Tropical Storm Katia, located deeper in the Atlantic, could also pose a threat if it intensifies and swings toward the Gulf.
Wednesday's modest decline means Nymex futures fell 7.2% during the month of August. The contract remains well off its high of almost $115 a barrel last reached in May, amid concerns about the recovery.
Market participants found little encouragement in data released Wednesday by payroll giant Automatic Data Processing, showing private-sector jobs in the U.S. rose by 91,000. Economists expected an August gain of 100,000.
Traders will be closely following Friday's nonfarm payrolls report from the Bureau of Labor Statistics for confirmation of the reading.
Front-month September reformulated gasoline blendstock, or RBOB, settled up 3.62 cents, or 1.2%, to $3.0320 a gallon. September heating oil settled up 0.9 cents, or 0.3%, to $3.0782 a gallon. Both contracts expired with the close of trading Wednesday.