I need to locate a list of all Oil & Gas / Energy Companies based in Denver or operate offices out of Denver.?

I need to locate a list of all Oil & Gas / Energy Companies based in Denver or operate offices out of Denver. I'm trying to do a hard target search for all of the companies however Hoover's won't let me search by location without an annual subscription. Does any one have a list or know where I can get a list of this sector?