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 nearly 910,000 producing oil and natural gas wells operating onshore and approximately 4,900 oil and natural gas platforms operating offshore in U.S. state and federal waters. These wells produce almost 1.9 billion barrels of oil and 23.5 trillion cubic feet of natural gas annually.
   
• Refining: The nation's 149 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: Petroleum 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 164 thousand service stations operating across the United States. In recent years, the petroleum industry has made a major investment to upgrade underground storage tanks and install leak detection devices, helping to protect communities and groundwater supplies.
   
• Other: Non-sector specific corporate programs such as planting trees or junking old cars. Also includes Research & Development expenditures to reduce pollution and emissions.