Formation

There are two theories about how oil is formed within the Earth’s crust, the Abiogenic (or Inorganic) Process and the Organic Process. The Abiogenic theory arose in the sixteenth century, but gained prominence in the early nineteenth century when scientists like Alexander von Humbolt and Dimitri Mendele’ev used the concept to explain the creation of petroleum. It holds that hydrocarbon fluids originate deep within

the Earth, having been either generated by the Earth’s own volcanic activity or deposited there by meteorites during the formation of the planet. Over the course of millions of years, these fluids have made their way towards the surface. The Abiogenic theory fell out of favour in the early 1900s, largely because it did not predict where oil deposits would be found.

The Organic theory likewise originated during the sixteenth century, and it is the most widely adhered to theory today. It holds that oil is derived from organic matter, the decomposed remains of prehistoric plant and animal life, beginning about 560 million years ago. As these plants and animals, primarily algae and zooplankton, died, they settled to the bottoms of ancient depressions, lakes and oceans. The decaying matter was covered with layers of mud and silt, which subjected them to high levels of heat and pressure. The organic material was converted to kerogen (a mixture of

organic compounds within sedimentary rocks). Increasing heat and pressure over a long period of time led to a process called catagenesis, which converts the organic matter into hydrocarbons by breaking the carbon bonds and forming new chemical bonds. Hydrocarbons in different states are created depending on the depth, period of time and amount of pressure. Oil is formed at relatively shallow depths over a period of about one million years and at temperatures ranging from 50° to
150°C (122° to 302°F).

Although most organic material deposits settled in horizontal layers, they seldom stayed that way. Pressures from below force oil deposits upwards, sometimes to the surface where they might form a “seep”, but usually they are trapped under a layer of non-porous or impermeable rock (typically shale, salt

and clay), where the oil forms reservoirs. These reservoirs are then subjected to tectonic pressures as the sedimentary rock of the Earth’s crust moves, folds, twists and lifts. These pressures force the reservoirs into a wide variety of formations, which are accessed by drilling.