Understanding the deposition of evaporites and how the subsequent deformation history can influence salt basin petroleum systems is crucial for successful resource exploration.
Many of the most prolific and prospective basins in the world are characterized by salt-related deformation. "These include the greater Gulf of Mexico basin, the South Atlantic basins of offshore Brazil and West Africa, the North Sea, the Gulf of Suez, Red Sea, the Persian Gulf, the Zagros Mountains of Iran and Iraq, and the north Caspian area. Not only does salt-related deformation define many of the traps in these and other provinces, but it also serves as the framework for other aspects of the petroleum systems," says Dr. Mark Rowan, an oil industry consultant in Boulder, Colorado.
Salt basins are primarily found in rift basins and along passive margins. Most are formed during the early post-rift phase, such as the offshore Brazil and West African basins. Some were formed during rifting or lulls between rifting episodes such as those of the northern Atlantic Ocean basins. A few may be older than the main rifting phases, for example the North Sea, where the Permian salt is older than the Triassic and Jurassic rifting.
Salt basins can also form in any restricted intracratonic basin. Some examples include the Amazon Basin in Brazil and the foreland basins such as the Paradox in the western U.S. Other basins are characterized by narrow entrances that become closed off, such as the Red Sea, the Mediterranean Sea, and the Pre-Caspian Basin.
"Rift basins share similar characteristics and histories, one that is conducive to evaporite deposition," says Dr. Rowan. "They typically form during extension of the earth's crust with a distinct basement architecture made up of grabens and half-grabens segmented by transverse structures. Salt deposition can be restricted to individual half-grabens or deposited regionally depending upon the rift geometry, sedimentation rates, and time of evaporite formation."
"The increased understanding of the geometry and evolution of salt bodies has greatly advanced in the past two decades," says Dr. Rowan. "This has been primarily through improved seismic data acquisition and processing, forward modeling, structural restoration, and field studies."
"Salt behaves much differently than other, more typical sedimentary rock types. One of the key factors is that salt is much weaker than other lithologies. Salt deforms as a viscous material that effectively flows. Also, salt has a nearly constant density irrespective of burial depth. This contrasts with sediment that becomes denser upon burial."
"Salt does not drive salt tectonics; instead salt reacts passively to external forces."
Typically, salt deformation is highly complex and variable. Within the evaporite layer, even when salt is interbedded with other lithologies, as is typical, the deformation is intense. Above the salt, the structural style results from an interplay between local withdrawal and diapirism and regional loading, extension, and contraction. (See GEO ExPro v. 5, no. 3, pp. 56-58 for a review of salt structures.)
"The history of salt deformation plays a large role in the spatial and temporal distribution of sedimentary facies (reservoirs) and in the generation, migration, and entrapment of hydrocarbons," says Dr. Rowan.
Common to salt basins that form along rifted margins, some of the best source rocks were deposited during the pre-rift phase, followed by a thick sequence of evaporite deposition. "Because salt has a high thermal conductivity (2 to 4 times that of other sediments), thermal maturation of sub-salt source rocks can be retarded while maturation of the supra-salt strata can be accelerated." says Dr. Rowan. "The changing geometry of salt also plays a major role in controlling the evolution of fluid migration pathways through time and provides both migration pathways and the seals to trap hydrocarbons."
"This is certainly the case for the plays in the Santos and Campos basins", says Marcio Mello of High Resolution Technology in Rio de Janeiro. "The source rock for both basins is the same and lies below the salt layer. Some of the oil in the Campos Basin escaped through faults and fractures in and along the salt into post-salt reservoirs. For the Santos Basin, the salt acted like a barrier, keeping the oil in pre-salt reservoirs. The thermal and elastic properties of the salt also help preserve liquid hydrocarbons in the deep, pre-salt reservoirs."
While a salt layer has little effect on the deposition of pre-salt reservoirs, the elastic nature of salt can help preserve porosity by acting like a "mattress, not letting pressure to be transmitted to the lower layers," claims Dr. Mello.
"Salt deformation can have a profound effect on the deposition of post-salt reservoirs."
Just as sedimentation can control salt flow in many ways, salt deformation impacts the geometry and distribution of sediment transport. Most growing salt bodies form bathymetric highs that, when combined with salt withdrawal from beneath basins resulting from sediment loading, control sediment transport pathways and depocenters. The effects can be dramatic, at times completely blocking sediment transport; once sedimentation fills the seafloor relief, the effect is minimal on sediment transport. Therefore, it is critical to understand how the salt-controlled topography changes over time.
"A better understanding of the interactions between salt deformation, sedimentation, and fluid flow can only aid in the exploration for, and exploitation of, hydrocarbons in salt basins." concludes Dr. Mark Rowan.
Updated: 15.12.2008 11:24 by Alf Kvassheim
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