Tetracyclic polyprenoids: Indicators of freshwater (lacustrine) algal input

Because of the great variety of lacustrine-source depositional characterist ics and biological consortia, identification of a universally applicable in dicator of freshwater or brackish-water organic input for crude oils and ro ck extracts has proven elusive. Tetracyclic polyprenoid compounds (TPP) are present in relatively high concentrations in oils and associated source ro cks 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 c hromatography-tandem mass spectrometry peak areas of a C-30 tetracyclic pol yprenoid divided by the sum of the peak areas of the C-26 27-norcholestanes , revealed great specificity in identifying freshwater or brackish-water al gal input, particularly in crude oils.