BP has raised estimates for Saudi Arabia's crude oil reserves by 12%, marking the first major change to the country's estimated reserves since 1989.

In its benchmark 2019 Statistical Review of World Energy, BP recalibrated some Saudi gas reserves as oil, allowing Riyadh to close in on Venezuela's top spot as the world's largest reserves holder.

BP said Saudi Arabia's proved oil reserves were revised to 297.7 billion barrels at the end of 2018 from 266.2 billion a year earlier, only slightly behind 303 billion in Venezuela.

Canada was third with 168 billion barrels, followed by Iran with 156 billion and Iraq with 147 billion.

The increase came after Saudi Arabia started separate reporting of oil, gas and natural gas liquids (NGL) reserves, BP Chief Economist Spencer Dale told reporters.

NGLs were previously included in gas reserves, Dale said. As a result, these fell to 208.1 trillion cubic feet from 283.8 trillion cubic feet in 2017.

Saudi Arabia has become more open in reporting its reserves as it prepared to float its energy company Saudi Aramco. The listing was postponed and is now planned for early next decade.

Riyadh has rarely changed its oil reserves estimates in the past, despite pumping 8-10 million barrels per day.

The last big change in reserves happened in 1989, when they were raised without explanation from 170 billion barrels to 260 billion.

BP also said oil reserves for the United States, which became the world's top producer in 2018, were revised upwards by 22% to 61.2 billion barrels from 50 billion barrels at the end of 2017.

Overall, global reserves were little changed at 1,729.7 billion barrels, roughly equivalent to 50 years of the world's current demand.