"We lost our esteemed Dr Muhammad Sanusi Barkindo," NNPC CEO Mele Kyari wrote on Twitter, adding that he died late on Tuesday.
Kyari said Barkindo died hours after meeting Nigerian President Muhammadu Buhari and giving the main speech at an energy summit in Abuja.
His tenure as the head of OPEC has been marked by global shocks that have battered the industry.
"Serving as Secretary General of OPEC for two terms has been the honour of a lifetime. Over the past six years, we have witnessed both challenging and historic moments, which have underscored time and again the importance of cooperation and teamwork," Barkindo said in his keynote speech on Tuesday just hours before his death.
Barkindo said the oil and gas industry is "under siege" and still reeling from the enormous investment losses of recent years.
"In a very short timespan, the industry has been hit by two major cycles – the severe market downturn in 2015 and 2016, and the even more far-reaching impact of the COVID-19 pandemic," Barkindo said.
He was acting OPEC secretary general in 2006 before returning to the position 10 years later and has led the organisation through a turbulent oil market period including steering it towards greater cooperation with non-OPEC oil producers.
Barkindo, 63, a veteran of the oil industry, was due to step down at the end of this month after six years in the top job at the Organization of the Petroleum Exporting Countries (OPEC).
After leaving OPEC, Barkindo was due to join U.S. think tank the Atlantic Council's Global Energy Center as a distinguished fellow, the Council recently announced.
"This tragedy is a shock to the OPEC Family. We express our sorrow and deep gratitude for the over 40 years of selfless service that Mohammad Sanusi Barkindo gave to OPEC. His dedication and leadership will inspire OPEC for many years to come," the OPEC Secretariat said in a Tweet.
Beyond the platitudes, just days ago, Barkindo promoted the idea of allowing Iran and Venezuela to resume oil sales. Both nations have been under strong international sanctions for years because of their roles as state sponsors of terrorism and other acts.
Barkindo was known to have engagement with Iran, and with Russia, where he appeared at the St. Petersburg International Economic Forum - the so-called "Russian Davos" in 2018.
