The future of OPEC is on shaky ground, an analyst told CNBC on Monday, after Qatar abruptly announced it would sever ties with the influential oil cartel after almost six decades. Qatar's Energy Minister Saad al-Kaabi said at a news conference Monday that Doha would leave OPEC on January 1, 2019. The decision comes just days before OPEC and its allies are scheduled to hold a much-anticipated meeting in Vienna, Austria. "This is big," Andy Critchlow, head of EMEA energy content at S&P Global Platts, told CNBC's "Squawk Box Europe." "In the 20 years that I've been covering OPEC, I can't think of anything that is bigger than this (and) that is a more systemic risk to the future of OPEC."

'OPEC doesn't really exist anymore'

Qatar's energy minister said the country would leave OPEC and focus on gas production. He denied the move was linked to an 18-month political and economic boycott of the country. Since June 2017, OPEC kingpin Saudi Arabia — along with three other Arab states — has cut trade and transport ties with Qatar, accusing the country of supporting terrorism and its regional rival Iran. Qatar denies the claims, saying the boycott hampers its national sovereignty. The Middle East-dominated group's final meeting of the calendar year is now expected to be Qatar's last. It has been an official OPEC member since 1961.

Qatar is not a major oil producer when compared to other OPEC members. But, Critchlow said, when looking at the small Gulf country's total energy output, it is on course to produce more than 6 million barrels of oil equivalent per day by 2022. "This is a major supplier of energy to the world," Critchlow said, highlighting Qatar's status as one of the world's largest producers of liquified natural gas (LNG). He also underlined the importance of Qatar's tactful approach to negotiating energy policy with regional rivals in recent years, saying the country had often acted as a "diplomatic bridge" in the Middle East-dominated group. "OPEC really doesn't exist anymore, it is a two-member organization — Russia and Saudi Arabia," Critchlow said. WATCH: Here's what drives the price of oil

Crucial meeting

Expectations are high that OPEC and non-OPEC members will orchestrate a fresh round of supply cuts later this week. It comes after Russian President Vladimir Putin said he and Saudi Arabia's Crown Prince Mohammed bin Salman "agreed to extend" a deal to limit production. "I think OPEC is still alive," Ayham Kamel, head of Eurasia Group's Middle East and North Africa research team, told CNBC on Monday. "(But) I think it has become a different club. Does it become a less effective diplomatic bridge? It has been. The Iranians are not consulted, the Saudis act on their own with the Russians and try to bring everyone along," he added.