Petroleum reservoir

Not to be confused with Petroleum reserve.
A structure map, looking downward, generated by contour map software for an 8,500-ft-deep gas and oil reservoir in the Erath Field, Erath, Louisiana. The left-to-right gap near the top indicates a fault line between the blue and green contour lines and the purple, red, and yellow lines. The thin red circular line in the middle indicates the top of the oil reservoir. Because gas rises above oil, this latter line marks the gas-and-oil contact zone.

A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in porous or fractured rock formations. Such reservoirs form when kerogen (ancient plant matter) is created in surrounding rock by the presence of high heat and pressure in the Earth's crust.

Reservoirs are broadly classified as conventional and unconventional reservoirs. In conventional reservoirs, the naturally occurring hydrocarbons, such as crude oil (petroleum) or natural gas, are trapped by overlying rock formations with lower permeability, while in unconventional reservoirs the rocks have high porosity and low permeability, which keeps the hydrocarbons trapped in place, therefore not requiring a cap rock. Reservoirs are found using hydrocarbon exploration methods.