The flammable gas that occurs naturally underground is known as natural gas, a fossil fuel that serves as a major global energy source for heating, electricity generation, and industrial processes. This gas is trapped in porous rock formations deep beneath the Earth’s surface, often alongside crude oil or coal deposits. The combustion of this gas releases significant amounts of energy.
Defining the Gas and Its Main Ingredient
Natural gas is a mixture of gaseous hydrocarbons, but its primary component is methane, a chemical compound with the formula \(\text{CH}_4\). Methane typically makes up 70 to 90 percent of the total volume of raw natural gas extracted from the ground. The methane molecule is the simplest alkane, consisting of a single carbon atom bonded to four hydrogen atoms in a tetrahedral structure.
This simple molecular structure is responsible for methane’s high flammability and energy density. When methane reacts with oxygen in combustion, it produces carbon dioxide and water while releasing a large amount of heat. The gas is highly combustible, but only within a specific range of concentrations in the air, known as the flammability limits. Methane-air mixtures must have a methane concentration between approximately 5 percent and 15 percent by volume to be explosive, which is a key safety consideration.
The Geological Process of Formation
The creation of natural gas deep within the Earth occurs through two distinct processes: thermogenic and biogenic formation. Thermogenic gas, which constitutes the majority of commercially extracted reserves, is formed under conditions of immense heat and pressure over millions of years. The process begins with the deep burial of ancient organic matter, primarily marine microorganisms, which is then chemically transformed into a waxy substance called kerogen.
As the sedimentary rock layers containing the kerogen are buried deeper, temperatures rise, causing the kerogen to crack and break down into oil and gas. Temperatures between 150 and 200 degrees Celsius are necessary to convert this organic material into methane and other heavier hydrocarbons.
Biogenic gas, in contrast, is formed at much shallower depths and lower temperatures through the actions of microorganisms. Certain types of bacteria, known as methanogens, consume organic material in oxygen-poor environments and produce methane as a metabolic byproduct. This process is common in swamps, landfills, and shallow subsurface sediments, which is why biogenic methane is sometimes referred to as “marsh gas.” Biogenic gas is almost entirely methane, while thermogenic gas contains a higher proportion of other hydrocarbons.
Components Found in Raw Natural Gas
Raw natural gas, as it emerges from the wellhead, is rarely pure methane and contains a complex mixture of other substances that must be separated before distribution. The distinction between “dry gas” and “wet gas” is based on the presence of heavier hydrocarbons alongside the methane. Dry gas consists predominantly of methane, while wet gas contains significant quantities of these heavier compounds.
These heavier components are collectively known as Natural Gas Liquids (NGLs) and include:
- Ethane (C2H6)
- Propane (C3H8)
- Butane (C4H10)
- Pentane (C5H12)
NGLs are valuable commodities used as petrochemical feedstocks, heating fuels, and gasoline additives, and are stripped from the methane stream at processing plants. The raw gas mixture also contains impurities such as nitrogen, carbon dioxide (CO2), and water vapor, which must be removed to prevent pipeline corrosion and meet quality standards. A particularly concerning impurity is hydrogen sulfide (H2S), which is highly toxic and corrosive, classifying the raw gas as “sour gas” when present in high concentrations.
Safety Concerns Related to Flammability and Leaks
The flammability of natural gas presents a significant hazard, primarily the risk of explosion or fire if a leak occurs and the gas-air mixture reaches its explosive limits. Because methane is lighter than air, it tends to accumulate in confined spaces near ceilings, and a spark or open flame can trigger combustion. Additionally, natural gas can act as a simple asphyxiant, as a massive leak in a poorly ventilated space can displace oxygen, leading to suffocation.
Methane is naturally colorless and odorless, making leaks virtually impossible to detect by human senses alone. For public safety, regulatory requirements mandate that a distinctive odorant, typically a sulfur-based compound like mercaptan, be added to the gas before it is delivered to consumers. This odorant provides the unmistakable “rotton egg” smell that alerts people to a leak even at very low concentrations, well before the gas reaches dangerous flammability levels. Beyond immediate safety risks, fugitive emissions from leaks are also a potent environmental concern, as methane is a powerful greenhouse gas that contributes to global warming.