For all the talk of a transition away from fossil fuels, players in the energy sector are still willing to bet there is more money to be made in oil and coal. Major oil companies such as BP BP 0.73% PLC and Royal Dutch Shell RDS.A 0.84% PLC are selling billions of dollars of assets to bolster their finances and reduce carbon emissions, while mining companies including Anglo American NGLOY 0.94% PLC and Rio Tinto RIO 1.51% PLC have exited coal projects.

Snapping up those unloved assets is a band of smaller competitors that wager that fossil fuels will remain the world’s main energy source for years to come, particularly in developing countries, and that underinvestment by larger rivals will further boost commodity prices. For the big companies, these sales generate funds typically used to pay down debt and help them make the case that they are unloading polluting projects as they face growing investor pressure to map out their future in a lower-carbon economy. But, these projects—and their emissions—aren’t going away. Instead, they are being managed by smaller players that often face less environmental scrutiny. Buyers are also betting their fossil-fuel projects have plenty of room to run.

“While I agree that the direction of traffic is one way, toward renewables, I think it’s going to take longer than people think,” said Blair Thomas, chairman of Harbour Energy PLC.

Harbour, which has bought U.K. assets from Shell and ConocoPhillips, COP 2.86% recently went public in London through a merger with Premier Oil HBR -0.80% of the U.K., adding fossil-fuel projects in Asia and Latin America.

Mr. Thomas said the tie-up would make it easier to tap funds to fuel further growth, and that he expects a pullback in investment by major oil companies will support higher crude prices.

“Capital that is not being spent now, is production that the industry won’t have two or three years from now,” he said. “The question is, will renewable penetration happen fast enough so that as demand comes back you don’t create a pinch? I tend to think it won’t.”

The world’s thirst for fossil fuels remains high. In 2019, before the coronavirus pandemic hammered global transportation and industry, oil, natural gas and coal accounted for 81% of global energy consumption, according to the International Energy Agency. That figure is forecast to drop to 76% by 2030, though rising overall demand means using even more carbon-intensive energy.