated would be better sources for gas than oil.
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