Published 1996 as part of Memoir 66
Copyright © 1996 The American Association of Petroleum Geologists.   All Rights Reserved.
 

In most basins, lateral subsurface petroleum migration occurs over tens or even hundreds of kilometers between source rock and trap and between accumulations and the surface. In general, this means, first, that seepage can only provide information for risking petroleum charge at the basin scale, and second, that there is no direct spatial relationship between filled prospects and surface seepage. Understanding the geology, and hence petroleum dynamics, of a basin is the key to understanding and using seepage in exploration. A spectrum of seepage styles can be used to focus exploration thinking. The spectrum ranges from prolific seepage (e.g., offshore California), through focused point-source seepage (e.g., offshore Colombia), to basins where long-distance lateral migration concentrates seepage on basin margins (e.g., Western Canada Sedimentary Basin). Related controls on fluid flow and seepage range from active tectonism, through high fluid potential gradients resulting from rapid muddy deposition, to fault and salt structures and basinwide carrier bed systems. Case studies of offshore oil seepage from the Gulf of Mexico, Central North Sea, Haltenbanken (offshore mid-Norway), and North Viking Graben are used to illustrate the spectrum of seepage styles and the factors that control different styles. Understanding seepage in terms of basin geology and petroleum dynamics is not only necessary for interpreting seepage for exploration but is also critical for planning seep collection, particularly in offshore areas. In all but basins with prolific seepage, likely seepage sites, which occur at the surface end of migration pathways, need to be targeted if seeps are to be sampled for analysis.