CARBON ISOTOPE COMPOSITION OF HYDROTHERMAL PETROLEUMS FROM GUAYMAS BASIN, GULF OF CALIFORNIA

Petroleum hydrocarbons are a major emanation from the active hydrothermal vent fields of Guaymas Basin. The samples we describe here occur in hydrothermal mound crusts which have been collected in situ with a submersible. The relative concentrations of the compound classes in the hydrothermal petroleum are highly variable with the following ranges: saturate hydrocarbons 1-60%; aromatic hydrocarbons 18-38%; NSO-compounds 10-27%; and asphaltenes 0.2-25%. The sulfur concentrations range between 1.2 and 3.4%. The S-rich hydrothermal petroleums tend to be low in saturated hydrocarbons and high in NSO and asphaltenes components and vice versa. Stable C isotope concentrations in whole hydrothermal petroleum vary little δ13C = -21.2 to -22.2%0 vs. PDB. carbon isotope patterns for the compound classes are similar to typical crude oils with the saturated hydrocarbons being depleted in 13C (-22.9 to -21.7%0) and asphaltenes being relatively enriched (-20.5 to -20.8%0). The variability decreases from saturates to aromatics, NSO and asphaltenes. This indicates that the hydrothermal petroleums oirginate from one source. The higher variability in the non-asphaltic compound classes is attributed to distillation and/or biodegradation effects. Organic matter in recent unaltered sediments in the immediate vicinity of the hydrothermal vents has an average δ13C of approximately -20.5%0, in a range consistent with their being the source for the oils. This simlarity supports the contention that the hydrothermal petroleums are pyrolysis products of the immature sediments which are permeated by the hydrothermal fluids. © 1990.