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Abstract: High Molecular Weight Polycyclic Aromatic Hydrocarbons in Hydrothermal Petroleums-Indicators of High Temperatures and Water Washing

Bernd R. T. Simoneit, John C. Fetzer

Hydrothermal petroleums from active sedimented oceanic spreading axes contain polycyclic aromatic hydrocarbons (PAH) as characteristic components of their rapid high temperature genesis. The unsubstituted PAH dominate over their alkyl-substituted homologs and occur at high concentrations relative to the aliphatic components, which distinguishes hydrothermal petroleums from conventional reservoir petroleums. Some of the samples contain PAH with higher molecular weights than coronene (M.W. 300) and here we describe the structures and distributions of these PAH in typical examples of petroleums from Guaymas Basin, Escanaba Trough and Middle Valley.

Some of the aromatic fractions (F2) show PAH of up to nine rings [e.g., dibenzo-(e,ghi)perylene, benzo(a)coronene, benzo(pqr)naphtho(8,1,2-bcd)perylene, naphtho(8,1,2-abc)coronene]. Fraction 3 in some of these samples has ovalene, a condensed ten-ring PAH. These PAH are the most thermodynamically stable structures for each ring number, and are most likely indicative of a one-ring build-up mechanism. The samples also contain a few specific PAHs [e.g., the seven-ring PAH dibenzo(cd,lm)perylene] which are not members of this stable class. Its formation mechanism must be very specific through the condensation of a pair of the three-ring phenalene radicals or through the Scholl-condensation of pyrene to a nine-ring PAH with its subsequent hydrogenation and cracking down to dibenzo(cd,lm)-p rylene.

This is the first report of the occurrence of heavy PAH (M.W. >300) in geological samples. They are concentrated in the bitumens by removal of the lower molecular weight PAH due to mixing during hydrothermal fluid/oil transport and selective deposition/solidification of the heavy ends at the seabed with concurrent loss of the more water soluble and lower weight products to the ambient seawater. Additional post-depositional reworking by hot fluids and biodegradation further enhance the enrichment of the heavy PAH.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada