Page 362 - Petroleum Geology

The formation of a diapir involves the displacement of diapiric material
from the mother layer to the diapir under dynamic forces acting over long
spans of time. The flow lines in the mother layer around a well-developed
diapir are centripetal, and the material moves down an energy gradient anal-
ogous to the fluid potential gradient around a producing oil or water well. If
the mother layer is horizontal, the pressure in it adjacent to the diapir is less
than that further away, because this is a necessary condition of horizontal
flow. The rim syncline, or peripheral sink, around a diapir is therefore to be
regarded as an expression of the potential energy of the mother layer anal-
ogous to the drawdown of the water table around a producing water well
(Ramberg, 1981). This sink is terminated on the inside by the upward drag
of the diapir. This is clear for a well-developed diapir.
During stages of incipient diapirism, unequal loading of a potential mother
layer (with low equivalent viscosity) creates a disequilibrium that may be re-
stored by flowage from the more heavily loaded areas to the less heavily loaded
areas. With mudstone diapirism in mind, this is generally away from the marine
margin of the physiographic basin. Locally, however, differences will exist,
some of which will be minor, others more important. Because subsidence
may locally increase the capacity to accumulate sediment, a further inequality
of loading may follow consequentially on initial inequality.
Diapirism, like many geological processes, is not easily reduced to simple
statements of cause and effect. There is, however, general agreement on the
main factors that contribute to diapirism, even if there is disagreement on
the relative importance of each. The main factors are: (1) low equivalent vis-
cosity in the material that contributes to, and forms the diapir; (2) the load
on the mother layer, and the variations of the load in space and time; (3) the
bulk density of the diapiric material relative to that of the overburden; and
(4) the thickness of the mother layer (but perhaps this is only a matter of
scale).
None of these factors by itself necessarily leads to diapirism. Not all salt
layers feed diapirs, for example, and there are many density inversions with
depth in the geological column that do not lead to diapirism. But physical
and mathematical models have been constructed that reproduce the essential
features of diapirs, both individually and collectively. In physical models
there are problems of scaling, but a wide variety of materials leads to struc-
tures that resemble real diapirs and incipient diapirs closely.
At the onset of instability, the interface between the overburden and
mother layer becomes wavy, and both types of models indicate that some
wave lengths become more strongly amplified than others (see, for example,
Biot and Od6, 1965). The more strongly amplified wavelength, called the
dominant wave length, is affected by the viscosity ratio and the thickness
ratio of the overburden and mother layer. Overburdens of larger equivalent
viscosity tend to be deformed with a longer dominant wavelength, and the
rate of diapiric growth is slower. As the thickness ratio is increased, Biot and