Offshore exploration has tested passive, convergent, and transform margin types. Passive margins are characterized by an initial rift structure, commonly involving nonmarine sediments, sometimes evaporites, and an overlying mostly marine sedimentary terrace of clastics and/or carbonates. Potential has proved variable. Significant hydrocarbons have been found in MacKenzie Delta, Gulf of Mexico, offshore Newfoundland, northwest Europe, Gulf of Suez, India, Indonesia, Australia/New Zealand, Brazil, and central west Africa from Ivory Coast to Angola. To date, little success has been obtained on the passive margins of the rest of Africa, South America south of Brazil, the east coast of the U.S., and most of Australia/New Zealand.

Convergent margins are characterized by an arc-trench system with intervening fore-arc basin and subduction complex. Sediments of fore-arc basins are predominantly marine volcanogenic clastics derived from the magmatic arc. The subduction complex is a tectonically imbricated package composed predominantly of volcanogenic clastics, but may incorporate significant amounts of deep-sea cherts, limestones, red clays, and slices of oceanic crust. Potential appears poor with the only significant hydrocarbons discovered in this setting being in southern Alaska and northern Peru.

Transform margins are relatively limited worldwide and are characterized by sharp fault-bounded basins with clastics derived from adjacent sides and carbonates developed in situ. Compressive structuring locally accompanies basin evolution. Potential is variable. Significant hydrocarbons have been found in this setting off southern California, Trinidad, northern Brazil, Ivory Coast, and Sakhalin Island, but none so far off western Canada, the northern Caribbean, or western Madagascar.

End_of_Article - Last_Page 571------------