Hydrothermal pyrolysis of organic matter in Guaymas Basin: I. Comparison of hydrocarbon distributions in subsurface sediments and seabed petroleums

Abstract

The bitumens of a selection of subsurface sediments from Guaymas Basin, Gulf of California, have been analyzed in order to assess the hydrothermal effects on the organic matter and to better delineate the processes influencing the generation, deposition, and alteration of the hydrothermal petroleums at the seabed. Although the thermally generated subsurface bitumens are generally rich in polar/asphaltic materials, as are the seabed oils, the hydrocarbon transformations parallel the extent of hydrothermal alteration as reflected in the downcore mineralogy at DSDP Sites 477 and 478. At Site 477, a progressive change from n-alkane- to aromatic hydrocarbon-dominated patterns in the high resolution gas chromatography-resolvable fractions of the bitumens accompanies decreases in organic carbon and bitumen yields with depth. These transformations are indicative of sequential hydrothermal alteration of the sediments with depth, due to increasing temperature and/or progressive passage of hydrothermal fluid through the sedimentary column. The polycyclic aromatic hydrocarbon (PAH) distributions undergo extensive transformations (e.g. dealkylation) consistent with hydrothermal alterations, are dominant at depth, and have the broadest range as thermal maturation indices. The aliphatic and aromatic components of the seabed oils can be transported to the seabed independently as a result of their generation under different temperature or depth regimes and sequentially over time during pyrolysis. The concentrations of the parent PAH in the thermally altered subsurface bitumens are comparable to those in the seabed oils. The differential dissolution of the more soluble aromatics and selective solidification/condensation of the higher molecular weight PAH out of the high-temperature fluids exiting at the seabed (chimneys) result in aromatic distributions of the oils significantly skewed towards the larger components [e.g. benzo(ghi)perylene] relative to the subsurface bitumens. Comparison of the downcore hydrocarbon distributions with those in the seabed petroleums support the contention that the hydrothermal oils are mixtures of pyrolysates from different depths or thermal regimes and are significantly altered by differential solubilization during transport and at the seabed.