Seeps Give a Peek Into Plumbing

A Natural Phenomena

Oil and gas seeps have provided resources that have been recorded everywhere from Biblical references to Jed Clampett of Beverly Hillbillies fame.

Beyond being a key indicator of hydrocarbon presence, seeps also provide a key study point of the earth's "plumbing system."

Indeed, a growing body of information from seep studies indicate that seep phenomena are not restricted to hydrocarbon provinces but are a fundamental feature of the geohydrologic system of continental margins.

Fluid seeps, both terrestrial and submarine, record the flux of fluids from the earth's lithosphere to the atmosphere and hydrosphere.

The overall impact of seepage on cycling of chemicals between the earth, oceans and atmosphere is unknown, but could significantly contribute to the supply of greenhouse gases.

The importance of seepage at continental margins led to a symposium, field trip and workshop on this topic at this year's AAPG Pacific Section meeting.

Supported by the Energy Institute at the University of California, the meeting was conceptualized as a West Coast, active continental margin meeting -- and still it drew earth scientists from across the United States and from Germany, France, Russia, New Zealand, Japan, Norway and Turkey.

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Oil and gas seeps have provided resources that have been recorded everywhere from Biblical references to Jed Clampett of Beverly Hillbillies fame.

Beyond being a key indicator of hydrocarbon presence, seeps also provide a key study point of the earth's "plumbing system."

Indeed, a growing body of information from seep studies indicate that seep phenomena are not restricted to hydrocarbon provinces but are a fundamental feature of the geohydrologic system of continental margins.

Fluid seeps, both terrestrial and submarine, record the flux of fluids from the earth's lithosphere to the atmosphere and hydrosphere.

The overall impact of seepage on cycling of chemicals between the earth, oceans and atmosphere is unknown, but could significantly contribute to the supply of greenhouse gases.

The importance of seepage at continental margins led to a symposium, field trip and workshop on this topic at this year's AAPG Pacific Section meeting.

Supported by the Energy Institute at the University of California, the meeting was conceptualized as a West Coast, active continental margin meeting -- and still it drew earth scientists from across the United States and from Germany, France, Russia, New Zealand, Japan, Norway and Turkey.

It also served as a forum for the development of a plan for the global analysis of fluid seeps that emphasizes quantitatively measuring the flow of fluids from seeps into the oceans and atmosphere over the entire earth.

(A detailed statement of this plan can be found at the Web site for the U.S. Margins Program. The AAPG Pacific Section published the field trip guide and related papers.)

Facts -- And Questions

Submersibles and remotely operated vehicles recently discovered numerous fluid seep sites along the continental margins around the world, resulting in an explosion of research.

Fluid seeps range from the oil- and gas-dominated systems, as mentioned above, to more water-dominated systems with dissolved hydrocarbons, carbon dioxide, nitrogen and hydrogen sulfide, to brine-dominated systems commonly derived from subsurface salt.

The discovery that chemicals in seep fluids support organisms that live off the chemical effluents electrified groups studying life in extreme environments, including the early earth and other planets.

Fluid seeps also form precipitates at the surface and in the subsurface, which affect the surface geomorphology of continental margins, and influence water and air quality.

Distributions of seeps can be interpreted tectonically showing structural control on fluid flow in the subsurface. Several presentations at the Pacific Section meeting showed evidence for large accumulations of precipitates, including carbonates, sulfates, and gas hydrates.

Several papers also showed that seeps bleed large quantities of methane into the oceanic water column, altering water chemistry. Monitoring studies showed that lowered reservoir pressure in production wells reduces rates of gas seepage and reduces the localized air pollution.

The question of how much seep gas reaches the atmosphere globally remains open. Geochemical studies traced seep hydrocarbon migration paths and demonstrated that some beach tar balls along the central California coast come from natural sources in areas not known for surface seepage of hydrocarbons.

Researchers showed that biological communities at seeps are sensitive recorders of the chemistry of fluid flow. Several talks emphasized that seep-related biological communities and precipitates are extensive in the stratigraphic record and provide unparalleled detail on the geometry, structural control and evolution of seep systems.

Both talks and posters related seepage and associated gas hydrates to geomorphic features of continental margins, including pockmarks, brine pools and landslides.

Important Goals

Determining the flux of fluids from the earth into the oceans and atmosphere emerged as one of the workshop's most important research goals.

This requires extensive surveying of continental margins, establishing observatories and calibration of remotely sensed images in terms of geomorphology, biology, fluid chemistry and fluid flow rate.

Once the distribution of and flux from seeps is known, earth scientists will have to ascertain how the fluids interact with and alter the rocks below and affect the overlying hydrosphere and atmosphere.

Evaluating fluxes and understanding fluid interactions will allow estimation as to the influence of fluid seeps on biological resources and on the chemistry of the ocean and atmosphere, as well as the role of seeps as geological hazards.

In addition to the major question of their flux, seeps offer exciting opportunities for fundamental research on microbiology and biological control of mineral formation.

To understand the dynamics of seeps, earth scientists need to study their subsurface plumbing, measure the pressure and density variations that may drive them, and model the fluid flow. Careful studies of seeps in ancient rocks are necessary to evaluate their influence on the geologic and paleontologic record.

Finally, fluid seeps of all kinds directly reflect fluid migration and offer clues to the understanding of the principles of hydrocarbon migration. For example, industry studies of microseepage of light hydrocarbons demonstrate that associated precipitation and mineral alteration can signal significant accumulations of hydrocarbons at depth.

This type of industry perspective has to be brought to the ongoing academic studies of fluid seeps at continental margins.

Conversely, results from the academic studies need to be carefully considered by industry scientists involved in both exploration and hazard surveys.

Exploration and production activities produce a wealth of surface and shallow sub-surface data that could be profitably analyzed by the academic community. These analyses in turn might provide valuable information to further industry goals.

Some effective collaborations already have occurred in the Gulf of Mexico and offshore California. The scope to expand such cooperative programs is large, and we hope that this can be accomplished for the benefit of both industry and academia.