Petroleum (oil and gas) is so ordinary that it takes an effort to see what an unlikely and marvelous substance it is: raw liquid oil and high-quality flammable gas, pumped in enormous quantities right out of the ground. Only a few generations ago, oil for all uses—fuel, lubrication, nutrition, medicine—was pressed from plant crops or rendered from animal fat. Gas was manufactured from coal. And the geologic resource was almost totally hidden.
Natural seeps of crude oil are not especially rare. But the oil leaking from the ground is usually a highly degraded substance, close to tar. It was at first used locally as a substitute for pitch or as a crude medicine. When drillers learned to tap petroleum at depth starting in 1859, its virtues began to be discovered, and over the next century oil transformed civilization. Natural gas came into prominence at the same time.
How Petroleum Forms
Geologists have learned a lot about petroleum, but we still don't know in complete detail how it forms. Clearly it is derived from the remains of living things, just as coal is. But before dead organic matter becomes petroleum or coal it exists as a material called kerogen. With time in the ground, kerogen matures into an assortment of hydrocarbon molecules of all sizes and weights. The lightest (smallest) hydrocarbon molecules waft away as natural gas, and the heavier (larger) ones make up an oily liquid.
Let's look closer. Petroleum source rocks form at sea, usually in mud that washes offshore (forming shale) or in limestones. A thick rain of dead planktonic algae adds organic remains to this sediment. (On land, woody plant matter predominates and becomes coal.) In both settings, the mixture is buried under conditions of no oxygen. Only a few percent of the world's dead organic matter is preserved this way.
Under these anaerobic conditions underground, the kerogen is transformed into a flammable substance called bitumen. Certainly heat is part of this process; so is the action of anaerobic microbes in the sediment and natural mineral catalysts. The possible combinations are intricate.
Most of the bitumen is eventually cooked into tarry asphalt, releasing hydrocarbon molecules (as well as water and carbon dioxide) out of the source rock as it heats. Heavy oils form first, then light oils. As temperatures rise to and above 100° C, source rocks produce more gas. Being lighter than rocks, petroleum tends to rise upward through fractures and the pores of coarse sandstone beds.
A small fraction of that leakage, perhaps 2 percent, is preserved in large pools wherever layers of impermeable rock like shale or limestone put a tight lid on top of it. In a nutshell, that's the basis of prospecting for oil: locating (1) source rocks, (2) migration pathways and (3) formations that trap the oil. What isn't trapped continues to rise until it enters, and is destroyed in, the oxidizing conditions of the atmosphere or the soil or the water column.
The conventional petroleum reservoir is in a structural trap—a dome or vault of impermeable rock, formed by folding or faulting of the rock layers or by the rise of salt domes, with permeable rocks beneath it. In those permeable rocks there may be a layer of natural gas on top, with petroleum below. Beneath the oil is usually a layer of rock soaked with water or brine.
There are also other unconventional types of reservoirs that are not trapped this way. Shale gas is an example, in which the gas is tightly trapped in the rock until it is fractured.
The key to a reservoir is sponge-like rock with open space between its grains—porosity. The porosity may have existed from the rock's original sediment; it might also arise as groundwater dissolves pores in the rock or as minerals undergo alteration. One major source of porosity is the transformation of calcite to dolomite, which takes up less space, by fluids rich in magnesium.
Besides porosity, there must be high permeability—the connectedness of pores that allows fluid to move easily through the reservoir rock. Permeability, porosity and geologic structure are all of great interest to petroleum geologists.
Reservoirs may come to be under excessive pressure due to tectonic forces. Modern equipment and practices can almost always handle this pressure, but in the past drilling sometimes produced gushers. The 2010 Gulf of Mexico blowout demonstrated that our skill and technology still have their limits.
Producing oil is an intricate art. Oil can be pumped out of the sponge at a certain maximum rate, determined by the viscosity of the oil and the quality of the reservoir. Oil production must be managed carefully to avoid clogging or collapsing the pores, which can prevent a well from accessing much of the reservoir. Pumping too fast, pumping too slowly or interrupting production can all damage an oilfield. It means that more wells must be drilled to fully exploit the reservoir, raising the expense of production.
Drilling curved and horizontal wells into reservoirs is a common technique to increase production. Another involves fracturing the reservoir rock by pumping fluids and sand into it under high pressure. The fluids open cracks, and the sand keeps them open to let out the petroleum. This can overcome low permeability, and in the eastern U.S. states large new reserves of natural gas have been found through such techniques. Treating the wellbore with various acids or solvents can also raise permeability.
Oil Chemistry and Classification
Petroleum is called a fossil fuel, and indeed crude oil has microfossils including pollen and algal cells in it. There are also what you might call chemical fossils—for instance biological chemicals that did not evolve until recently have been found in young oils. But on the whole, oil and gas are purified, transformed products of countless dead organisms from past ages. You might consider petroleum a sort of geologic compost.
Crude oil contains a large set of liquid and solid hydrocarbons. In producing fuels, refineries focus on the alkanes, ranging from pentane (C5H12) to the heaviest long-chain alkanes. The lighter alkanes (methane, ethane, propane and butane) make up natural gas. Refiners sort out and purify these and other petroleum compounds to produce fuels, lubricants and tars. (See more about alkanes from About.com Chemistry.)
Crude oil is classified as light or heavy according to its density. However, it may also be described by its API gravity, a counterintuitive measurement, developed by the American Petroleum Institute (API) and denoted in degrees, that gives heavy oil a low number and light oil a high number. Heavy and light crude are below and above 20 °API, respectively, and the tarriest, densest petroleum is under 10. Oils range from around 10 to around 70 °API, and the most productive light crude is around API gravity 45. Lighter oils than that yield less of the most valuable products.
Crude oil is also called sweet if it has little sulfur in it, or sour if it has a lot. Sweet crude is more desirable because it takes fewer steps to process into fuel and chemical feedstocks.