A decline in crude processing could increase global gasoline shortages.
Oil firms Shell and Repsol have slashed natural gas use at European refineries as the region seeks to curb demand in case of further supply disruptions from major supplier Russia.
A restriction on flows through the Nord Stream 1 pipeline to Germany prompted the European Union to ask countries to save gas and store it for the northern winter, fearing that Russia will cut supplies altogether in retaliation for sanctions due to the war in Ukraine.
In addition to being burned for heating and power generation, gas plays an important role in refining crude oil into products such as gasoline, jet fuel or diesel.
Shell cut its use at Europe's largest oil refinery, the 404,000 barrel-per-day Pernis site in Rotterdam, by 40% and at its German refineries by 70% in light of the crisis, its chief executive said.
"These are very significant reductions in a material flow and are therefore very welcome as we prepare for winter," top executive Ben van Beurden told reporters.
Further south in Spain, oil and gas group Repsol reduced its refinery gas use by 1 billion cubic meters (bcm), top executive Josu Jon Imaz said.
"We are working hard to focus on security of supply and that is why we have started to drastically reduce gas consumption in our refineries, in our industries," Imaz told a group of analysts on a conference call.
Repsol was able to make the reduction, which represents 3% of total Spanish gas consumption, through measures including the substitution of naphtha and LPG (liquefied petroleum gas) for natural gas in hydrogen plants, Imaz said.
"One bcm is a huge figure," he added.