Oil and Gas Modeling Differences??

Hey,

At the highest level, what are the key inputs/differences in building an earnings model for an oil and gas company (e&p) versus a typical model.

I.e. Are cash flows driven simply by expected production based on proven, probable and potential reserves and attributing a probability percentage for those less op. expenses? What are the key metrics? FCF, NAV, Earnings???

I want to model a company and know if im headed in right direction.

Thanks.