Additional (Independent) Companies – U.S.
- Abraxas Petroleum Corp.
- Anadarko Petroleum Corp.
- Apache Corp.
- Berry Petroleum Co.
- Burlington Resources Inc.
- Cabot Oil & Gas Corp.
- Callon Petroleum Co.
- Chesapeake Energy Corp.
- Citation Oil & Gas Corp.
- CMS Energy Corp.
- Denbury Resources Inc.
- Devon Energy Corp.
- Eagle Plains Resources Ltd.
- El Paso Corp.
- Energen Corp.
- EOG Resources Inc.
- Equitable Resources
- Forest Oil Corp.
- Houston Exploration Co.
- Koch Industries Inc.
- National Fuels
- Newfield Exploration Co.
- Noble Energy
- Pioneer Natural Resources Co.
- Pogo Producing Co.
- Ruwco Oil & Gas Corp.
- Questar Corp.
- Stone Energy Corp.
- Swift Energy Co.
- Unocal Corp.
- Vintage Petroleum Inc.
- Williams Companies Inc.
- XTO Energy