Chlorophyll and its porphyrin derivatives have been shown to be sensitive markers of organic diagenesis and serve as indicators of maturation. It has previously been suggested that coals of the HVC to HVB bituminous rank are at a maturation stage equivalent to early catagenesis in petroleum source rocks. However, comparisons of the mass spectrometric data of coal porphyrins with that of petroporphyrins result in some striking differences. Coal porphyrins lack carbon numbers above C₃₂, do not contain the DPEP (308 + 14n monocyclano-alkano) porphyrin series, and have an irregular mass spectral envelope owing to the predominance of even numbers of carbons (C₃₂, C₃₀, C₂₈) in the 310 + 14n etioporphyrin series. Furthermore, the centr id of the envelope systematically shifts from C₃₂ to C₃₀ to C₂₈ with increasing coal rank. In contrast, petroporphyrins usually have carbon numbers ranging from C₃₆ to C₂₅ and a broad symmetrical envelope with a centroid at C₃₂ or C₃₁. Both the DPEP and the etio series are typically present. From the data it is speculated that early chlorophyll diagenesis in coals follows an oxidative rather than a reductive pathway. During early catagenesis, dealkylation of coal porphyrins causes a shift in the centroid to lower carbon numbers. "Transalkylation," the free-radical mechanism by which alkyl groups are randomly added and removed from the tetrapyrrole ring during petroleum genesis, must not be operative in coal porp yrin catagenesis.

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