Comparison of organic matter in unmetamorphosed rocks with organic matter in metamorphosed rocks shows three consistent differences: (1) the absolute amounts of hydrocarbon material present are less in metamorphosed rocks than in unmetamorphosed rocks, (2) the relative amount of the total organic matter present as hydrocarbon material is less for metamorphosed than unmetamorphosed rocks, and (3) the extractable material and the isolated hydrocarbons from metamorphosed rocks have a higher proportion of saturated (compared to aromatic) species, and have lighter colors, lower melting points, and lower average molecular weights than analogous material from unmetamorphosed rocks. In addition, anomalously high carbon isotope ratios (^dgrC¹³) were observed in two samp es believed to have undergone higher degrees of incipient metamorphism.

These results suggest that the nature and characteristics of organic matter in sedimentary rocks may provide a useful new basis for recognizing and classifying degrees of incipient metamorphism.