An oil and gas reservoir is a subsurface pool of hydrocarbons contained in porous or fractured rock formations.  These reservoirs are typically composed of a porous rock such as sandstone or limestone that has the ability to hold significant amounts of oil and/or natural gas within its pore spaces or fractures.   Oil and gas is extracted from these reservoirs through drilling wells and then processing the extracted fluids to separate the oil, gas, and water.  The efficiency of extraction depends on the characteristics of the reservoir rock, the properties of the hydrocarbons, and the technology used in the extraction process.

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Reservoir Classification

Oil and gas reservoirs are classified based on several criteria, including the nature of the hydrocarbons they contain, the geological formations, and the types of traps that contain them.

Based on Hydrocarbon Type

Oil Reservoirs

• Light Oil Reservoirs  -  Contain light crude oil with a high API gravity and low viscosity.
• Heavy Oil Reservoirs  -  Contain heavy crude oil with a low API gravity and high viscosity.
• Extra Heavy Oil Reservoirs  -  Contain even more viscous oil that is more challenging to extract.

Gas Reservoirs

• Dry Gas Reservoirs  -  Contain primarily methane with little to no condensate.
• Wet Gas Reservoirs  -  Contain significant amounts of condensates like propane and butane along with methane.
• Condensate Reservoirs  -  Produce natural gas along with significant amounts of liquid hydrocarbons.

Combination Reservoirs

• Oil and Gas Reservoirs  -  Contain both oil and gas, often with a gas cap above the oil due to the difference in density.

Based on Geological Structure

Structural Traps

• Anticlines  -  Upward-arching folds in rock layers.
• Fault Traps  -  Created by movements along faults that juxtapose permeable and impermeable rocks.
• Salt Domes  -  Formed by the upward movement of salt, which can create traps by deforming surrounding rock layers.

Stratigraphic Traps

• Unconformities  -  Erosional surfaces that create traps by juxtaposing different rock types.
• Pinch-Outs  -  Formed where reservoir rocks thin out or "pinch out" against an impermeable barrier.
• Reefs  -  Ancient carbonate reefs that create traps due to their porous nature.

Based on Reservoir Drive Mechanism

Based on Fluid Phases

Understanding the type of reservoir is needed in determining the appropriate extraction methods and predicting the production behavior of the reservoir.