The generation of petroleum is a thermal process dependent on the maximum temperature and on the duration of the maximum heating phase attained by the source rock. The coalification process, which is governed by the same factors, offers a convenient means of determining the stage of thermal alteration of organic matter, because coals form a continuous metamorphic series ranging from peat to graphite.

The LOM scale (level of organic metamorphism) devised by Hood and Gutjhar is used to correlate a variety of measures of

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organic metamorphism; under ideal conditions, LOM is nearly linear with maximum burial depth. Comparison of reflectance data of vitrinite obtained from coal-bearing with other strata provides the most satisfactory method of describing the progression of organic metamorphism through a major segment of the coal-rank scale. Vitrinite reflectance studies in noncoal-bearing sequences are commonly complicated by the presence of higher reflectance vitrinite, which may result from the physical incorporation of vitrinite from older rocks or from oxidation caused by winnowing action or bioturbation. Interpretation of the reflectance data strongly hinges on a sequence of samples, a knowledge of the lithology, and knowledge of the depositional environment.

Geochemical maturity parameters, such as the n-paraffin ratio and the naphthene-ring index, indicate that the generation of petroleum is at a maximum for a source rock with a thermal history corresponding to a high-volatile B to medium-volatile bituminous coal rank (LOM 9 to 11.5), which falls within a vitrinite reflectance (Ro) of 0.72 to 1.20%. Studies in California and Alaska Tertiary basins show that the onset of geochemical maturity occurs at LOMs of 9-11. The depths to this zone range from 10,000 to 17,000 ft; the differences depend chiefly on variations in geothermal gradient.

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