In this blog post, we’ll examine the characteristics and uses of fossil fuels—such as oil, coal, natural gas, and oil sands—as well as their depletion, the environmental issues they raise, and trends in the development of alternative energy sources.

What Is Fossil Energy?

Fossil energy, or fossil fuels, refers to fuels formed over long periods of time as the remains of plants and animals buried in the Earth’s crust undergo fossilization; these are primarily hydrocarbons composed of carbon and hydrogen. Just as traces of ancient life—such as dinosaur footprints—are not found evenly distributed across the globe, the reserves of fossil energy, formed from the remains of ancient organisms, also vary significantly by region. Furthermore, fossil energy is a finite resource whose reserves diminish as it is consumed. However, despite this regional disparity and price volatility, fossil energy has served as the primary fuel driving the development of modern society—including the Industrial Revolution—and continues to be utilized as a vital energy source across a wide range of industries. Fossil energy sources are broadly categorized into fossil fuel resources, including oil, coal, natural gas, and unconventional oil resources such as oil sands.

How is petroleum formed and used?

First, the term “petroleum” refers to a mixture of hydrocarbons formed underground in liquid, gaseous, or solid states; however, it generally refers to crude oil in its liquid state and the petroleum products obtained by refining it. The term “petroleum” is derived from the Greek and Latin words “petra” (rock or stone) and “oleum” (oil), and is believed to have been first used by the German mineralogist Georgius Agricola in 1556, when he wrote a book on the extraction and refining of petroleum. Oil is concentrated in specific regions and is produced not only in the Middle East but also in North and South America, Russia, and other areas. In particular, political conflicts or shifts in the international landscape in major oil-producing regions have historically influenced oil supply and prices. For example, geopolitical events such as the Libyan Civil War have served as factors driving fluctuations in international oil prices. However, oil prices do not fluctuate solely due to political factors. Advances in crude oil production technology, the development of new oil fields, production costs, global supply and demand, investment, and the international economic situation also influence oil prices and supply. Furthermore, when producing crude oil from an oil field, it is not possible to extract all the crude oil present underground to the surface. The actual recovery rate varies significantly depending on the characteristics of the oil field and the production methods used; various enhanced oil recovery (EOR) technologies are employed, such as injecting fluids like water or gas, or utilizing carbon dioxide and polymers. Therefore, rather than making a blanket statement that only about 25% of the crude oil can be extracted from a specific oil field, it is more accurate to view the amount of crude oil that can be economically recovered as varying with advances in production technology. In fact, resources that were previously difficult to produce due to low economic viability can now be included in recoverable reserves through the application of new technologies and production methods.
Crude oil is heated at a refinery and then separated into various products—such as petroleum gas, naphtha, kerosene, diesel, and heavy oil—through refining processes like fractional distillation, which utilizes differences in boiling points. Depending on their properties, these substances are used to produce gasoline and diesel for automobiles, asphalt as a raw material for road paving, and liquefied petroleum gas (LPG) for household fuel. Since petroleum is widely used not only as a fuel but also as a raw material in the petrochemical industry, it remains a vital resource in the global economy. However, it is difficult to conclude with certainty that the use of petroleum-based energy will continue to rise sharply, as it did in the past, driven by increased automobile ownership and electricity consumption resulting from the economic development of China and India. Although global oil demand increased in 2025, the rate of growth has slowed compared to previous years, and in China, demand for petroleum products for road transportation is showing signs of stagnation due to factors such as the spread of electric vehicles and increased use of railways. Conversely, in emerging economies, including India, oil demand is projected to continue rising due to factors such as industrialization and growing transportation needs.

How did coal drive the Industrial Revolution?

Second, coal refers to a dark brown, combustible rock formed when terrestrial or aquatic plants from geological eras were deposited, buried, and then metamorphosed over a long period under heat and pressure. Coal is classified into various types based on its properties and degree of carbonization; generally, it is categorized as peat, lignite, subbituminous coal, bituminous coal, and anthracite. Coal was a key driving force behind the Industrial Revolution, which began in 18th-century Britain. This was because coal was used to boil water, and the steam energy generated in that process was used to move pistons and power steam engines. From that point on, coal was extensively used as a vital power source in the industrialization and modernization processes of many countries. Unlike petroleum, coal has also been produced in South Korea. Coal development in South Korea began in earnest during the Japanese colonial period, when Japan developed coal mines for the purpose of exploiting coal resources, and it subsequently served as a crucial energy source during the country’s industrialization. Furthermore, as a relatively abundant resource capable of yielding large amounts of energy, coal was used for a long time as the primary fuel for thermal power generation. However, as various oil-based industries developed and the energy structure shifted, South Korea’s coal industry continued to decline, and regions such as Taebaek in Gangwon Province—which had grown based on coal mining—were significantly affected by these changes in the industrial structure. Meanwhile, although coal remains a globally significant energy source today, its combustion emits not only carbon dioxide but also fine particulate matter, sulfur oxides, and nitrogen oxides; in particular, coal is considered the most carbon-intensive energy source among fossil fuels. In 2024, carbon dioxide emissions from global coal combustion were estimated at approximately 15.8 billion metric tons.

Why has natural gas garnered attention as a clean energy source?

