Petroleum reservoir
（重定向自Petroleum system）
![Screenshot of a structure map generated by Contour map software for an 8500ft deep gas & oil reservoir in the Erath field, Erath, Louisiana. The left-to-right gap, near the top of the contour map indicates a Fault line. This fault line is between the blue/green contour lines and the purple/red/yellow contour lines. The thin red circular contour line in the middle of the map indicates the top of the oil reservoir. Because gas floats above oil, the thin red contour line marks the gas/oil contact zone.](/Images/godic/202502/02/Contour_map_software_screen_snapshot_of_isopach_map_for_8500ft_deep_OIL_reservoir_with_a_Fault_line1828.jpg")
A petroleum reservoir or oil and gas reservoir is a subsurface pool of hydrocarbons contained in porous or fractured rock formations. Petroleum reservoirs are broadly classified as conventional and unconventional reservoirs. In case of conventional reservoirs, the naturally occurring hydrocarbons, such as crude oil 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 no need for any cap rock. Reservoirs are found using hydrocarbon exploration methods.