A good slide on gravity index

1. GRAVITY INDUCED
STRUCTURES
DIAPIRS, DIAPIRISM & GROWTH
STRUCTURES
•1. Diapirs, in the context of petroleum geology, are
intrusions of sedimentary rocks, primarily salt or mudstone,
into the overlying sedimentary sequence.
•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. GRAVITY INDUCED
STRUCTURES
•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 sedimentary rocks overlying them.
•Hence, once a diapir has been initiated (particularly a salt diapir),
the forces of buoyancy tend to elongate the deformation vertically.
•The upward movement of a diapir is relative to the surrounding
sedimentary rocks. The accumulation of a sedimentary sequence
over a diapir indicates that it was subsiding with the development
of the sedimentary basin.

3. GRAVITY INDUCED
STRUCTURES
•The upward movement is only absolute if the relative movement is
faster than the subsidence of the surrounding sedimentary sequence.
•This differential subsidence may influence the accumulation of
sediments, contributing to the variations of loading on the diapiric
mother bed.
•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.

4. GRAVITY INDUCED
STRUCTURES
•A diapir commonly, but not invariably, shows a gravity minimum.
This indicates a deficiency of mass.
•Failure of the overburden by faulting may accompany diapiric
development; but diapiric development may also inhibit subsidence
locally at the surface of accumulating sediment and so lead to a local
stratigraphic hiatus.

5. GRAVITY INDUCED
STRUCTURES

6. GRAVITY INDUCED
STRUCTURES
•2. Diapirism is a dynamic process that takes place under the force of gravity
during the accumulation of sediment in a developing sedimentary basin. Its
significance for the petroleum geologist is that it is a process that deforms the
sedimentary strata while they are compacting, during fluid expulsion from the more
compactible lithologies.
•Mudstone diapirism is probably more significant than salt diapirism because the
mudstone itself may be a petroleum source rock, and mudstone diapirism is
essentially contemporaneous with fluid expulsion from the mudstone.
•Diapirism involves the flow of diapiric material. When referring to the flow of rocks,
we use the term “equivalent viscosity” because the term “viscosity” may suggest that
Newtonian viscosity is involved, with the velocity of flow at a point proportional to
the distance of the point from a static boundary.

7. GRAVITY INDUCED
STRUCTURES
3. Growth structure –Growth fault. - A fault that separates correlative sequences
of which the thicker is on the downthrown side. More generally, it is used of faults
that are inferred to have been moving during the accumulation of sediment in at
least the downt hrowing block.
•As a consequence of this thickness contrast, the throw tends to increase with
depth. This is not a diagnostic criterion because antithetic faults may reduce the
throw.
•Growth faults results from compaction disequilibrium and are found most often
in deltas with prolific net sediment accumulation and high shale/sand ratios.
•In the Niger delta, Nigeria, the roll-over anticlines form traps rather than the
growth fault that give rise to them. Almost all the oil field discovered so far in
Niger delta complex are associated with roll-over anticlines.

8. GRAVITY INDUCED
STRUCTURES
3. Growth structure –Listric fault. – Are curved normal fault surface in which
the fault surface is concave upwards; its deep deceases with depth.
The formation of a roll-over anticline will occur when listric fault collapsed.
assignment draw a growth fault and a listric fault