Reservoir Rocks: Types and Properties

A reservoir rock is a rock that can store oil, gas and water in its pores and let those fluids flow toward a well. Every oil and gas field depends on the right rock being in the right place, sealed by a rock that stops the fluids from escaping. In this article you will learn which rock types make good reservoirs, which properties control their quality, and how traps keep hydrocarbons in place.

What Makes a Good Reservoir Rock?

Four properties decide whether a rock can become a producing reservoir:

• Porosity: the share of the rock volume that is empty space. It tells you how much fluid the rock can hold. See porosity and the classification of porosity for details.
• Permeability: how easily fluids move through connected pores. A rock can have high porosity and still be a poor reservoir if the pores are not connected. Read more in permeability and the relation between porosity and permeability.
• Fluid saturation: the fraction of the pore space filled with oil, gas or water.
• Thickness and extent: the rock body must be thick and wide enough to hold a commercial volume of hydrocarbons.

Rock Types and Their Role in Reservoirs

Rocks are grouped by the way they form: igneous, sedimentary and metamorphic. Almost all of the world’s oil and gas comes from the sedimentary group.

Igneous Rocks

Igneous rocks form when magma or lava cools and solidifies. Granite and basalt are common examples. Their crystals are tightly interlocked, so they usually have very low porosity and permeability and rarely act as reservoirs. Fractured or weathered igneous rocks, especially in buried basement highs, can hold and produce hydrocarbons in a few fields.

Sedimentary Rocks

Sedimentary rocks form from broken rock fragments, the remains of living organisms, or minerals that precipitate from water. These rocks are laid down in layers and keep open spaces between grains, which makes them the main source of reservoirs. Two types matter most:

• Sandstone: made of sand grains cemented together. Porosity is commonly in the range of 10 to 25 percent, and permeability can be high when grains are well sorted. Algeria’s Hassi Messaoud field produces from Cambrian sandstones.
• Carbonates (limestone and dolomite): formed mainly from marine organisms. Their pore systems are more complex, with pores that form during deposition and others created later by dissolution or dolomitization, and natural fractures often improve flow. A large share of the world’s reserves is found in carbonate reservoirs.

Shale is also sedimentary, but its pores are extremely small, so it normally acts as a seal rather than a reservoir.

Metamorphic Rocks

Metamorphic rocks, such as marble, quartzite, hornfels and gneiss, are existing rocks changed by heat and pressure. They are generally dense and tight, so they rarely store hydrocarbons, except where fractures allow oil or gas that has migrated from nearby source rocks to collect.

How Oil Gets Into the Reservoir

Oil and gas are formed in a source rock, then move out of it because they are lighter than the water in the surrounding rock. This upward movement, driven mainly by buoyancy, is called migration. You can follow the full story in The Hidden Origins of Oil. The hydrocarbons rise through connected pores and fractures until an impermeable layer stops them. There they displace part of the formation water and fill the pores of the reservoir rock. For a general introduction, see petroleum.

Seals and Traps

A reservoir needs a seal, an impermeable rock above it. Shale is the most common seal, and evaporites such as salt and anhydrite are also excellent. The shape of the rock layers that holds the hydrocarbons is called a trap. The main types are:

• Anticline traps: layers folded upward into a dome shape.
• Fault traps: a fault places the reservoir against an impermeable rock.
• Salt dome traps: rising salt bends the layers around it upward and seals them.
• Unconformity traps: a reservoir is truncated by an erosion surface and covered by a sealing layer.
• Pinch-out (lenticular) traps: the reservoir thins out and its permeability falls to zero, enclosing the hydrocarbons.

The fluids inside a trap are under pressure from the weight of the fluid column, which you can study in formation pressure.

Key Takeaways

• A reservoir rock must store fluids (porosity) and let them flow (permeability).
• Most reservoirs are sandstones and carbonates, which are sedimentary rocks.
• Igneous and metamorphic rocks are reservoirs only when fractured.
• A seal and a trap are needed to keep the hydrocarbons in place.