Page 357 - Petroleum Geology

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Salt diapirs

Salt diapirs occur in large areas of the Gulf Coast province of North America,
north-west Europe, Russia, and around the Arabian or Persian Gulf. Some
have reached the surface, or are at very shallow depths, from a mother layer
more than 5 km deep. Others take the self-explanatory shape of salt pillows,
forming the core of diapiric anticlines. The truly intrusive forms are known
as salt domes, plugs, or stocks.
The mining of salt near the surface has shown that the salt is intensively
deformed, with complicated flow patterns (rather than folds) but very rare
faults. The external form, however, tends to be more regular. Many salt
domes, particularly those at shallow depth, have developed a cap rock that
consists mainly of anhydrite or gypsum. The cap rock is the less soluble residue
from leaching of the salt by ground water. Salt domes may be sheathed in a
thin skin of anhydrite or mudstone “gouge”. Some cap rock contains sulphur
that was commercially extractable until the cleaning of sour gas provided a
cheaper and more abundant source. The sulphur of salt domes is associated,
probably biogenically, with petroleum-bearing diapiric structures.
Although a salt-dome is usually roughly circular in plan, the cap rock com-
monly extends laterally over a wider area than the main part of the dome,
which may be more or less cylindrical, or narrowing downwards to the mother
layer of salt from which the dome was supplied. Characteristically, the upper
surface of the mother layer is deformed near the salt dome into a rim syncline,

Fig. 15-2. Diagrammatic cross-section through top of well-developed salt diapir.