Nigeria ? Chevron Corp. temporarily shut down some operations Friday in Nigeria’s offshore waters as the second-largest U.S. oil company scrambled to protect its workers and equipment from rampant violence that threatens to drive up gasoline prices.

The California-based company’s lockup in the Niger Delta came just a few days after gunmen seized four of its American subcontractors from an offshore vessel amid an outbreak of militant attacks that have disrupted Africa’s biggest oil-producing country.

Chevron’s U.S. workers are particularly vulnerable. Nearly 100 foreigners have been kidnapped in Nigeria since the beginning of the year. Most were released after a ransom was paid.

The temporary suspension of some drilling, logistical support and maintenance work won’t immediately affect Chevron’s production in the Niger Delta, according to Friday’s statement.

Chevron produced an average of 137,000 crude oil barrels per day from 30 fields in the region during 2006. That accounted for about 5 percent of Chevron’s total production last year.