Download Traps and Seals

Traps and Seals
They are not what you think
Where does oil and gas come from?
What is a trap?
• A trap is a place where oil and gas
are barred from further movement,
and remember that a trap is trap
whether it is barren or productive.
A simple anticlinal trap
Facts on traps
• The highest point of a trap is the crest, or
culmination.
• The lowest where hydrocarbons may be
contained is the spill plane
• The vertical distance between from crest to
spill plane is the closure
Seals and Cap Rocks
• For a trap to have integrity it must be overlain
by an effective seal.
• Any rock may act as a seal as long as it is
impermeable
• Seals will commonly be porous, but they must
not permit the vertical migration of petroleum
or gas from the trap
Trap Rocks
• Shales are the commonest seals
• Evaporites are the most effective seals
• Shales are commonly porous, but because of
their fine grain size, they have high caplillary
forces that prevent fluid flow.
• Sometimes shales may selectively trap oil,
while permitting the upward migration of gas.
Leakage of gas
• Several mechanisms allow the leakage of gas
from a trap.
• These include: the compressible Darcy flow of
a free gas phase, the transport of dissolved
gas in aqueous solution, and the diffusive
transport through the water-saturated pore
space of the cap rock.
Darcy’s Law
• Darcy’s law is an equation that defines the
ability of a fluid to flow through a porous
media such as rock. It relies on the fact that
the amount of flow between two points is
directly related to the difference in pressure
between the points, the distance between the
points, and the interconnectivity of flow
pathways in the rock between the points. The
measurement of interconnectivity is called
permeability.
Info on Darcy’s law
• In the subsurface, rock is deposited in layers.
• Fluid flow within and between the rock layers is
governed by the permeability of the rocks. However, to
account for permeability, it must be measured in both
the vertical and horizontal directions.
• For example, shale typically has permeabilities that are
much lower vertically than horizontally (assuming flat
lying shale beds). This means that it is difficult for fluid
to flow up and down through a shale bed but much
easier for it to flow from side to side.
Darcy’s law example
• A good example of this
characteristic is shown in the
picture at right; which clearly
indicates that it would be
much easier for water to flow
along the horizontal bedding
planes in the shale where
there are natural flow
pathways instead of vertically
where there are few flow
pathways.
Distribution of oil and gas in a trap
• A trap may contain oil, gas or both. The
oil:water contact is the deepest level of
producible oil.
• Similarly, the gas:oil contact or gas:water
contact is the lowest limit of producible gas
• Where oil and gas occur together in the same
trap, the gas overlies the oil because the gas
has a lower density
Classification of traps
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There are two major genetic groups of traps
Structural
Stratigraphic
There is a third group called combination but
we will not go into those
Structural Traps
• Structural traps are those traps whose
geometry was formed by tectonic processes
after the deposition of the beds involved.
• A “structural trap” is one whose upper
boundary has been made concave, as viewed
from below, by some local deformation, such
as folding, or faulting, or both, of the reservoir
rock.
Examples of Structural traps
Anticlinal Traps:
These may be divided into two classes:
Compressional anticlines (caused by crustal
shortening)
and Compactional anticlines (developed in
response to crustal tension)
Anticline structural trap
Fault Type trap
Compressional anticlines
• These are most likely to be found in, or
adjacent to, subductive troughs, where there
is shortening of the earth’s crust
• Thus fields in such traps are found within, and
adjacent to, mountain chains in many parts of
the world
Compressional anticlines
Compactional anticlines
These anticlines are formed by crustal tension.
Stratigraphic Traps
• These are traps whose geometry is formed by
changes in lithology.
• The lithological variations may be;
• Depositional (e.g., channels, reefs and bars)
• Postdepositional (e.g., truncations and
diagenectic changes)---changes undergone by
a sediment after its initial deposition
Examples of stratigraphic oil traps
What is Diagenesis?
• Diagenesis is changes to sediment or
sedimentary rocks during and after rock
formation, at temperatures and pressures less
than that required for the formation of
metamorphic rocks or melting. It does not
include changes from weathering
What is Postdepositional
• Post-depositional processes that can change
or even remove deep-sea sediments include:
physical transport, disturbances by organisms,
chemical alteration, dissolution and
diagenesis, gravity-driven sediment transport
mechanisms, carbonate dissolution, and
authigenic mineral precipitation.