Interpretation of Traps Related to Salt Structures
In the last decade, many of the important oil-field discoveries have been a result of exploration of Gulf of Mexico salt traps. Possible salt-related petroleum traps also exist in onshore Texas and Louisiana, the Paradox Basin of Utah, the North Sea, North Africa, the Hannover Basin of Germany, and the Caspian Sea area. Other areas such as the east coast of Canada have potential petroleum traps associated with salt intrusions but have yet to provide significant production.
Salt is unusual in that it does not follow the normal seismic velocity-density relationships of many other rocks. Salt generally has a lower density and often higher seismic velocity than surrounding sediments. With the low density and “buoyancy” of salt buried under sediments, salt usually will “float through,” or intrude through denser sediments, and it often will pierce overlying sediments, providing traps along flanks.
Various salt shapes can result from these intrusions. Salt (halite, or NaCl) often provides a permeability barrier for petroleum in sediments that dip against the salt flank. In addition to this type of trap, significant faulting often is associated with intrusion. Sealing faults can serve as traps above salt intrusions. In other cases, petroleum may accumulate beneath intrusions.
Levorsen (1967) classified salt domes according to their age. Young domes are characterized by anticlines with relatively little deformation. Mature domes are vertical salt stocks on which cap rock has begun to accumulate. Older domes have thicker accumulations of cap rock that is a disklike body of anhydrite, gypsum,
Figures & Tables
Fundamentals of Geophysical Interpretation, SEG Geophysical Monograph Series No. 13, is a practical handbook for the petroleum geophysicist. Fundamental concepts are explained using heuristic descriptions of seismic modeling, deconvolution, depth migration, and tomography. Pitfalls in processing and contouring are described briefly. Applications include petroleum exploration of carbonate reefs, salt intrusions, and overthrust faults. The book includes past, present, and possible future developments in time-lapse seismology, borehole geophysics, multicomponent seismology, and integrated reservoir characterization.