The Secretary-General of the Organization of Petroleum Exporting Countries has died, Nigerian authorities announced Wednesday.

Mohammad Barkindo, 63, died late Tuesday, a spokesperson for Nigeria’s petroleum ministry told The Associated Press. The reason for his death was not immediately known. The Vienna-based oil cartel known as OPEC that he oversaw did not immediately comment.

Mele Kyari, Managing Director of the Nigerian National Petroleum Company Limited, tweeted the news of Barkindo’s death which he described as “a great loss to his immediate family, the NNPC, our country Nigeria, the OPEC and the global energy community.”

Barkindo, a Nigerian, led the crude oil bloc since August 2016, seeing it through some of its most turbulent times including during the pandemic when oil prices plummeted due to declining demand. During his tenure as head of OPEC, he guided the group, working to keep the positions of its various members unified.

The 13 OPEC member-states have 1.24 billion proven crude oil reserves among them, or 80% of the world's share. Of the world’s total crude oil, OPEC producers’ share of that is just under 38%. OPEC member-states, however, contributed to around 48% of all world crude oil exports last year.

Barkindo's legacy, however, might be most tied to his final years overseeing OPEC as the group entered into an agreement known as OPEC+ with major non-OPEC producer, Russia. That agreement, which is set to expire this year, helped to steady the volatile oil market during the pandemic, though it has come under increased scrutiny and criticism amid current high oil prices and as the U.S. and other Western nations try to squeeze Russia's economy over the war in Ukraine. Brent crude has traded at over $100 a barrel.

Barkindo was wrapping up his tenure at OPEC when he died. He began his career with the Nigerian Mining Corporation in 1982 before holding multiple roles over more than two decades at the Nigerian National Petroleum Corporation. Before leading OPEC, Barkindo served as the Deputy Managing Director of Nigerian Liquefied Natural Gas, a joint venture between NNPC and multinational oil giants Shell, Total and Eni.

Born in Yola, Nigeria, he attended undergraduate university in Nigeria before earning a post-graduate degree in petroleum economics from Oxford University in the U.K. and an MBA from Washington University in the U.S.

In March, Barkindo was named a distinguished follow of the Atlantic Council, which holds an annual global energy forum that he frequently headlined and spoke at.