Leakage of deep, reservoired petroleum to the near surface on the gulf of Mexico Continental slope

Abstract

Reservoired oils, shallow sediment cores (2m), sea slicks and tar balls were collected in the Green Canyon Lease area of the northern Gulf of Mexico continental slope. The gaseous and liquid hydrocarbons associated with near surface sediments and water have migrated from deep (2000–3000 m) subsurface reservoirs and/or source rocks. This conclusion is based on molecular (GC/FID, GC/FPD, GC/MS) and carbon isotopic evidence. Visual observations at two locations on the continental slope confirm the presence of massive amounts of active liquid as well as gas seepage. Hydrate gas recovered in sediment cores originates from deep, oil-associated gas. This gas has migrated to shallow sediments with little or no isotopic fractionation. In contrast, near surface hydrocarbon liquids (shallow bitumens and sea slicks) are depleted in aliphatics, 4-ring or larger aromatics, naphthalene, C1-naphthalenes and C2-naphthalenes as compared to the reservoired fluids.These near-surface fluids are extensively altered by the concurrent processes of migration, dissolution and microbial degradation. However, the distributions of highly alkylated (> C2) naphthalenes, phenanthrenes and dibenzothiophenes, triterpanes, steranes and triaromatized steranes are similar to the precursor reservoired oil. This study documents, for the first time, a direct link between natural seepage in a deep water marine setting and sea slick and tar ball formation. This and other studies suggest that the natural seepage of oil and gas can be a significant process in the deep ocean.