Natural Gas
Where Does Natural Gas Come From?
Natural gas is a highly flammable, clean-burning fossil fuel. It is a mixture of simple hydrocarbon compounds, predominantly composed of methane (usually 70% to 90%), along with smaller amounts of ethane, propane, and butane.
Because it produces significantly less carbon dioxide and harmful pollutants compared to coal and petroleum, it is widely considered an important “bridge fuel” in the transition toward renewable energy.
The Formation Process (Thermogenic Gas)
The majority of the natural gas we use today is found deep underground and shares its origin story with petroleum. This type of gas is known as thermogenic natural gas. The step-by-step process of its formation is as follows:
- Accumulation of Biomass: Millions of years ago, the remains of microscopic marine plants and animals (like algae and plankton) died and settled on the ocean floors.
- Burial and Compression: Over time, these organic remains were buried under massive layers of silt, sand, and rock. This deep burial completely cut off the oxygen supply (creating an anaerobic environment).
- Heat and Pressure: As the layers of rock thickened over millions of years, the pressure increased immensely. Combined with the intense heat from the Earth’s core, the organic matter was “cooked.”
- Chemical Breakdown: Depending on the temperature, the organic matter turned into different fossil fuels. If the heat was moderate, it formed liquid crude oil. If the temperature was extremely high, the carbon bonds broke down further, resulting in lighter, gaseous molecules—creating natural gas.
Where is it Trapped Underground?
Because natural gas is exceptionally light and less dense than surrounding rocks and water, it naturally migrates upwards through the tiny pores of sedimentary rocks (like sandstone).
However, its upward journey is eventually blocked by a layer of dense, non-porous rock (impermeable rock), creating an underground trap or reservoir. In these geological formations, natural gas is frequently found trapped in a layer directly above liquid crude oil, floating on top of it.
Biogenic Gas (An Alternative Origin)
While thermogenic gas forms deep underground over millions of years, there is another type called biogenic natural gas. This gas is created much closer to the Earth’s surface by methanogens (microorganisms). These tiny organisms break down organic matter in oxygen-poor environments, such as marshes, bogs, and even modern-day garbage landfills. While biogenic gas is the same chemical substance (methane), it is formed much faster and is often captured directly from landfills to generate local electricity.
Conventional vs. Unconventional Natural Gas
Natural gas is broadly classified into two categories based on the geological location where it is found and the technology required to extract it: Conventional and Unconventional natural gas. Chemically, both are the same (primarily methane), but they differ greatly in how they are mined.
Conventional natural gas:
- Conventional gas is accessible because it is trapped under a permeable layer of rock. In most cases, it occurs together with oil reservoir deposits and sometimes it floats on top of the oil, forming a layer on it.
Unconventional natural gas:
- Unconventional gas hiding deep below (4500 meters or more) the surface, making extraction difficult. Economically, it is not always viable to extract unconventional gas types, because it is not cost-effective. Yet, new technologies are developing every day to make natural gas extraction easier. Thus, today’s unconventional natural gas types will become conventional in the future.
Important Types of Unconventional Gas:
Deep Natural Gas types belong to the unconventional category. These are hard to extract for different reasons.
- Shale: Shale is a fine-grained, clastic sedimentary rock, composed of mud and it is a massive mineral. It is so tough that drilling into it is a rather costly process.
- Tight gas: It can be extracted only by fracking and acidizing because tight gas is trapped underground in an impermeable rock formation.
- Coalbed Methane: It can occur aside seams of coal. Historically it was the waste product of coal mining, but in recent decades it has become an important source of energy.
- Gas in Geopressurized Zones: Geopressurized Zones form 3000-7600 meters below the surface. It is hard to extract, but gas processing can be viable because a high amount of natural gas hides in this zone.
- Methane Hydrates: It is trapped in places like the Arctic with low temperature and high pressure. Climate change incites the release of methane hydrates into the atmosphere.
Biogas: It contains less methane gas than natural gas, but still a reliable energy source. It is a type of gas that is produced when organic matter decomposes without oxygen being present.
