Hydrothermal effects on organic matter—high vs low temperature components

Abstract

Hydrothermal activity and sea-bed mounds have been explored in Guaymas Basin, Central Gulf of California by the Deep Sea Drilling Project, piston coring, dredging and diving with the D.S.R.V. Alvin. The deep-seated magmatic heat flow causes extensive thermal alteration of both organic and mineral matter, especially in the case of the south rift of Guaymas Basin. This results in hydrothermal circulation with concomitant migration of organic pyrolysate to the seabed. The bitumen at depth of DSDP Sites 477 and 481 contained thermogenic components such as primary olefins, thermodynamically stabilized molecular markers [17α(H) hopanes], and elemental sulfur, with traces of polynuclear aromatic hydrocarbons (PAH). The major components were n-alkanes with no carbon number predominance and unresolvable complex branched and cyclic hydrocarbons (hump), which are typical of mature petroleum and were generated by the heat from the intrusives and magma. Dredge samples recovered from a mound in Guaymas Basin contained petroleum of a thermogenic origin as confirmed by the presence of: gasoline range hydrocarbons; a broad distribution of n-alkanes (C 13-C 33, no carbon number predominance), hump, pristane and phytane; olefins and PAH; and large amounts of aromatic/naphthenic and asphaltic material. Similar petroleums, subsequently recovered from other active mounds in the southern rift by the D.S.R.V. Alvin, had analogous compositions, ranging from volatile to heavy oils. In pyrrhotite chimneys the extractable organic matter consisted of mainly higher molecular weight PAH. These petroleums are derived from the immature, primarily marine organic matter at depth by thermal alteration. Rapid quenching by hydrothermal removal and migration, followed by condensation at the sea-bed, results in the manifestations of oil as part of the sulfide formations. Exterior, exposed oil samples are degraded and leached, whereas interior samples are essentially unaltered. The PAH are high temperature resynthesis-aromatization products that condense in the hottest parts of the vent systems.