What is the OPEC unit of measure?

OPEC, the Organization of the Petroleum Exporting Countries, primarily uses the unit of measure known as "barrels" to quantify oil production and reserves. One barrel is equivalent to 159 liters or approximately 42 U.S. gallons. In its reports, OPEC often provides data in terms of millions of barrels per day (mb/d) to reflect production and consumption levels in the global oil market.