Oil and gas currently provide more than half of the world's energy supply, and according to the International Energy Agency, they—along with coal—will continue to be the major sources of energy well into the 21st century.6 OPEC provides about 40% of the world's oil, although its share is slated to grow. While output of conventional oil and gas from areas like Russia and the Caspian region could rise, North America and the North Sea are expected to decline gradually.7

Over the longer term, where oil and gas are found and in what form will undergo a transformation. Where costs were once prohibitive, energy companies are using new technologies to extract oil and gas from existing reservoirs.8 And offshore technologies are allowing companies to find and extract oil and gas in deep ocean water—a location that was considered inaccessible just a few years ago.9 Fossil fuels also exist in unconventional forms—hydrocarbons contained in oil-sands, and even shale are believed to have more energy content than all the oil in Saudi Arabia.10 The catch is that it may currently take more energy and may cost more to extract and produce oil from some of these unconventional forms than would be gained.

Then there's natural gas. Getting natural gas to market takes time and investment. But gas is a cleaner source of energy than oil or coal, and emits fewer greenhouse gases.11 And natural gas will only increase in significance. Demand for gas is projected to grow 2.8 % annually through 2025,12 somewhat faster than demand for oil.

Renewable sources of energy (biomass, solar, wind, hydro and geothermal) are self-replenishing and clean, and currently contribute 15% of the world's energy supply.13 Solar power is expanding as technology improves and its cost becomes more competitive.14 Commercial wind turbines are now harnessing energy in over 65 countries, making wind power the world's fastest growing source of energy after solar.15 And geothermal energy—literally "heat from the earth"—is meeting a significant portion of electrical power demand in several developing countries.16

But each of these sources has its limitations—technologies are still developing and costs are high in many cases. While renewables will continue to play a growing role, they aren't expected to overtake fossil fuels in the near future in meeting global energy demand.17

Nuclear power has its proponents and skeptics, but it may reemerge as a significant source of energy.18 It has been developed extensively in several European countries, and currently provides 75% of France's electricity.19 While concerns over operating safety and waste disposal linger, some environmentalists and governments find nuclear power appealing because it is relatively clean and emits no greenhouse gases.20

Hydrogen also holds some promise in powering fuel-cell vehicles. The issue is finding sources of pure hydrogen. Currently, the least expensive way of deriving hydrogen is by reforming natural gas, which means we will continue to be reliant on fossil fuels. Another way to produce hydrogen is through electrolysis of water, using electricity or sunlight.21

The challenge for the energy sector is to optimize the development and use of all sources of energy, and drive commercialization of new and emerging sources through successful business—while continuing to meet the world's growing energy demands efficiently and with minimal environmental impact.

Oil, gas, and coal will continue to be a significant energy source for decades to come.22 But, over time, they will likely share more and more of the market with other sources of energy.23

The sources of energy are changing as energy companies continue to meet the challenge of fulfilling the world's demand in the near term and well into the future.