The head of Russia's state-owned gas company Gazprom says it has a deal to build a pipeline to China, but there are many unanswered questions about the details of the agreement.

On paper, the project — known as the Power of Siberia 2 — would give Russia a way to replace some of the revenue from its decades of selling natural gas to Europe that was lost over its invasion of Ukraine. The pipeline would carry gas from reserves in western Siberia through Mongolia to China.

And what Gazprom CEO Alexei Miller called a “legally binding” memorandum to build the pipeline with the China National Petroleum Co. is a chance for Moscow and Beijing to underline their deepening ties against the United States.

Here are key issues surrounding the Power of Siberia 2 and why it can't completely replace Russia's lost revenue from Europe:

The pipeline would run 6,700 kilometers (4,163 miles) from gas fields in the Yamal Peninsula in western Siberia, past Lake Baikal in eastern Siberia, and then across Mongolia into China. For more than 50 years, Russia earned fat profits sending Yamal gas to Europe through pipelines leading west.

But Russia cut off most pipeline gas to Europe over the war in Ukraine, and the European Union wants to end the remaining trickle of supplies by 2027.

So the new pipeline would be a way to shift those lost gas sales to a big new customer.

Power of Siberia 2 would carry 50 billion cubic meters a year to China, compared with the up to 180 billion cubic meters a year that went to Europe — meaning the new pipeline could only make up part of the lost business. It would supplement a previous, smaller Power of Siberia line that carries gas from different fields in eastern Siberia with a capacity of 38 billion cubic meters per year.

Miller's announcement, which came during a meeting between Russian President Vladimir Putin and Chinese President Xi Jinping, left out key details. There was no agreement on gas prices or even who would finance the pipeline's construction.

Analysts say the announcement was primarily a chance for Russia and China to underline their closer relationship, and for China to snub supplies of U.S. liquefied natural gas that comes by ship.

India is buying Russian oil despite U.S. President Donald Trump retaliating with 25% tariffs on imports, and China's purchases of U.S. liquefied natural gas are blocked by tariffs imposed as part of China's trade dispute with the Trump administration trade. Meanwhile China has started taking LNG shipments from Russia's Arctic-2 terminal, which has been the target of U.S. and EU sanctions.

China's future demand is part of a complex equation involving a shift away from coal, which emits more carbon dioxide, as the swing fuel used to cover peaks in electricity demand that can't be met by renewables such as wind or hydro power.

A faster move away from coal means more gas use over the short term, while a slower coal exit could increase gas consumption. Battery storage to cover demand peaks and nuclear power could also play a role.