Distributions of ratios of odd- to even-carbon-numbered heavy n-paraffins (carbon preference indices) for shales and mudstones in oil provinces show that thirty percent of the samples contain petroleum-like mixtures of heavy n-paraffins; sixty percent contain n-paraffins intermediate between oil and recent sediment n-paraffins; and ten percent have n-paraffins similar to those in recent sediments, indicating that the freshly deposited sediments of the geologic past contained heavy n-paraffins with strong preferences for odd-carbon numbers. Carbon preference indices are inversely related to the proportion of hydrocarbon in the organic material of shales and mudstones and, consequently, may be indicative of conversion of organic material to hydrocarbons. They also provide a unique clue for recognizing petroleum-like mixtures of heavy n-paraffins in non-reservoir rocks.

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