The policy of keeping unused oil production capacity of close to 2 million barrels per day could be a hurdle in listing the upstream parts of Saudi Aramco’s business.

The global oil giant is more likely to list the downstream business, according to news agency Reuters, which cited the unnamed industry sources and analysts.

“Including upstream oil in a listing would be nigh-on impossible,” Reuters cited an analyst as saying, who declined to be identified. “The spare capacity is a benevolence that is not commensurate with what investors would want a listed company to be doing.”

Aramco for many years has carried a government policy of 1.5 to 2 million barrels per day of spare capacity to be turned on at short notice. No commercial company would do that unless they were paid a fee, an unnamed source familiar with the matter told the news agency. “So there are a few complexities,” he said of the possible listing. “What the outcome will be I don’t know, but I suspect it will be more related to the downstream.”

Aramco, did not respond to an emailed request for comment from the news agency.

Saudi Arabia produced 10.14 million barrels per day of oil in December and held another 2.12 million barrels per day, according to the International Energy Agency.

The kingdom, the biggest oil producer in the Opec’s basket, is waging a price war against the shale producers for its share of the world’s oil trade. It has tapped into the spare capacity to fill supply shortages caused by outages or regional conflicts to control spiking in prices in the past.

Saudi Deputy Crown Prince Mohammed bin Salman al-Saud in an interview with The Economist on 4 January said that a share sale of Aramco is being considered as part of the country’s privatisation drive to raise money amid a slump in oil prices. Crude, the benchmark for more than half of the world’s oil has lost about three quarters of its value since June 2014.

Aramco has crude reserves of about 265 billion barrels, over 15 per cent of all global deposits. Analysts estimate Aramco’s value anywhere between $1 trillion to $10 trillion. There is no timeline specified for selling shares in Aramco. The government will keep a controlling stake if it goes ahead with an IPO, according to its chief executive Amin Nasser said.