Geochemists have made considerable progress in recent years in evaluating source beds for crude oil and natural gas. Petroleum is generated from disseminated sedimentary organic matter by thermochemical processes. A certain amount of time and temperature (thermal energy) is essential to produce the thermal cracking reactions causing the generation of petroleum. As the temperature is raised, the time for the reactions to occur is decreased. The same thermochemical processes that result in the generation of oil and hydrocarbon gases also contribute to the maturation and the ultimate destruction of oil and natural gas. Methane and graphite are the stable end products of these reactions. The exposure time-temperature relations necessary for the generation of petroleum and its expulsion from the source bed have been determined with sufficient accuracy from geochemical data to permit predictions of the approximate stage of generation or diagenesis in advance of sample analysis. Also, the conditions under which oil and condensate are thermally destroyed have been reasonably well established from empirical data. Geochemists are gradually improving their understanding of the factors that control source-bed performance. Factors such as the minimum amount and quality of organic matter necessary for effective oil source beds have been quantified rather accurately.

A balanced program includes evaluation of both the extractable organic matter and the residual organic matter in a source bed. The evaluation of extractable organic matter includes conventional analyses of organic carbon, total extractable organic matter, and extractable hydrocarbons. Evaluation of the extractable organic matter and extractable hydrocarbons include the study of heavy (C₁₅-C₃₀) hydrocarbon distributions and infrared spectra. Elemental analysis of carbon, hydrogen, nitrogen, and oxygen is a method used to investigate the diagenetic (carbonization) stage of residual organic matter. Pyrolysis techniques have been developed to investigate organic matter from the standpoint of its remaining generating capability.

Examples from the literature confirm that, in general, young source beds must be exposed to sustained higher temperatures than old source beds to attain peak generation. If 2 source beds of the same age and with identical burial histories were subjected to significantly and uniformly different temperatures, the hotter would be in a more advanced stage of organic matter diagenesis.

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