Nigeria's influence in OPEC is determined by the volume of her oil revenue. OPEC, which stands for the Organization of the Petroleum Exporting Countries, is a group of oil-producing nations that seek to coordinate and unify their petroleum policies. Nigeria is a member of OPEC, and its influence within the organization is largely determined by the amount of oil it produces and the revenue it generates from oil exports.
As one of the largest oil-producing countries in Africa, Nigeria is a significant contributor to OPEC's total oil output. The country's oil exports account for a significant portion of its GDP and government revenue, which gives it a vested interest in ensuring stable oil prices and market conditions. Therefore, Nigeria's influence in OPEC is determined by the volume of its oil revenue, which gives it the leverage to push for policies that are favorable to its economic interests.
While the other factors listed (sizes of her refineries, accessibility of her oil fields, low sulfur content of her crude) can also affect Nigeria's oil production and export capabilities, they are not the primary determinants of its influence within OPEC. Instead, it is the country's oil revenue that gives it a voice and bargaining power within the organization, allowing it to influence the policies and decisions of the group.