It is very simple task. You just plot historic production rates verses time. Make sure units are same i.e. years vs years
fit a line through historic production data and see the trend into future years. This will give you how many more years the field has in production before it is depleted. Production rates decline as a function of time. If you do not have stable production rates then results are not reliable either. Also what ever event drops the production data in the past, it will reflect forecasted results trend. There generally three curve types namely: Exponential, Hyperbolic, Harmonic. Really you can do it yourself. Just plot the data.