Page 368 - Petroleum Geology

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(1968) studied the stratigraphy around the salt domes and found that in
groups of salt domes that he called salt-stock .families, there was a sequential
development from the “mother salt stock” outwards in space and time (Fig.
15-10), with progressively younger sedimentary rocks in the rim synclines. In
this manner, he elucidated the development of the region (Fig. 15-11).
In general, the development of salt diapirs takes place during the accumu-
lation and compaction of the overlying sedimentary rocks, and so is contemp-
oraneous with fluid expulsion from them. The deformation of the sequence
creates traps for any petroleum generated and expelled from the source rock;
and ultimately such accumulations may be displaced to the flanks when pene-
tration of the overburden occurs. Because there is no known causal relation-
ship between salt and petroleum, we must regard these accumulations as co-
incidental, the deformation happening to take place while petroleum was
being generated. This is not necessarily true of mudstone diapirism.
Mudstone diapirism has not received the same attention as salt diapirism,
perhaps because it is not so distinctive; but it is probably more important in
petroleum geology - particularly the geology of regressive sequences. This
diapirism, from both observational and theoretical points of view, can begin
soon after the more permeable, sandy, part of a regressive sequence begins to
accumulate. Mud volcanism is mudstone diapirism at the surface, and incipient
mudstone diapirism at depth is inferred for many parts of the world.
The apparent rarity of penetrative mudstone diapirs at depth (but see Gil-
reath, 1968; and Bishop, 1978) and their common occurrence at shallow
depths is consistent with relative viscosity (and perhaps density) considerations
because undercompaction is more pronounced at shallow depths. Mechanical
instability in the sequence may remain until the mudstone is buried under 4

2 Generahon 1 Generation 2 Generation 3 Generatton L Generation
(Master Salt Stock)

Fig. 15-10. Schematic diagram of development of a salt-stock family. (After Sannemann,
1968, p. 264, fig. 3.)