China appears to have stopped filling up its strategic petroleum reserve (SPR) – removing a big source of incremental demand from the global crude oil market.

Early this year, Beijing shored up oil consumption and prices by diverting millions of barrels of crude oil into its burgeoning emergency reserve. But new data suggest that the buying spree ended in early September.

China does not divulge official information about how it is filling up its reserve, forcing analysts to use rudimentary proxies. The International Energy Agency, for example, measures the stockpiling activity by analysing the gap between oil consumption – as measured by refinery throughputs – and total oil supply – the sum of domestic crude oil production plus net imports.

The IEA says that in the first quarter the gap between China's measure of demand and supply stood at 650,000 barrels a day – equivalent to storing 58m barrels over the three-month period.

In the second quarter, the gap narrowed to 510,000 b/d – or 45m barrels. And in July it fell to 100,000 b/d – or 3m barrels. Data for August and September show that the gap has closed. The Paris-based IEA concludes the figures "imply that filling has come to an end".

"Evidence suggests that the filling of recently completed Chinese strategic storage capacity has ceased," the IEA's oil market report said.

The estimates by the IEA suggest that China diverted 106m barrels into its SPR in the first seven months of the year.

The assessment is in line with estimates in the private sector of around 100m barrels. Industry executives believe that Kazakhstan and Russia provided the bulk of the oil for the reserve.

China's SPR will be the world's second largest by size, behind the US, after its completion in 2020. Beijing ordered a decade ago the construction of its stockpile – often considered a "war reserve" – because of its use by Western countries during conflicts, such as the Gulf War in 1990-91 and the Libyan war in 2011.

In 2001, China's tenth five-year plan (2001-2005) called for the establishment of a national strategic oil stockholding system to improve the country's energy security.

The Chinese SPR is being developed in three phases. Beijing built the tank farms for the first phase, with a capacity of more than 100m barrels, between 2004 and 2008. By the end of 2010, China had filled up the four storage centres, all located near big refining centres on the east coast.

The construction of the second phase, with a capacity of over 200m barrels, is under way – and tanks are being filled up as they are built. Liu Tienan, head of the China National Energy Administration, said that China would "push forward" with building storage sites for the second phase.

The construction – which involves several major storage centres deep in the interior of the country – is expected to be completed in 2013, with crude arriving in 2014.

The third phase, with capacity of around 200m barrels, is scheduled to be built between 2015 and 2018, with crude filling up the tanks until 2020.

By then, China's reserve would contain around 500m barrels of crude, compared with the 700m barrels that the US stores in several caverns.