Hydrothermal alteration of organic matter in sediments of the Northeastern Pacific Ocean: Part 1. Middle Valley, Juan de Fuca Ridge

Samples from the Ocean Drilling Program Leg 169 in Middle Valley, NE Pacific Ocean, were analyzed to study maturation by accelerated diagenesis and catagenesis of the sedimentary organic matter to hydrothermal petroleum. Some data from Site 858 in Middle Valley (Leg 139) are also discussed for completeness. Sediments at Sites 858 and 1036 in Middle Valley were altered either by the full variety of high temperature fluids, which generated hydrothermal petroleum, or by accelerated thermal diagenesis of the immature organic matter in situ without specific migration of those petroleum (bitumen) products. The hydrothermal petroleums from the deeper sediments have migrated and accumulated in discrete intervals at shallower depths within a contemporary temperature window of 60–135 °C at both of these sites. Maturation for the n-alkanes proceeded from high CPI values (immature) to <1.0 (mature), i.e., strong even C number preference. Due to their low TOC contents, these sediments did not generate high amounts of hydrocarbons from the kerogen and thus revealed this unique even C number preference. Furthermore, the significant content of terrestrial organic matter yielded petroleums low in gas and volatiles. Biomarkers were converted from the biological natural product precursors to the geological derivatives with maturities reflecting the thermal stress. High-temperature alteration also yielded polynuclear aromatic hydrocarbons.