Schlumberger

Schlumberger Limited is the world's largest oilfield services corporation.

read more here : http://en.wikipedia.org/wiki/Schlumberger.

Baker Hughes

Baker Hughes is the world's third-largest oilfield services company behind Schlumberger & Halliburton, its main competitors.

read more here http://en.wikipedia.org/wiki/Baker_Hughes.

EOG Resources

EOG Resources, Inc. is one of the largest independent (non-integrated) oil and natural gas companies in the United States with proven reserves in the United States, Canada, offshore Trinidad and the United Kingdom North Sea.

see more at http://www.eogresources.com/operations/division_operations.html