Although oil and gas are legally classified as minerals, they differ from most other minerals in several important respects.
Most minerals (ie. gold, copper, salt, etc.) are defined by their unique chemical composition. Neither oil nor gas has a unique chemical composition. Both oil and gas are complex solutions consisting primarily of organic compounds.
With few exceptions, the structure of an organic compound is based on a carbon atom linked to other atoms (either carbon atoms or atoms of another element) with four covalent bonds (a sharing of electrons between atoms). The simplest such compound found widely in nature is methane (CH4), which consists of a single carbon atom and 4 hydrogen atoms bound together by 4 single covalent bonds. In ethane (C2H6), one of the single carbon-hydrogen covalent bonds in methane is replaced with a carbon-carbon covalent bond. Methane and ethane are the 1st and 2nd members of the hydrocarbon family known as the paraffins whose chemical formula can be expressed as CnH2n+2, and whose structure is based on a continuous chain of carbon atoms linked with single covalent bonds. This continuous chain structure is usually illustrated as follows:
Once the number of carbon atoms in the chain exceeds three, different compounds with the same chemical formula and fundamental binding mechanism but linked through various branching chains become possible. These hydrocarbon compounds, which have different physical properties, are known as "isomers". The number of ways of arranging the branching chains to create isomers increases with the number of carbon atoms in the particular paraffin. Butane (C4H10) has 2 common isomers, pentane (C5H12) has 3, decane (C10H22) has 75. As paraffins containing up to 30 carbon atoms (triacontane (C30H62) have been recognized, the paraffin family itself contains tens of thousands of individual organic compounds each with different physical properties.
The number of carbon atoms in a member of the paraffin series determines whether the hydrocarbon will exist as a gas, a liquid, or a solid, when in isolation. Methane (CH4), in isolation, is a gas under all naturally occurring temperature and pressure conditions. Ethane (C2H6), propane (C3H8), butane (C4H10) and its isomers, in isolation, are gaseous under standard surface conditions (60o Fahrenheit and 14.4 pounds per square inch (psi) pressure) but may exist in either gaseous or liquid phase under the elevated temperature and pressure conditions found in subsurface reservoirs. In isolation, members of the paraffin series with between 5 and 17 carbon atoms in their molecular structures (pentane - C5H12, hexane - C6H14, heptane - C7H16, octane - C8H18, etc. and their isomers) are liquid under standard surface conditions and most temperature and pressure conditions encountered in subsurface reservoirs. Members of the paraffin series with more than 17 carbon atoms are solid under most naturally occurring temperatures and pressures.
In addition to the family of hydrocarbons whose structure is based on continuous chains of carbon atoms linked with single covalent bonds, families based on double covalent bonds, 2 or more double bonds, and triple covalent bonds exist. Furthermore, there is an entire additional group of hydrocarbons in which the carbon atoms are attached to each other in rings rather than chains.
Because of their similar molecular structures, most hydrocarbons are soluble in each other. A typical subsurface 'oil' or 'petroleum' contains hundreds of thousands of different hydrocarbon compounds in a complex liquid solution. Subsurface 'gas' is a complex solution of the same hydrocarbons, but in different proportions. The solubility of the various hydrocarbon compounds in each other depends on the temperature and pressure under which they are confined. Subsurface temperatures and pressures typically increase at the rate of approximately 1 degree and 45 psi per hundred feet of depth. Under the relatively high confining temperatures and pressures of the subsurface, all oil contains normally gaseous hydrocarbons such as methane dissolved in liquid solution, and most natural gas contains normally liquid hydrocarbons such as pentanes dissolved in gaseous solution.
As subsurface oil or gas moves from its point of recovery at the bottom of a producing well bore to the surface, it is subject to a rapid decline in confining temperature and pressure. In the case of oil, the reduction in pressure causes normally gaseous hydrocarbons dissolved in the oil to evolve from solution in the well bore and to be produced at surface as gas. A simple analogy would be the bubbles of carbon dioxide observed when one removes the cap from a bottle of soda pop. In the case of gas, the reduction in temperature associated with the production process causes normally liquid hydrocarbons dissolved in the gas at the bottom of the well bore in the subsurface to condense from solution in the well bore and to be produced at surface as liquid 'condensate'. A simple analogy would be the droplets of water that condense on a cold pane of glass when one blows on the glass.
Because the gas that evolves from oil as it moves to surface in the production process consists primarily of 'lighter' hydrocarbon compounds such as methane and ethane, the composition of the oil changes significantly between the subsurface and the surface. These changes have been summarized as follows1:
Thirty-four percent of the molecules in the 'typical' subsurface oil described above were methane. Some 'heavy' oils contain less than 5% methane whereas many 'volatile' oils contain more than 50% methane. Because all subsurface oils contain methane and other hydrocarbon compounds which are gaseous under standard surface conditions, all oil wells produce measurable volumes of gas at surface. Because most gas in the subsurface contains some normally liquid hydrocarbon compounds such as pentanes, gas wells usually produce some liquid hydrocarbons. In some cases, the quantities of liquid hydrocarbons produced from a gas well are too low to be measured. In these cases, the subsurface pool is referred to as 'dry gas' pool. In other cases, the quantities of normally liquid hydrocarbons in the subsurface gas are so great that the production at surface is dominantly liquid (see "1920's: The Turner Valley Controversy"). If measurable quantities of liquid hydrocarbons are produced at surface from a gas well, the subsurface pool is referred to as a 'wet gas' pool.
A complete compositional spectrum of single phase conventional hydrocarbon pools exists with dry gas pools forming one end of the spectrum and heavy oil pools the other.
If there are more normally gaseous hydrocarbons trapped in a subsurface pool than can be dissolved in the liquid hydrocarbons within the pool, the oil is said to be saturated with gas and both a liquid and a gaseous phase will exist in equilibrium in the pool. The gas phase, being lighter, will overly the 'oil leg' as a 'gas cap'. Due to the fact that oil was the focus of early exploration and development, such pools have come to be known as oil pools, irrespective of whether the gas cap or the oil leg is volumetrically larger or economically more valuable.
The oil and gas 'pool' terminology may conjure up visions of vast subterranean caverns or lakes filled with oil or gas. In fact, subsurface hydrocarbons are normally found in the tiny voids or pores of subsurface rock formations. Even the solidest and most dense rocks have some internal void space or porosity. The degree to which this void space is connected so as to allow fluids to flow through the rock is referred to as the rock's 'permeability'. It is generally accepted that oil and gas pools are formed when hydrocarbons migrating from areas of high hydrostatic pressure deep in the earth to areas of low pressure through porous and permeable rock formations over geological time encounter a permeability barrier which prevents their further migration and results in their becoming 'trapped'.
When a well is drilled into a subsurface pool and perforated, pressure communication is effectively established between the surface and the subsurface and the equilibrium of the pool is disturbed. The pressure in the pool in the area immediately surrounding the well bore is reduced and this causes the oil or gas present in the pool to begin to flow toward the well bore through the tiny interstices connecting the voids or pores in the permeable reservoir rock formation.
Oil and gas are referred to as fugacious minerals because of their ability to flow from one area of a subsurface pool to another in response to production-induced pressure differentials. It is this fugacious character, together with their non-unique chemical composition, which distinguishes oil and gas from hard minerals.
1. Petroleum Reservoir Engineering, Amyx J.W., Bass D.M. and Whiting R.L., Texas A. & M., 1960, p. 295