Oil prices fell to their lowest point in 11 years Wednesday, continuing a plunge that has hit U.S. exploration and production companies hard.

At least nine oil and gas companies accounting for more than $2 billion in debt filed for bankruptcy in the fourth quarter, according to OilPrice.com, and falling energy prices have contributed to the collapse of at least one notable law firm, Texas-based energy boutique Burleson. Other firms with major energy practices have laid off lawyers and staffers.