Figure 2–1 An anticlinal reservoir containing oil and (associated) gas.

Table 2–1

Range of Composition of Natural Gas

2.1 Origin

There are many different theories as to the origins of fossil fuels, specifically natural gas and petroleum. The most generally accepted theory is that natural gas and petroleum are formed when organic matter or debris (such as the remains of a plants or animals) is compressed under the earth, at very high pressure for a very long time. Millions of years ago, the remains of plants and animals decayed and built up in thick layers. Over time, as sediment, mud, and other debris piled on top of the organic matter, metamorphosis occurred and the sediment, mud, and other debris were changed to rock, causing pressure to be put on the organic matter. The increasing pressure compressed the organic matter and, combined with other subterranean effects, decomposed the individual constituents into gas and petroleum.

Thus, the events that are believed to have occurred in the formation of natural gas and petroleum are:

The deep-lying source rock that contains the organic precursors is often referred to as the kitchen. In theory, the deeper and hotter the kitchen, the more likelihood of gas being produced. The increase of heat with depth is due to the presence of a geothermal gradient that is generally on the order of 25 to 30°C/km (0.008 to 0.009°C/ft of depth or 8 to 9°C/1,000 ft of depth; 0.015 to 0.016°F/ft of depth or 15 to 16°F/1,000 ft of depth), i.e. about one degree for every one hundred feet below the surface of the earth.

However, this does not mean that non-associated gas will always be found at greater depths than crude oil. Gas and crude oil have migrated from the kitchen, sideways and upwards, until they are trapped in reservoirs in the subsurface formations. Thus, a field may have a series of layers of crude oil/gas and gas reservoirs over a thousand meters subsurface.

The methane (natural gas) produced in this manner is referred to as thermogenic methane (natural gas), because it is presumed that the pressure effects and the increased depth of the precursors include the influence of heat to convert the organic matter to natural gas and petroleum includes the effects of heat. However, the actual temperature is not known and, like the remainder of the theory, is at best speculative. Experimentalists will note that laboratory studies at specified temperatures will convert organic materials to methane and other hydrocarbons gases. Be that as it may, the temperatures used in the laboratory are often high on the basis that the increase in temperature makes up for the lack of geological time in the laboratory. The experimentalists often ignore that higher temperatures change the chemistry of the process and the findings may not be completely true. The fact remains that the thermogenic origin of natural gas and other fossil fuels is not conclusively proven.

This theory answers many questions, namely the fact that gas and oil are only found under a small portion of the earth. While there are other theories, this one seems to be the most widely accepted.

Methane (natural gas) can also be formed through the transformation of organic matter by tiny microorganisms. This type of methane is referred to as biogenic methane (biogenic natural gas). In this process, the methane-producing microorganisms (methanogens) chemically break down organic matter to produce methane. The methanogens are commonly found in areas near the surface of the earth that are lacking oxygen. Formation of methane in this manner usually occurs close to the surface of the earth, and the methane produced is usually lost into the atmosphere. In certain circumstances, however, this methane can be trapped underground, recoverable as natural gas. An example of biogenic methane is landfill gas. Waste-containing landfills produce a relatively large amount of natural gas from the decomposition of the waste materials that they contain. New technologies are allowing this gas to be collected to add to the supply of natural gas.

A third way in which methane (and natural gas) may be formed is through abiogenic processes. Extremely deep under the earth’s crust, there exist hydrogen-rich gases, and as these gases rise towards the surface of the earth, they may interact through the catalytic activity of minerals in the absence of oxygen. This interaction may result in a reaction to form products that are found in the atmosphere (including nitrogen, oxygen, carbon dioxide, argon, and water). If these gases are under very high pressure as they move towards the surface of the earth, they are likely to form methane (abiogenic methane), similar to thermogenic methane.

Because of the manner in which it is formed, and the diverse nature of the precursors, natural gas contains constituents other than methane. While the major constituent of natural gas is indeed methane, there are components such as saturated hydrocarbons (CnH2n+2), aromatic hydrocarbons (derivatives of benzene), carbon dioxide (CO2), hydrogen sulfide (H2S), and mercaptans (thiols, R-SH), as well as trace amounts of gases such as carbonyl sulfide (COS)...