ABUJA (Reuters) - Nigeria used $1 billion of its oil savings to supplement government spending in October after revenues for the previous month fell short of its budget forecast, the accountant general said on Friday. Africa's biggest oil and gas producer shares its revenues among three tiers of government each month - federal, state and local - and tops the disbursal up with a withdrawal from its windfall oil savings if there is a shortfall. Government revenues reached 539.55 billion naira in September, up slightly from 525.62 billion naira in August but short of the forecast 623.77 billion, due to oil supply outages, Jonah Otunla said. Oil revenues from Africa's biggest producer, which account for around 80 percent of total government revenue, have been hit by persistent oil theft and pipeline outages this year. The total revenue distributable to the government for October was 568.41 billion naira, plus the $1 billion from the excess crude account, Otunla said. The Excess Crude Account, where the government holds its oil savings, stood at around $4.3 billion prior to the latest withdrawal, down from $9 billion in December last year. Africa's second-biggest economy is becoming increasingly attractive to portfolio investors, but they worry about the government's tendency to squander its oil windfall. Nigeria still has a relatively low debt-to-GDP ratio of around 20 percent, compared with around 50 percent for fellow West African commodities exporter Ghana.