If the source rock is the kitchen, the reservoir rock is the pantry — where the hydrocarbons collect after migration and wait to be produced. It is the element of the petroleum system that actually holds the oil and gas we drill into.
A reservoir rock must have two properties: porosity (void space to store fluid) and permeability (connected pores so fluid can flow). Sandstones and carbonates are the most common reservoir rocks.
The two essential properties
For a rock to make a good reservoir, it has to do two different jobs, governed by two different properties:
The percentage of the rock's volume made up of pore (void) space. Porosity determines how much fluid the rock can store.
A measure of how well the pores are connected, controlling how easily fluid can flow through the rock toward a well.
Both are required. A rock can be highly porous yet barely permeable — holding plenty of fluid that cannot easily flow out — or permeable but with too little pore space to store a worthwhile volume. Only when a rock has both adequate porosity and permeability does it make a productive conventional reservoir. The distinction is so important it has its own guide: porosity vs permeability.
Common reservoir rocks
Two rock types account for the great majority of the world's conventional reservoirs:
- Sandstone — made of sand grains with pore space between them. Porosity and permeability depend on grain sorting, packing, and the cement that binds the grains.
- Carbonate (limestone and dolomite) — often built from shells and reef material. Carbonate porosity can come from original pores, fractures, or later dissolution, making these reservoirs more variable and complex.
For hydrocarbons to accumulate in a reservoir rather than pass through, the reservoir must sit within a trap capped by an impermeable seal. Where the rock holding the hydrocarbons is itself too tight to flow — like a shale — it is no longer a conventional reservoir but an unconventional resource that requires hydraulic fracturing. Reservoir quality also drives how much oil recovery methods can ultimately extract.
Built by the team behind OpsFlo — field service & billing software for oilfield service companies. Capture tickets at the wellsite and bill in days, not weeks.
Frequently asked
A good reservoir rock has both porosity to store hydrocarbons and permeability so those hydrocarbons can flow to a well. Sandstones and carbonates with well-connected pore space make the best conventional reservoirs.
Sandstones and carbonates (limestone and dolomite) are the most common reservoir rocks. Sandstones store fluid between sand grains, while carbonate porosity often comes from original pores, fractures, or later dissolution.
Porosity gives the rock space to hold fluid, but without permeability the pores are not connected and fluid cannot flow out. A rock needs both to store hydrocarbons and deliver them to a producing well.