Oil and gas is booming in Colorado, and that is leading energy companies to rent more and more office space in Denver. These well-off companies want real estate with lots of amenities: prestigious addresses, high floors with views and easy access to public transportation. That's driving up prices and tightening up the market. It's true of cities like Houston, Dallas, Philadelphia and Calgary. But some fear ghosts of the past. Denver experienced a big boom and bust in the 1980s with energy companies retreating along with the price of oil.