I worked at a natural gas fired power plant years ago. It was also permitted to run oil. Had about 4 days worth of oil on site and of course tankers could always bring in additional oil if it had to run longer. We only ran it once as a test to make sure it worked. Other times when we got curtailed in the winter due to the shortage of natural gas in the cold, the plant always shut down, wasn't any need to switch to oil as we got the lowest rate in the winter so it was cheaper to just shut it down and the grid didn't need the power. The best rates were in the summer but you never got curtailed on gas during the summer because people weren't using natural gas to heat their homes in the summer. So yeah, you could easily design your plant to run on oil if you wanted to and get it permitted, but then that means you need those storage tanks and the oil to be on standby and they didn't want to pay to do that.