Formation of Oil and Gas Deposits
Oil and natural gas are formed out of organic matter from dead plants and animals decomposing deep within the Earth. Under very specific pressure and temperature, the biologic material is first converted to kerogen, which is then transformed into bitumen and eventually into oil or gas. Hydrocarbon molecules then rise via migration paths – permeable rock – to the surface where they form oil and gas deposits.
For hydrocarbon deposits to exist, the entire system is essential – i.e. source rocks, migration paths and cap rocks or traps. All of these elements were formed by historical geological processes.
Source rocks are represented by claystone and marlstone rich in organic matter (long-decomposed animal and plant material), while migration paths are tectonic disturbances and porous horizons deep under the Earth’s surface.
Petroleum traps consist of porous rocks (reservoirs) that either form vaulted structures or are lithologically limited. These traps are sealed by impermeable rock which prevents hydrocarbons from escaping to the surface.