Crude oil and natural gas exist in the ground in the pore spaces of reservoir rock, typically sedimentary rock.
Crude oil and natural gas accumulate in the ground in structures known as hydrocarbon reservoirs.
There are five elements that are required for a hydrocarbon reservoir. The absence of just one element will preclude a geological feature from acting as a hydrocarbon reservoir. The timing of the formation of these elements is critical; all elements must be in place before the migration of oil or gas begins. These elements are:
the source rock
the migration path
the cap rock
reservoir quality rock
The source rock is an organic-rich sedimentary rock where catagenesis converts dead organic material, predominantly dead algae and zooplankton, into kerogen and eventually hydrocarbons.
The migration path is a pathway where mature oil and gas migrate from the source rock to the trap. During the migration process, buoyancy is the dominant force acting on the hydrocarbons.
The cap rock is an impermeable rock that prevents lighter oil and gas from migrating vertically away from the trap and towards the surface.
The trap is a specific geological structure that keeps the crude oil and natural gas in place over geological time (i.e., provides four-way closure to the oil and gas). Traps can either be stratigraphic in nature, where the trapping mechanisms are dominated by the layering of sedimentary rocks, or structural in nature, where the trapping mechanisms are dominated by the folding and faulting of the rocks.
The reservoir rock is a rock that has sufficient reservoir quality to allow for commercial crude oil and natural gas accumulations:
sufficiently high porosity to allow for the storage of crude oil and natural gas in commercial volumes;
sufficiently high permeability to allow for the subsurface transport of crude oil and natural gas at commercial production rates.
The prevailing theory is that the hydrocarbons that make up crude oil and natural gas are organic in nature.
Crude oil and natural gas are complex mixtures of four hydrocarbon types:
The phase behavior of the complex mixtures of hydrocarbons are determined by the composition of the mixture and are governed by Gibbs Phase Rule.
The original pressure and temperature of a hydrocarbon reservoir at discovery are determined by the local hydrostatic gradient and thermal gradient, respectively.
Where the original pressure and temperature of the reservoir lies in the phase envelope of the phase diagram determines the type of crude oil or natural gas system that will be produced from the reservoir.