Petroleum is a complex mixture of organic compounds divided into crude oil and natural gas. Formed hundreds of millions of years ago, it occurs quite naturally and is the result of plant and animal remains trapped in fine-grained sediments being transformed by heat and pressure to produce gas and oil. Historical records show that as early as 3000 BC, the Sumerians, Assyrians and Babylonians used natural asphalt and bitumen seepages in building mortar, constructing roads and caulking ships. Nowadays, it has even greater import.

Crude oil varies between coalfields, manifesting as a pale yellow, low-viscosity liquid to a dense black ‘treacle’ consistency. Crude oils fall into one of three classifications: tar sands, heavy oils and medium/light oils. These categorisations are determined by density and relative ability to move between sediment grains. Tar sands contain oil which does not flow into a well drilled into the sands, whereas heavy oils do so, albeit slowly and with some human assistance. Medium/light oils freely move through sediments and are readily recoverable through wells.

Petroleum consists of three primary groups of hydrocarbons: paraffins, napthenes and aromatic hydrocarbons. These categorisations are determined (basically) by the complexity of their structure. In less abstract terms, most paraffins are liquids (and very stable compounds), napthenes more complex (with similar properties to paraffins) and aromatic hydrocarbons more complex yet. If crude oil contains appreciable sulphur content (up to 5%) it is known as a sour crude, whereas mere traces or a complete absence would see it described as a sweet crude.

The formation process is similar to that of coal, although chemically many petroleum sources are hydrocarbons, while most of the hydrogen in coal is lost. Coal usually contains more sulphur than petroleum, and so has lost favour in light of recent concerns over air pollution. As a fluid, petroleum can move and so is rarely found in the rocks it formed in (the source rock) unless it is ‘trapped’ by surrounding rocks (reservoir rocks, which assist large-scale excavations). Petroleum-holding rocks are sandstone (est. 60%) and limestone (est. 40%) - the same sedimentary rocks associated with coal. Only infinitesimal amounts have been discovered in fractured igneous or metamorphic rocks as they lack the porous structure needed to successfully hold petroleum.

No-one is entirely sure how sufficient amounts of organic matter could have been so heavily concentrated as to form the large oil and gas deposits that have been found. A popular geological theory is that floating plankton produced much of it - oxygen deficiency, so the theory goes, caused the death of vast amounts of these organisms near the sea floor, causing remains to be subjected to methane-producing decay. The presence of fossil remains in the rocks which contain petroleum could be indicative of this - the debris is rich in organic fats and oils.

Burial depth is divided into three zones which denote increasing pressure and temperature. Diagenesis occurs to depths of only a few hundred metres, where temperatures are generally below 50°C. Highly-active anaerobic bacteria are responsible for the conversion of organic matter into a sticky liquid. Considerable amounts of biogenic gases are produced. Catagenesis occurs at depths of 3.5-5 Kilometres, where temperatures range from 50°C -150°C and pressure to 1500 atmospheres. These conditions compact rock and expel water. The organic matter is continually altered, becoming kerogen and liquid petroleum. Thermogenic gases prevail over here. Below 3.5-4 Kilometres, the early stages of metamorphism occur in a process known as metagenesis (when applied to organic matter). This stage sees the leftover organic matter converted into nearly-pure thermogenic methane or a carbon-rich solid residue.

After determining the conditions in which oil is likely to form and selecting a site, geologists map the above-ground rock formations in order to infer the specific location where geological structures (structural/anticline or fault traps, which rely on the deformation of rock and stratigraphic traps, which rely on varying porosity of sedimentary layers) hold oil. Pre-drilled wells often have sensors lowered into them and logs of mud, electric resistance and acoustics are taken. These indicate the presence of gases and rock type, the level of water saturation and the amount of pore space in and density of a rock layer. Artificially-generated seismic waves are used to provide a map of subterranean structures and modern computers can generate three-dimensional maps, given enough data.

Once a prospective site has been selected, it is drilled to determine whether it is economically viable. A variety of methods are employed, from straightforward rotary drilling (direct and simple boring through rock) to offshore rigs, which often employ slant drilling in order to save expense on the construction of multiple drilling platforms and to reach several oilfields. Once oil or gas is discovered, the drilling rig is usually replaced by a production platform of some variety, which also contains storage facilities.

The first stage is distillation into different parts (fractions). These are then divided into commercially-desired products, most prominently petrol. Crude oil is a mixture of hydrocarbons with many different boiling points, so it can be separated by distillation into different hydrocarbon groups (which will eventually produce gas, petrol, kerosene, diesel/distillate, lubricating oils and asphalt).

Atmospheric distillation occurs in a column at or near atmospheric pressure. The heated (350-400°C) crude oil is piped into the column and liquid and gaseous components separated. Gases settle on a variety of perforated trays - heavier hydrocarbons settle on lower trays. Petrol is formed in the top trays, kerosene and gas oils in the centre and fuel oils at the base. These heavy fuel oils can also be subjected to vacuum distillation, which can separate them without producing unwanted by-products.

Catalytic cracking refers to the process whereby heavy hydrocarbon fractions obtained through vacuum distillation are converted into useful products, such as petrol and diesel. The larger hydrocarbons are chemically broken down under the influence of a catalyst (such as a number of silicates), which is itself not changed. Catalytic reformation utilises heat and a usually-platinum catalyst to bring about chemical reactions which produce high-octane petrol.

We are most familiar with fuels and lubricating oils, but petroleum actually helps in the manufacture of many plastic, rubber and other synthetic products, as well as paints, dyes, adhesives and fabrics. Modern industry is based on a ready and never-ending supply of cheap raw materials - this way of life is not sustainable. New technologies aid in the further exploration and uncovering of new fields, although petroleum is a highly-sought commodity and little practical consideration is given to supply and demand. It is estimated that at our current consumption rate, there are approximately 50 years of crude oil reserves and 70 of natural gas remaining.

Source: senior high school environmental science notes.