Page 355 - Petroleum Geology

CHAPTER 15

DIAPIRS, DIAPIRISM AND GROWTH STRUCTURES

SUMMARY
(1) Diapirs, in the context of petroleum geology, are intrusions of sediment-
ary rocks, primarily salt or mudstone, into the overlying sedimentary se-
quence. Incipient diapirs are salt pillows and the analogous mudstone pillows
or “shale masses”. Deformation of the sedimentary rocks around and above
diapirs and incipient diapirs creates potential petroleum traps.
(2) Diapirs are initiated by unequal loading of a layer of material of small
equivalent viscosity. The common diapiric materials - salt and abnormally
pressured mudstone - may be less dense than the normally compacted sedi-
mentary rocks overlying them. Hence, once a diapir has been initiated (parti-
cularly a salt diapir), the forces of buoyancy tend to elongate the deforma-
tion vertically.
(3) The upward movement of a diapir is relative to the surrounding sedi-
mentary rocks. The accumulation of a sedimentary sequence over a diapir in-
dicates that it was subsiding with the development of the sedimentary basin.
The upward movement is only absolute if the relative movement is faster
than the subsidence of the surrounding sedimentary sequence.
(4) This differential subsidence may influence the accumulation of sedi-
ments, contributing to the variations of loading on the diapiric mother bed.
(5) The mechanical properties of the diapiric material change ‘with time
and position. Salt becomes less viscous with increasing temperature. Mudstone
viscosity is a function of pore pressure and depth as well as temperature.
(6) A diapir commonly, but not invariably, shows a gravity minimum. This
indicates a deficiency of mass.
(7) Failure of the overburden by faulting may accompany diapiric devel-
opment; but diapiric development may also inhibit subsidence locally at the
surface of accumulating sediment and so lead to a local stratigraphic hiatus.
(8) Diapirism is necessarily contemporaneous with the expulsion of pore
fluids from compacting mudstones. Diapiric mudstone may also be a petro-
leum source rock.

DIAPIRS

Diapirs are essentially intrusions of deeper material into the overlying
material of the Earth’s crust. The processes of diapirism are dynamic, and