A dec is a short term way of saying e3/m3, which means 1000m3 of gas. 1 e3m3 = 1 dec 1 e3m3 = 1000m3 of gas at standard conditions.
E3 equals 10^3, which is 1000. Therefore, e3m3/day would be 500000m3/hr. times 24 hours/day, which equals 1200000m3/day. The final result would be 1200 e3me/day.
One Deck=35Mcf or 35000 Cubic Feet of gas. I believe Deck or DEK is a slang term for the metric measurement of 1 E3M3 which is equivalent to 35 MCF.