File:Texaco bus oil fluorescing.jpg

English: Vintage Texaco bus oil fluorescing under ultraviolet light.

"Petroleum" refers to crude oil and natural gas. The word literally means "rock oil". The term is often used to refer only to crude oil. Historically, petroleum follows coal as the world's principal energy source. Before coal, it was wood from trees. Crude oil is usually measured in arbitrary units called barrels, which are 42 gallons each. Metric-defined units are used in some countries. In contrast, natural gas is measured in cubic feet of gas (cfg). In terms of energy, 6000 cfg is equal to 1 barrel of oil.

Crude oil is a complex mixture of hydrocarbon chemicals in fluid form. Natural gas is the same, except in gaseous form. Crude oil consists of paraffins (= straight-chained & branched-chain hydrocarbon molecules such as heptane), naphthenes (ringed hydrocarbon molecules such as cyclopentane or cyclohexane), and aromatics (such as benzene and toluene). Crude oil and natural gas usually occur together. A specific reservoir can have oil with very little gas, or gas with very little oil.

The origin of petroleum is complex. Considering petroleum makes society function (& without it, society stops), it's unfortunate that most people don't have a clue where it comes from. Petroleum ultimately comes from source rocks, which form by burial and lithification of sediments rich in organic matter - usually muds (clay and silt). The organic matter is from partially decayed, small planktonic algae and protists in ancient oceans. The resulting rock is usually black shale (especially oil shales, such as kerogenites).

With burial and heating, the organic matter in the source rocks "cracks" into simpler chemicals - hydrocarbons. At relatively low temperatures, crude oil is generated. At relatively high temperatures, natural gas is generated.

Once liberated, petroleum migrates, or moves, due to high pressures at depth. Petroleum usually migrates upward and updip. Oil and gas move along fractures, bedding planes, and through porous rocks. Migrating petroleum can reach the land surface or the seafloor to form seeps. Natural seeps can be observed at several sites in California - for example, McKittrick Tar Pits and La Brea Tar Pits.

Often, oil and gas will accumulate in the subsurface in reservoir rocks - porous rocks that have space for the petroleum to reside. Reservoir rocks are usually sandstones, but they can be limestones. In parts of the Middle East, the famous "superperm" reservoir rocks are highly porous limestones with numerous "open" trace fossils.

Petroleum will stop migrating if it is trapped by geologic structures. Petroleum trap geology can be complex. The rocks over a trap have to be tight and impermeable - they are called cap rocks (also called the seal). Cap rocks are usually shales. Once in traps, natural gas sits above the crude oil, which sits above water (usually brine). The simplest type of trap is an anticline - an uparched fold. Other trap types include fault traps, unconformity traps, reef traps, salt dome traps, sandstone pinchout traps, sandstone lens traps, etc.

Discovering and extracting petroleum is complicated, expensive ($$$), and time-consuming. Petroleum exploration usually starts with mapping surface geology to identify structures, such as folds and faults. In modern times, seismic stratigraphy is done, using charges or thumpers (Vibro-Seis vehicles). Waves propagate through layers of the Earth - some wave energy is reflected back to the surface, where it is detected at listening devices called geophones. Computer processing of data results in seismic lines, which show the subsurface geometry of rock layers.

Historically, oil and gas were found by drilling near seeps. Other methods of locating wells included luck, superstition, and pseudoscience (woo).

If a well encounters economically significant quantities of petroleum. the oil is pumped out. Not all petroleum can be extracted, however. it's easier to get most of the natural gas out. Injection of water or carbon dioxide at nearby wells can help flush petroleum into the area of a producing well.

Petroleum has numerous uses in society. The number one use of crude oil is refining to make gasoline, kerosene, diesel fuel, asphalt, plastics, and chemicals. Without it, there's no trucks, no planes, no trains, no anything.

The container seen here contains bus oil-grade petroleum, produced by Texaco by refining natural crude oil.

The fluorescence is caused by electron excitation under ultraviolet light, but the electrons do not stay in an energetically excited state. The electrons quickly give off energy and resume their normal energy levels. If electron energy release is in the visible spectrum of light, the material glows, or fluoresces.