1 Exxon Mobil Irving
2 ConocoPhillips Houston
3 Valero Energy San Antonio
4 Marathon Oil Houston
5 Dell Round Rock
DFW influence will continue to rise due to gas drilling/extraction on the Barnett Shale
There are more energy companies (oil/gas, seismice, oil field service) concentrated in Houston than in Dallas. You are looking at only the largest oil & gas companies. You need to consider medium size companies and the whole spectrum of energy related companies.