from SFGate.com

Oil prices have hit levels not seen in nearly a year, threatening drivers with another round of increases at the gas pump.

Crude oil prices have topped $70 per barrel for the last week, pushed higher by violence against oil workers in Nigeria and fears of tightening supplies worldwide. Contracts to buy crude for future delivery closed Friday at $72.81 on the New York Mercantile Exchange, the highest price since last August. They slid 0.8 percent Monday to close at $72.19.

Rising crude prices usually mean that motorists will soon pay more for gasoline. The effect isn't immediate and can take weeks to be noticeable. But there are signs it is already happening.

The national average price for gasoline peaked at $3.23 for a gallon of regular in late May and then started tumbling. But the drop stalled around the start of July. In recent days the national average has started to rise, although by mere fractions of a penny. It now stands at $2.97, according to the AAA auto club.

California's average plunged from a record $3.49 per gallon in May to $3.13 now. It too has flattened and could be poised to rise.

"They'll either reverse or at least hold," said John Kingston, who directs oil coverage for the Platts energy information service.

Kingston said many factors are keeping oil prices high, pressuring gas prices. Worldwide crude oil inventories did not build in the year's second quarter the way they usually do at that time of year. Attacks on oil facilities and workers in Nigeria have blocked about 714,000 barrels of oil from reaching the international market every day. Four more oil workers there were kidnapped over the weekend.

"There are real problems in the crude market right now," Kingston said. "Supply is not rising."

The last time oil prices reached this level, California's gasoline prices were about 7 cents per gallon higher than they are today.

But that doesn't guarantee another substantial increase at the pump this summer. Gasoline and oil prices don't always move in lockstep.

Although the companies that refine and sell gasoline base wholesale prices partly on the cost of crude, other factors affect gas prices as well. Gasoline inventory levels and production rates, for example, affect the supply of fuel and can push prices up or down.

This spring's gas price spike was caused by problems at gasoline refineries, not a rise in crude oil costs. When the national average for gasoline peaked in May, crude cost about 11 percent less than it does now.

Michael Lynch, president of the Strategic Energy & Economic Research consulting firm, said gasoline appears to be in a holding pattern.

Prices could resume their fall, he said, if commodities traders and speculators start selling off some of their holdings, which they do when they believe prices have peaked or reached a plateau. Then again, if this year's hurricane season turns out to be more active than the last, oil prices could climb. August, after all, is just weeks away.

"Last year we got all hot and bothered about the hurricane season, and nothing happened," Lynch said. "But that don't mean nothing. Hurricane forecasting is about as reliable as forecasting the price of oil."