SURVEY OF HYDROTHERMALLY-GENERATED PETROLEUMS FROM THE GUAYMAS BASIN SPREADING CENTER

The Guaymas Basin spreading center and hydrothermal system are generating an assortment of petroleum-like bitumens by pyrolysis of organic matter in the overlying unconsolidated sediments. Samples collected by D. S. V. Alvin from hydrothermal fields in the Southern Trough exhibit large variations in the quantities and character of the solvent-soluble organic materials. Various pyrolytic regimes combined with fluctuating thermal gradients and migration velocities may impart a compositional fractionation, possibly by differential gaseous solubilization and hydrodynamic alteration of the multicomponent fluids. The data suggest that differential condensation/solidification, biodegradation, and water-washing cause selective removal of components at the seabed. Many hydrothermal oils are unlike normal reservoir petroleums, because they contain significant concentrations of polar material, due to their rapid genesis and transport and probable solubilization or advection of polar, immature components. The biomarker distributions confirm high-temperature pyrolysis at depth with variable entrainment of less thermally-mature bitumens during transport.