I'm guessing the wells that are currently on the property are conventional wells (i.e., not fracked shale wells)? As Vagabond76 noted, they're not offering you $1 MM for that $10k income stream that the property is currently producing. They probably think the shale can be fracked to produce many times the volume that is currently coming out of the wells. Either that, or the current density of wells is far below what the property could support.
How much would it cost you to hire a consulting geologist to evaluate the mineral potential? If you've already been offered $1 MM, it's probably safe to say the minerals are worth much more, so spending $10k, $20k, $30k, or whatever to find out the true value probably would be worth it.