Third, natural gas is a general term for hydrocarbons and non-hydrocarbon substances that occur naturally underground and exist in a gaseous state at surface conditions. Generally, it refers to flammable natural gas composed primarily of methane. Natural gas is widely used as a fuel for households, power generation, and industrial purposes; because it was first widely adopted in urban areas, it was historically referred to as “city gas.” Natural gas is also used in the form of liquefied natural gas (LNG), which is produced by cooling and liquefying gas composed primarily of methane. Unlike liquefied petroleum gas (LPG), which is primarily obtained during the crude oil refining process, natural gas has the advantage of generally emitting fewer air pollutants than coal or oil when burned. However, since natural gas is also a fossil fuel, it emits carbon dioxide, and methane leaks during production and transportation can contribute to the greenhouse gas problem.
Furthermore, since liquefying natural gas requires cryogenic equipment, the liquefaction, transportation, and regasification processes demand substantial infrastructure and costs. Natural gas is produced both onshore and offshore, and natural gas resources also include coalbed methane (produced from coal seams) and gas extracted from tight geological formations such as shale. In particular, advances in horizontal drilling and hydraulic fracturing technologies have played a key role in significantly expanding the production of natural gas in various regions, including the United States, where production was previously difficult.
In South Korea, the widespread adoption of natural gas-powered city buses began in earnest in 2000 as part of a policy to improve urban air quality, and their adoption expanded rapidly around the time of the 2002 World Cup. By June 2002, the number of natural gas buses in service had exceeded 2,000, and by 2009, a significant portion of the nation’s city buses had been replaced with CNG buses. Therefore, rather than viewing natural gas buses as a recently introduced mode of transportation, it is more appropriate to understand them as a mode of transportation that played a crucial role in South Korea’s urban air quality improvement policies of the 2000s.

Why did oil sands attract attention as an unconventional oil resource?

Finally, oil sands are a resource in which bitumen—a heavy hydrocarbon—is mixed with minerals such as sand and clay; they were formerly known as tar sands. Because oil sands consist of highly viscous bitumen mixed with sand and clay, they are more difficult to produce and process than conventional crude oil. Therefore, methods such as separating the bitumen after mining or, in the case of underground deposits, using heat or steam to reduce the bitumen’s viscosity before extraction are employed. These extraction processes may require more energy and cost than conventional crude oil production, and their environmental impact is also a significant consideration. Due to high production costs and environmental burdens, oil sands have historically been a more challenging resource to develop than conventional crude oil; however, they have garnered attention as a new oil resource whenever international oil prices rise or the stability of oil supply becomes a priority. Oil sands are found in large deposits particularly in the Canadian province of Alberta, and Canada is one of the world’s leading producers of oil sands. During past periods of high international oil prices, oil sands development played a significant role in the Canadian economy and energy industry; there were even times when the Canadian dollar was valued higher than the U.S. dollar. However, rather than attributing the rise in the Canadian dollar’s value solely to oil sands sales, it is more appropriate to view it as the result of a complex interplay of various factors, including international commodity prices, the Canadian economic situation, interest rates, and exchange rates.

Why Should Fossil Fuels Be Replaced?

Although fossil fuels have played a vital role in various industries and aspects of daily life, as described above, massive investments are currently being made in the development of renewable energy and low-carbon energy technologies to replace them. One reason for this is that the use of fossil fuels generates air pollutants and greenhouse gases. Furthermore, since fossil fuels are finite resources that take a very long time to form geologically, it is necessary to secure diverse energy sources and new technologies to ensure long-term energy supply. Air pollutants can be classified into primary air pollutants—which are emitted directly into the atmosphere through sources such as factory smokestacks or vehicle exhaust pipes—and secondary air pollutants—which are formed when primary pollutants undergo physical and chemical changes in the atmosphere. Emissions of sulfur oxides and particulate matter associated with coal combustion have declined in some regions due to emission reduction technologies and fuel switching; however, nitrogen oxides and ozone—which are influenced by transportation, industrial activities, and atmospheric chemical reactions—remain significant air pollution problems. Furthermore, today, not only air pollution but also greenhouse gas emissions and climate change resulting from the use of fossil fuels have become major reasons for reducing dependence on fossil energy. The energy sector is one of the largest sources of greenhouse gas emissions that cause climate change, and the combustion of fossil fuels is the primary cause of energy-related carbon dioxide emissions.

Can shale gas be an alternative to fossil fuels?

Recently, shale gas—one of the unconventional energy resources—has garnered attention as an alternative to these fossil fuels. Shale gas is natural gas trapped in rock layers—such as shale formations, which are composed of sand and clay that have solidified over long periods of time—and its main component is methane. Although shale gas is a type of natural gas, it is trapped in rock layers with lower permeability than those of conventional natural gas fields, requiring specialized technology for production. Advances in horizontal drilling and hydraulic fracturing technologies made it possible to access these rock layers and extract the gas, leading to a significant increase in shale gas production, particularly in the United States. In the U.S., in particular, the expansion of natural gas production from unconventional formations—such as shale layers—using horizontal drilling and hydraulic fracturing technologies led to a substantial increase in natural gas supply, which in turn influenced the energy structure of the power and industrial sectors.
In 2012, there were high expectations that this expansion of shale gas production could lead to a resurgence of U.S. manufacturing, and analyses suggested that traditional manufacturing sectors—such as the U.S. petrochemical and steel industries—could enhance their competitiveness by utilizing low-cost natural gas. However, from today’s perspective, it is more appropriate to view shale gas not as an energy source that completely replaces fossil fuels, but rather as an unconventional natural gas resource that has brought about significant changes in U.S. energy production and industrial structure by partially replacing existing coal and oil. Since natural gas is also a fossil fuel, it emits carbon dioxide, and methane leaks occurring during production and transportation are also cited as significant environmental issues. Therefore, in the future energy transition, we must consider not only the use of unconventional fossil fuels such as shale gas but also the expansion of renewable energy, improvements in energy efficiency, and technologies to reduce greenhouse gas emissions. In 2025, demand for coal, oil, and natural gas all increased, but the rate of growth slowed compared to previous years, while the share of low-emission energy sources—including solar power—in the global increase in energy demand continued to grow.