Interesting article on how an oil rig and natural gas generator powered entirely by natural gas could give energy companies a greater choice and benefit the environment: http://business.financialpost.com/2014/09/04/an-environmentally-friendly-drill-rig-new-engines-slash-emissions-costs-associated-with-petroleum-extraction/?elid=drillrig

How's the maintenance cycle on the natural gas gennies as opposed to the diesel?

Absolutely nothing new here, but NG engines normally burn cleaner than both gas and diesel engines and so need fewer oil changes. Engine life is about the same as diesel. Is NG straight from the well much dirtier than normal NG - I don't know.