ABSTRACT: Petroporphyrin Distributions as Indicators of Thermal Maturity
Janina K. Rafalska, Geoff Eglinton, Nils Telnaes
Porphyrin distributions, their classes, pseudohomologous series, and carbon ranges were determined from computerized gas chromatography-mass spectrometry of porphyrin derivatives. The materials studied were extracts of hydrous pyrolysates generated by heating marl slate samples at 280, 300, and 320°C.
Upon heating, all classes of porphyrins except the benzo-aetioporphyrins diminished in abundance relative to aetioporphyrins. The carbon number distribution of porphyrins shifted toward the lighter end. The release of porphyrins from kerogen, their thermal destruction, and dealkylation are suggested as the major cause of these changes.
The most striking effect of artificial catagenesis is the formation of a large number of the isomers whose porphyrin indices increase with increasing pyrolysis temperature. The aetioporphyrin isomers possessing high poyphyrin indices and found in both artificially and naturally matured materials, but not in immature oil shales, are considered to be genuine and not artifacts of pyrolysis. Therefore, these indices could be used as indicators of thermal stress.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990