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ABRAMS, MICHAEL A., Exxon Exploration Company, Houston, TX

ABSTRACT: Distribution of Subsurface Hydrocarbon Seepage in Near Surface Marine Sediments

Hydrocarbon seeps in surficial marine sediments are of two types: ACTIVE: Where gas bubbles, pockmarks, or bright spots are visible on seismic records and/or the presence of chemosynthetic communities in conjunction with large concentrations of migrated-hydrocarbons. Generally in areas where generation and migration of hydrocarbons from the source rock is ongoing today (i.e., maximum burial) and/or where significant migration pathways have developed from tectonic activity. PASSIVE: Where concentrations of migrated hydrocarbons are so low that few or no geophysical anomalies are seen. Typically in areas where generation and expulsion is relict (no longer at maximum burial) and/or regional seals prevent significant vertical migration.

The type of seep strongly controls the distribution of migrated hydrocarbons in the near surface sediments and should dictate the sampling equipment and approach required to detect seeps. Active seeps or macroseeps, usually can be detected near the water-sediment interface, within the water column, and at relatively large distances from major leak points. Most conventional sediment and water samplers will capture active seeps. Precise location of sampling is typically not critical to detect active seeps. The Gulf of Mexico, Santa Barbara Channel, and parts of the North Sea have active hydrocarbon seeps.

Passive seeps or microseeps, can only be detected relatively far below the water sediment interface and require samples to be collected very near the leak points. Sampling equipment must penetrate the zone of maximum disturbance or any shallow migration barriers. In areas where the surficial sediments are coarse grained or compacted, conventional gravity corers will not work. Other options for subsurface sampling include the vibracore (utilizes air or electric vibrating motors), jet core (utilizes high pressure water to break up sediments), or rotary core (open hole drilling). Precise location of samples (i.e., site-specific approach), using seismic profiles to identify and locate leak points, is critical to detect passive hydrocarbon seeps. The Beaufort and Bering Seas offshore Alask , and parts of the North Sea contain passive seeps.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.