The number of oil and gas rigs working in the United States dropped by 18 this week to total 968, Houston-based Baker Hughes announced. A year ago, the total was 806. Among the major oil-producing states, California, Oklahoma and Michigan each gained five rigs. The previous week, the rig count had recovered from recent losses by posting an increase of 22 to total 986. The rig count, which tallies the number of rigs actively exploring for oil and not rigs producing oil, usually declines during the first few months of the year.