The U.S. oil and natural gas industry's business operations are divided into four major business sectors: exploration and production, refining, transportation and marketing. Following is a brief overview of how environmental expenditures have been allocated across these sectors.

• Exploration and Production: The U.S. is the world's third-largest oil and natural gas producer, with more than 500,000 producing wells and approximately 4,000 oil and natural gas platforms operating in U.S. waters. Combined, they produce almost 2 billion barrels of oil a year. Technologies developed by the oil and natural gas industry have propelled the safety of drilling and exploration operations into the fringes of tomorrow.
   
• Refining: The nation's 144 refineries, which process more than 17 million barrels of crude oil every day, are upgrading their operations to produce ever-cleaner fuels and meet the growing variety of state and local mandates for fuel formulation.
   
• Transportation: Oil and natural gas products move from well to refinery to market through a network of tanker ships, pipelines, barges and tanker trucks. Among the industry's key environmental investments in this segment: by 2015, all tankers and barges operating in U.S. waters will feature double hulls, which help to contain cargo and prevent spills in the event of a collision or grounding.
   
• Marketing: There are more than 165,000 service stations operating across the United States. In recent years, the oil and natural gas industry has made a major investment to upgrade underground storage tanks and install leak detection devices, helping to protect communities and groundwater supplies.