Many petroleum traps have both stratigraphic and structural features. Some, in which both types of characteristics are essential in trapping petroleum, are difficult to classify as either primarily structural or primarily stratigraphic. For instance, originally horizontal formations that now pinch out updip can trap hydrocarbons that might not otherwise have accumulated. Secondary porosity in a shattered (brecciated) fault zone or anticlinal crest is a stratigraphic trapping mechanism caused by structural deformation. Most hydrodynamic trapping depends partly upon formation structural features and often upon stratigraphic variations within the reservoir formation.
Many types of traps can be found near salt domes. Most would be considered structural, although some could be classified as combination traps. Beneath the U.S. Gulf Coast are thick beds of salt that were deposited, during the opening of the modern Atlantic Ocean, in sedimentary basins with restricted circulation and high evaporation rates. Under pressure, this rock salt, light and easily deformed, is displaced by the weight of accumulating sediments, forming huge mushroom-like columns that rise toward the surface. In each of these diapirs, the overlying rocks are pushed aside or bulged upward into a dome. The penetrated layers are dragged upward by the rising salt core and depressed downward away from it as the overlying layers subside to replace the depleted salt bed. Leaching by groundwater prevents the salt from breaking through the surface, but leaves atop the column a residue of less soluble compounds, forming a dense, impermeable caprock. Overlying sediments break in a complex series of intersecting faults. The base of the salt core may narrow, creating a mushroom-shaped overhanging column.
Many types of petroleum traps are thus formed: a multi-layered dome on top, cut by faults; upturned drag folds that terminate against impermeable salt; upturned pinchouts where compression and other diagenetic changes have reduced permeability; and faults along the flanks. Oil may also collect beneath the impermeable caprock or beneath the overhanging salt. The multiple possibilities for traps and the high likelihood of finding petroleum have made salt domes popular places to drill.
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