European energy giants Shell and TotalEnergies reported record results Thursday from their refining divisions, but there are several factors signaling that these gains cannot be sustained.

Shell's products and chemicals segment earned $3.9 billion, up from $550 million a year ago. TotalEnergies earned $3.6 billion, up from $687 million. The windfalls allowed the companies to cut debt and send billions in cash back to shareholders. But there are already signs that the refining boom is starting to wane -- and that could limit how much more these companies' earnings can grow.

Shares of London-based Shell were up 2.2% in late trading, while French oil giant TotalEnergies was up 1.2%.

Refineries pump out fuels derived from crude oil. Crack spreads, which are the margins that refiners make from turning oil into fuels like diesel and gasoline, hit record highs last month. But in the last couple of weeks, there are signs that the spreads are falling. Diesel cracks have come down, to $93 per barrel on Wednesday from $113 on Oct. 13 and nearly $118 last month, according to data from Dow Jones Energy, which is owned by the same parent company as Barron's.

"Triple digit cracks for diesel was not going to last forever....and even with the declines, refining margins are still quite robust," wrote Denton Cinquegrana, chief oil analyst at Dow Jones Energy.

Margins can be impacted by several things. Refiners have to buy the crude oil they turn into fuel, so rising oil prices actually hurt their margins. Lately crude has gotten more expensive because people expect that the Iran War will flare up after the U.S. elections next week. Brent crude, the international benchmark, has been trading around $115 per barrel. Also, margins are narrowing because fuel has become slightly more plentiful as Europe has been using more of its stockpiles and Russia has been exporting more.

Energy companies will almost certainly earn a lot from refining fuel in the fourth quarter, but probably not quite as much as they did in the third. In response to a question about refining results on the company's earnings call Thursday, Shell CFO Sinead Gorman said that in October, the company's refining margins have "come down a little bit relative to where [they were] for the third quarter."

The refining division could have trouble matching its performance in the fourth quarter due to lower margins and the rising cost of shipping oil and products around the world. "We do believe we could see lower realized numbers coming through this quarter given the freight rates," Gorman said.