Because of the great variety of lacustrine-source depositional characteristics and biological consortia, identification of a universally applicable indicator of freshwater or brackish-water organic input for crude oils and rock extracts has proven elusive. Tetracyclic polyprenoid compounds (TPP) are present in relatively high concentrations in oils and associated source rocks deposited under freshwater or brackish-water conditions, but typically are present in relatively low concentrations in samples from marine or coal-forming depositional environments. A TPP ratio, constructed from the gas chromatography–tandem mass spectrometry peak areas of a C30 tetracyclic polyprenoid divided by the sum of the peak areas of the C26 27-norcholestanes, revealed great specificity in identifying freshwater or brackish-water algal input, particularly in crude oils.

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