An OPEC-led production cut may well be accelerating a drawdown in global oil stocks that began last year, but implementing the reduction for just six months means the producer group will fall short of achieving its objective of rebalancing the market.

The Organization of the Petroleum Exporting Countries and non-OPEC producers in December reached their first deal since 2001 to curtail oil output jointly, by around 1.8 million barrels per day, according to Reuters.

In the months leading up to the deal and after it was struck, OPEC ministers said tackling an overhang in crude and oil product inventories that has depressed oil prices for over two years was one of their main objectives.

So far, OPEC kingpin Saudi Arabia, which is contributing the biggest chunk of the cut, has said the deal does not need to be extended beyond a six-month period.

This contrasts with Iran, whose oil minister Bijan Zanganeh said OPEC should cut production further in the second half of 2017. Under the deal, Iran was allowed to boost output slightly above October levels.

The International Energy Agency (IEA) said inventories of crude, natural gas liquids and oil products in member countries of the Organisation for Economic Cooperation and Development (OECD) remained 286 million barrels above the five-year average of around 2.7 billion barrels. This is despite a draw of 800,000 bpd in the fourth quarter of 2016.

The overhang is almost evenly split between crude and liquids on one side and oil products on the other.

The agency forecasts a stockdraw of 600,000 bpd in the first half of 2017 if compliance with the output deal is maintained at January levels.

Oil supply is expected to grow year-on-year in the first half of 2017 in large non-OPEC producers such as Canada, Brazil and Kazakhstan, BNP Paribas said in a research note this week.

“The key question is the extent of the renaissance of the U.S. shale oil sector, given a continual rise in the rig activity since May 2016 and high hedge ratios for 2017 among producers,” the French bank said.

A threat to the deal’s success could also come from within OPEC as Libya and Nigeria, which are exempt from the cuts, raise output. Libya has added 190,000 bpd of production since October.

Therefore, analysts say the recent agreement may not be enough to secure the rebalancing in the oil market.