Marine Oil Seeps

MEYER, RICHARD F., U.S. Geological Survey, Reston, VA

Petroleum hydrocarbons of both biogenic and thermogenic origin are common constituents of the marine water column and sediment of the continental shelves. Approximately 0.25 million metric tons of oil per year, constituting about 8% of the oil input into the sea, is derived from natural seeps, the rest being anthropogenic. Seepage has occurred world-wide for millions of years and must have been many times greater in the past, when enormous oil deposits, such as the Orinoco Oil Belt, were first exposed to erosion. Although the amount varies from site to site with time, seepage is pervasive in polar and temperate seas. Marine-seep oil is intensely weathered and thus can be distinguished chemically from recent biogenic or undegraded crude oil. The degraded oil from seeps appears to have ittle deleterious effect on many marine organisms, which ingest and discharge the oil mostly unmetabolized. Chemical analyses suggest that a very large oil-rich layer in the Sargasso Sea originated from a large and as yet undetected seep. Oil seeps have long been used as guides for oil exploration onshore but have been underutilized for this purpose offshore because of oil-plume drift from the site of the seep and because natural oil slicks may be masked by spilled oil. At least one marine seep, in the Santa Barbara Channel, California, is producing oil and natural gas into two hollow steel pyramids from which the oil is collected by work boats and the natural gas is transported to shore by pipeline. This facility effectively reduces atmospheric pollution, controls marine oil pollution f om the largest seep in the area, provides emission credits, and yields a modest economic benefit, but the seep is not known to have been used directly in oil exploration.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)