Sea floor vents, seeps, and gas hydrate: relation to flux rate from the deep Gulf of Mexico petroleum system

Sassen, Roger, Geochemical and Environmental Research Group, Texas A&M University, College Station, TX; Harry Roberts, Coastal Studies Institute, LA State University, Baton Rouge, LA; A. V. Milkov, Geochemical and Environmental Research Group, Texas A&M University, College Station, TX; and Debra A. DeFreitas

Abstract

To study the Gulf of Mexico slope is to study the dynamic and ongoing birth of a leaky oil and gas province. There is a strong association between subsurface oil and gas fields, sea floor vents, seeps, and gas hydrate because all are products of the prolific subsurface petroleum system. The bulk of oil and gas in reservoirs beneath the Gulf slope originate from prolific Mesozoic source rocks that became generative in the recent geologic past. Driven by rapid sediment deposition, active salt deformation and faults provide efficient conduits for dominantly vertical migration from great depth. Many oil and gas fields are geologically transient. Episodic salt deformation from sediment loading causes multiple trap failures over time and multiple remigration towards more competent hydrocarbon traps updip. Trapping efficiency is poor. Much oil and gas are dispersed within the sediment section, or are lost at the sea floor.

Vents are point orifices, such as at mud volcanoes, from which subsurface fluids enter the water column either episodically or at a constant rate. Residence time at vent sites is so short that gas and oil may be relatively unaltered by bacterial oxidation. Vent clusters on the sea floor are associated with large gas plumes in the water column. Gas bubbles lined with crude oil reach the sea surface and maintain relatively persistent natural oil slicks visible from space. Geochemical analysis of thermogenic vent gas samples from the Gulf slope indicates that the molecular and isotopic properties of the gas are essentially unaltered during migration, preserving important geochemical information on source and maturity. Oil also may be characterized using biomarkers and isotopes, allowing inferences as to oil quality issues (i.e., high sulfur, low API gravity) prior to drilling. Vent fluids are sometimes associated with enhanced temperatures from fluid transport or heat flow above shallow salt. Brines from salt dissolution are common. The net effect of venting is rapid loss of hydrocarbons from the petroleum system to the water column and to the atmosphere.

Sea floor seep features include gassy and oil-stained sediments, unique chemosynthetic communities, and authigenic carbonate rock from hydrocarbon oxidation. Residence time of gas and oil is generally long enough in sediment to allow bacterial oxidation of hydrocarbons. Molecular and isotopic properties of thermogenic gas and oil are rapidly affected by bacterial oxidation. Bacterial hydrocarbon oxidation produces carbon dioxide, much of which precipitates as authigenic carbonate rock, effectively perturbing the carbon cycle. Some carbon dioxide at thermogenic seeps is recycled via methanogenesis to generate bacterial methane, explaining the observed association between bacterially oxidized oil and bacterial methane. The net effect of introducing free gas and oil to the sediment is rapid destruction by bacterial oxidation and sequestration of carbon as carbonate rock.

Gas hydrate is an ice-like crystalline mineral in which hydrocarbon and non-hydrocarbon gases are held within rigid cages of water molecules at the high pressures and low temperatures that prevail across much of the Gulf slope. Both thermogenic and bacterial gas hydrate are present in the Gulf slope. Gas hydrate occurs at vent sites as outcropping mounds and as vein-fillings and as nodules in sediment. A thin layer of gas hydrate is marginally unstable where it crops out on the sea floor because of variable seawater temperatures, but gas hydrate buried at a few meters depth is insulated from such effects. Stability of gas hydrate increases with thickness of the water column, perhaps to depths in sediment greater than a kilometer at 2 kilometers water depth. Hydrocarbon molecules held within the gas hydrate crystal lattice are relatively unaffected by bacterial oxidation, contributing to stability. The net effect of gas hydrate crystallization is to trap enormous volumes of thermogenic gas in sediment. It should be emphasized that the estimated volume of thermogenic and bacterial gas hydrate in the Gulf slope is significantly greater than conventional reserves of subsurface oil and gas. The Gulf slope is now an increasingly important oil and gas province but may attain second-stage economic viability later in this century as a gas hydrate province.