Engineering geology

Salt diapirism, salt geometry and the flow of salt

Salt diapirism, salt geometry and the flow of salt

Salt structures

Salt volumes in the crust have for a long time been known to take on a variety of geometric shapes, from elongated structures known as salt anticlines and salt pillows to more localized structures such as salt stocks. Although many of these structures are loosely referred to as diapirs they have different names, as illustrated schematically in Figure 1.

Figure 1 – Different types of salt structures, their names and geometries. Maturity increases from the central part of the figure to the left and right.

The termdiapir derives from the Greek words for through (dia) and pierce (peran) and is used in geology to describe a body, usually of salt, magma or water-saturated mud or sand, that gravitationally moves upward and intrudes the overburden.

Hence, some structures, such as salt pillows and salt anticlines that just bend and uplift the overlying layers, are not diapirs sensu stricto, because they do not intrude or pierce the overburden.

However, most of these structures represent various stages that could lead to the formation of a true diapir. The process through which a diapir develops is known as diapirism.

The flow of salt from a salt layer into a salt structure is usually referred to as salt withdrawal or, more correctly but less commonly, salt expulsion. In simple terms, there are two principal types or end-members of flow. One, Poiseuille flow, occurs when salt flows into a salt structure during the growth of a salt anticline or diapir (Figure 2a).

Figure 2 – The two principal types of flow occurring in deforming salt layers. Arrows indicate velocity and the velocity profile is parabolic in (a) and linear in (b).

In this case flow is restricted by the viscous shear forces acting along the boundaries of the salt, an effect known as boundary drag. This effect causes the salt to flow faster in the central part of the salt layer than along the top and bottom.

Hence, thin salt layers flow slower than thick ones, implying that flow in a thick (tens of meters or more) salt layer is slowed down when the salt is reduced to a thickness of a few tens of meters.

If the salt becomes completely exhausted the boundary layers become attached to each other, and the contact is referred to as a salt weld (Figure 1), typically indicated on geologic and seismic sections by pairs of dots (Figure 3).

Figure 3 – Salt structures imaged on seismic 2-D line. The two structures have surfaced (now covered by thin Quaternary cover) and developed reflective caps. Deeper parts of the salt structures are obscured by seismic noise. The source layer has been interpreted based on a salt weld structure. The growth history is recorded in the sedimentary record: thickness changes near the diapirs start above the blue horizon, after deposition of a 1.5–2 km sedimentary load. Note minibasin above the weld. Data courtesy of the Norwegian Petroleum Directorate.

Salt welds will always contain some remnant salt, and even if the remaining salt is only centimeters thick it will represent a weak zone that is prone to localized strain. The importance of a salt weld is that it terminates lateral flow of salt into adjacent salt structure(s).

The other type of flow is known as Couette flow (Figure 2b), which involves simple shearing within the salt layer as the overburden is translated relative to the substrate.

This type of flow is typical for salt layers acting as de´collements, but the two types of flow may well be superposed on each other. Ideally, in Couette flow there is no boundary effect of the type occurring in Poiseuille flow.

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