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