Houston-based operators Oasis Petroleum Inc., a Lower 48 explorer, and oilfield services (OFS) giant Superior Energy Services on Wednesday filed for voluntary bankruptcy protection to restructure and shrink their considerable debt loads.

Oasis, whose exploration and production operations are focused on the Permian and Williston basins, sought voluntary Chapter 11 protection in U.S. Bankruptcy Court for the Southern District of Texas to reduce debt by an estimated $1.8 billion. 

The list of U.S. explorers and OFS operators has grown substantially since the spring.

CEO Thomas B. Nusz said “historically low global energy demand and commodity prices” had led management to “take decisive action to strengthen our liquidity and overcome the headwinds now challenging both our...