Crude oil prices fell sharply Friday on word that a jailed Nigerian politician would soon be freed.

But the politician, Moshood K.O. Abiola, rejected the conditions of his release, clouding prospects for ending a month-old oil workers strike in Nigeria, the world's tenth-largest oil producer.Light sweet crude oil for September delivery settled at $19.31 per barrel, down 83 cents at the New York Mercantile Exchange.

Oil workers in Nigeria went on strike July 4 to protest the imprisonment of Abiola, the apparent winner of a 1993 presidential election that was annulled by the military government. The strike has paralyzed many basic services in the West African country and been a source of volatility to the world's energy markets.

Several large oil companies have said their production has been hurt in Nigeria, one of the 12 members of the Organization of Petroleum Exporting Countries. But doubts have grown about the severity of the decline, the specter of which had given a lift to prices.

For instance, Mobil Corp. reported Friday its daily Nigerian production had fallen from 310,000 barrels per day to just under 300,000.

A court on Friday said Abiola, who was to be tried on charges of treason, could be released if he promised not to address any political rally, travel abroad or do nothing to weaken the government. He refused.

In trading for refined products, unleaded gasoline for September delivery settled at 58.21 cents a gallon, down 1.93 cents. Home heating oil for delivery in September settled at 49.18 cents a gallon, down 2.87 cents.

Natural gas prices were down 4.2 cents, with contracts for delivery in September settling at $1.72 per 1,000 cubic feet. Gas prices have been depressed by lower utility usage because seasonal air conditioning demand has been down.

In London, North Sea Brent Blend crude oil for delivery in September settled at $17.86 per barrel, down 56 cents.