Hydrocarbon Migration and Its Near-Surface Expression

Abstract

Hydrocarbon seeps in surficial marine sediments are of two types: active and passive. Active seeps occur where gas bubbles, pockmarks, or bright spots are visible on seismic profiles and where chemosynthetic communities are present in conjunction with large concentrations of migrated hydrocarbons (macroseeps). These generally occur where generation and migration of hydrocarbons from source rocks are ongoing today (at maximum burial) or where significant migration pathways have developed from recent tectonic activity. Passive seeps occur where concentrations of migrated hydrocarbons are usually low (microseeps) with few or no geophysical anomalies. These occur typically in areas where generation and expulsion is relict (no longer at maximum burial) or regional seals prevent significant vertical migration. The type of seepage controls the distribution of migrated hydrocarbons in the near-surface sediments and should dictate the sampling equipment and approach used to detect seeps. Active seeps are usually detected near the water-sediment interface, in the water column or at the sea surface, and at relatively large distances from major leak points. Most conventional sediment and water samplers can capture active seeps. The Gulf of Mexico, Santa Barbara Channel, and parts of the North Sea have active hydrocarbon seeps. Passive seeps can only be detected relatively far below the water-sediment interface and require samples to be collected near leak points. Sampling equipment must penetrate the zone of maximum disturbance or any shallow migration barriers. In areas where surficial sediments are coarse grained or compacted, conventional gravity corers will not work. Other options for subsurface sampling include vibracores, jet cores, and rotary cores. Precise location of samples (site-specific) using seismic profiles to locate leak points is critical to detect passive hydrocarbon seeps. The Beaufort and Bering seas, offshore Alaska, and parts of the North Sea contain passive seeps.