Hydrous Pyrolysis of Polycyclic Aromatic Hydrocarbons and Implications for the Origin of PAH in Hydrothermal Petroleum

Abstract

Polycyclic aromatic hydrocarbons (PAH) are found at high concentrations in thermally altered organic matter and hydrothermally generated petroleum from sediment-covered seafloor hydro-thermal systems. To better understand the factors controlling the occurrence of PAH in thermally altered environments, the reactivities of two PAH, phenanthrene and anthracene, were investigated in hydrothermal experiments. The compounds were heated with water at 330 degrees C in sealed reaction vessels for durations ranging from 1 to 17 days. Iron oxide and sulfide minerals, formic acid, or sodium for-mate were included in some experiments to vary conditions within the reaction vessel. Phenanthrene was unreactive both in water alone and in the presence of minerals for up to 17 days, while anthracene was partially hydrogenated (5-10%) to di- and tetrahydroanthracene. In the presence of 6-21 vol % formic acid, both phenanthrene and anthracene reacted extensively to form hydrogenated and minor methylated derivatives, with the degree of hydrogenation and methylation increasing with the amount of formic acid. Phenanthrene was slightly hydrogenated in sodium formate solutions. The hydrogenation reactions could be readily reversed; heating a mixture of polysaturated phenanthrenes resulted in extensive dehydrogenation (aromatization) after 3 days at 330 degrees C. While the experiments demonstrate that reaction pathways for the hydrogenation of PAH under hydrothermal conditions exist, the reactions apparently require higher concentrations of H2 than are typical of geologic settings. The experiments provide additional evidence that PAH may be generated in hydrothermal systems from progressive aromatization and dealkylation of biologically derived polycyclic precursors such as steroids and terpenoids. Furthermore, the results indicate that PAH initially present in sediments or formed within hydrothermal systems are resistant to further thermal degradation during hydrothermal alteration.

Estimation and application of the thermodynamic properties of aqueous phenanthrene and isomers of methylphenanthrene at high temperature

“…• C and McCollom et al (1999) found small amounts of MP when P was reacted with formic acid in hydrous pyrolysis experiments at 330 • C. Pyrolysis experiments on pure compounds reported by Wu et al (2002) and on kerogens and bitumens by Garrigues et al (1990) and Brocks et al (2003b) did not include catalysts and were unlikely to have approached metastable equilibrium, which is evidenced by the reported increases of experimental MPR with temperature, in contrast to the decreasing trend apparent in Fig. 4a.…”

“…Two other experimental studies of MP isomerization using mineral catalysts are those of Alexander et al (1995) and McCollom et al (1999) but neither of these studies reported the relative abundances of the isomers of MP. Alexander et al (1995) reacted P in the presence of Ca-montmorillonite at 100…”

“…Despite the condensed or surface-bound nature of much sedimentary organic matter, quantifying the behavior of aromatic compounds dissolved in high-temperature aqueous fluids is important for understanding the sources and reactions of organic species in hydrothermal fluids (Price, 1993;Konn et al, 2012), the formation of petroleum in some environments by hydrothermal alteration of organic matter (Kawka and Simoneit, 1990;Ventura et al, 2012), possible formation via abiotic processes on other planets (Zolotov and Shock, 1999), and reactions occurring in hydrous pyrolysis experiments (Kawka and Simoneit, 1994;McCollom et al, 2001McCollom et al, , 1999 and in industrial processes (Karásek et al, 2007). Therefore, the thermodynamic properties of aqueous solute species of P and MP are considered in this study.…”

Effects of sulfide minerals on aromatic maturity parameters: Laboratory investigation using micro-scale sealed vessel pyrolysis

“…3D). Phenanthrene was reported to be totally stable during hydrous pyrolysis at 330°C for up to 17 days (McCollom et al, 1999) and even persisted after pyrolysis at 400°C for 30 days (Price, 1993), therefore destruction of the phenanthrene ring system is not expected to occur at the conditions used in this study. The increased PAH/P ratio is attributed to the increased production of high MW PAHs at higher pyrolysis temperatures.…”

Low-temperature zircon growth related to hydrothermal alteration of siderite concretions in Mississippian shales, Scotland

“…9c). Such products are not present in typical sedimentary organic matter and may only be formed under high-temperature aqueous conditions (McCollom et al 1999). For example, large amounts of hydrogenated derivatives of phenanthrene and anthracene formed during a 3-day hydrous pyrolysis experiment at 330°C (McCollom et al 1999).…”