Reservoir rock and Cap rock

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Reservoir rock and Cap rock
Where does oil and gas get trapped,
and the kind of rocks that allow it to
occur
Reservoir rock
• A permeable subsurface rock that contains
petroleum. Must be both porous and permeable.
• Reservoir rocks are dominantly sedimentary
(sandstones, shales and carbonates); however,
highly fractured igneous and metamorphic rocks
have been known to produce hydrocarbons,
albeit on a much smaller scale
• Understanding reservoir rock properties and their
associated characteristics is crucial in developing
a prospect
Examples of reservoir rocks
What is a source rock and what kind of
rock is it?
• A sedimentary rock in which petroleum forms.
• Source rocks are widely agreed to be
sedimentary.
Examples of Reservoir rock
• The first characteristic is “porosity”
• Porosity consists of the tiny spaces in the rock that
hold the oil or gas.
• Even though sandstone is hard, and appears very solid,
it is really very much like a sponge (a very hard,
incompressible sponge). Between the grains of sand,
enough space exists to trap fluids like oil or natural gas!
The holes in sandstone are called porosity (from the
word “porous”).
• The second characteristic is “permeability”
• Permeability is a characteristic that allows the oil and
gas to flow through the rock
Porosity
Info on Porosity
• Porosity of a rock is a measure of its ability to hold a
fluid. Mathematically, porosity is the open space in a
rock divided by the total rock volume (solid + space or
holes). Porosity is normally expressed as a percentage
of the total rock which is taken up by pore space.
• For example, a sandstone may have 8% porosity. This
means 92 percent is solid rock and 8% is open space
containing oil, gas, or water. 8% is about the minimum
porosity that is required to make a decent oil well,
though many poorer (and often non-economic) wells
are completed with less porosity
Three basic types of pores
• 1. Catenary: these are pores that
communicate with other pores by more than
one throat passage.
• 2. Cul-de-sac: or dead-end pores only have
one throat passage connecting to another
pore.
• 3. Closed pore: have no communication with
other pores.
Catenary pores
Cul-de-sac pores
Closed pore
What does this mean?
• Catenary and cul-de-sac pores constitute
effective porosity, in that hydrocarbons can
emerge from them. In catenary pores
hydrocarbons can be flushed out by water
where cul-de-sac pores are unaffected.
• The size, geometry of the pores and the
diameter of the connecting throat passages
all affect the productivity of the reservoir
Info on Permeability
• The permeability of a rock is a measure of the resistance to
the flow of a fluid through a rock.
• In the last 10 years, an increasing amount of US gas
production is coming from shale gas wells. Shale has a lot of
porosity (much more than sandstone), but extremely low
permeability.
• That means shale has historically been a poor producer of
hydrocarbons. While gas has been produced from shales for
over a hundred years, quantities were small. Two things
have changed the situation, allowing for increased shale
gas development. These concepts have allowed petroleum
companies to artificially induce more permeability into
shale gas rocks
Rocks encountered in oil fields
• The three sedimentary rock types most
frequently encountered in oil fields are:
• shales,
• sandstones,
• carbonates
• Each of these rock types has a characteristic
composition and texture that is a direct result of
depositional environment and post-depositional
(diagenetic) processes (i.e., cementation, etc.)
Permeability versus Porosity in rocks
• Limestone: High porosity, low permeability
• Most SandStone reservoirs: Porosity
proportional to permeability
• Fractured reservoir: Low porosity, high
permeability
• Shale: High porosity, extremely low
permeability
• Sandstones
• Conglomerates
• Halite (rock salt)
• Gypsum
• Anydrite
• Limestone
• Chalk
• Coquina
• Coal
Which one of these sandstone samples
would hold gas and be able to move that
gas to other areas

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