Conventional reservoir
- Also known as
- conventional well, conventional oil
A conventional reservoir is a body of porous, permeable rock in which oil or gas has migrated and collected beneath an impermeable seal, and from which it can flow to a well at economic rates without the large-scale stimulation that unconventional reservoirs require.
What is a conventional reservoir?
Oil and gas form in fine-grained source rocks. In a conventional petroleum system, they then migrate out of the source rock, move upward through more permeable rock and accumulate where a trap stops them: a structure such as a fold or fault, capped by an impermeable seal such as shale or salt. The rock holding the accumulation, commonly sandstone or carbonate, is the reservoir.
Because conventional reservoir rock has good porosity and permeability, oil and gas can flow to a well drilled into it, often a vertical well, without the extensive hydraulic fracturing that tight and shale formations require.
Conventional versus unconventional
| Conventional | Unconventional | |
|---|---|---|
| Where hydrocarbons sit | Migrated into a trap | Largely remain in or near the source rock |
| Permeability | Moderate to high | Very low |
| Typical well | Vertical or deviated, sometimes horizontal | Long horizontal with multistage fracturing |
| Production profile | Often longer-lived, slower decline | Steep early decline |
Producing a conventional reservoir
Conventional fields typically move through stages: primary recovery using natural reservoir energy; secondary recovery, usually waterflooding; and in some fields enhanced oil recovery. Many mature conventional fields produce large volumes of water and have a large population of low-rate wells, including stripper wells.
Frequently asked questions
Yes. Smaller fracture treatments and other well stimulation methods are used in conventional wells, but production does not depend on them in the way it does in shale.
Many older conventional fields are mature, but conventional reservoirs remain a major source of oil and gas worldwide.
Sources
- reservoir; trap; seal, SLB Energy Glossary
- Oil and natural gas resource categories, U.S. Energy Information Administration (EIA)