What is Petroleum?
Petroleum (or crude oil) is a complex, naturally occurring liquid mixture containing mostly hydrocarbons, which contains compounds of oxygen, nitrogen and sulfur as well. It is also termed fossil fuel, formed by natural anaerobic decomposition of buried dead organisms, the age of which is typically millions of years, at times exceeding 600 million years. There is a general conception that when it is found under solid ground, that ground will be part of a desert, as is mainly the case today, with most petroleum reserves found in the sands of Saudi Arabia and the Middle East countries.
What is not known is that only a small portion of this crude oil came about as a consequence of decomposing dead organisms buried under solid ground. Most of the crude oil actually migrated to underground locations from under the seas! 70 percent of the earth’s surface is covered by water and the living organisms therein far exceed those on and under solid ground. The undersea organisms comprise of ancient fossilized organic materials, such as zooplankton and algae. Vast quantities of these remains settled to sea or lake bottoms, mixing with sediments and being buried under anoxic
conditions. As the number of layers increased with time, their density increased, causing a build up of intense heat and pressure in the lower regions. These conditions changed the organic matter into a waxy material known as ‘kerogen’. Petroleum is formed by the breaking down of large molecules of fats, oils and waxes that contribute to the formation of kerogen.
Because petroleum is a fluid, and also due to continuous geologic tectonic movements, it is able to migrate through the earth as it forms. This migration is slow, over millions of years. Hydrocarbons migrate because oil and gas are less dense than water, so they try to rise toward the Earth's surface to
get above groundwater. Natural gas, being less dense, floats above the oil. This buoyancy tends to drive both oil and gas upwards. Typically, a hydrocarbon system must have a good migration pathway, such as a set of permeable fractures, in order for large volumes of hydrocarbons to move6. Oil companies pray for the absence of migration pathways, so that the oil and gaseous bodies become static pools.
Geological surveys look for large pools; extraction from small pools is not cost-effective, given the extremely high rates for leasing deepwater oil rigs (between US$ 450,000- 800,000 per day) and other working costs. To remain static, the pool or reservoir must be trapped by a non-porous rock
formation. The reservoir needs to have a cover of impervious rock that will prevent the passage of hydrocarbon fluids to the surface. This impervious rock covering the reservoir is called a cap rock.
A hot and wet climate is conducive for the growth of large amounts of organisms. If this growth takes place in a shallow sea, the drying out of the environment and evaporation of the sea water leaves behind large deposits of salt. Salt is impervious to hydrocarbon fluids and makes an excellent cap rock. If migration is prevented by a geological folding of subsurface rocks, very large reservoirs are formed.
These were precisely the conditions that prevailed for eons in the Middle East, resulting in the enormous deposits of oil found in that region of the world10. Again, these are precisely the conditions that prevail in most tropical and subtropical continental shelves. The atmospheric and subsea conditions
in the Gulf of Mexico are ideal for oil pools from which the ‘black gold’ can be extracted gainfully.