MCF stand for 1,000 cubic feet of gas.
so 290694 MCF = 290 694 000 cubic feet of gas (ft3)
mass = volume * density
You have a volume and you want a mass, so you must obtain natural gas density to find the response.
The natural gas density depend of the pressure and temperature of the gas according the gas law PV=MRT we find that density = M/V = P/RT. Without this data it's hard to find real density.
I found here:
http://www.engineeringtoolbox.com/gas-density-d_158.html that density of natural gas at 1 atm and 0 deg C (32 deg F) is between 0.044 - 0.056 lbm/ft3.
So mass = 290 694 000 ft3 * 0.044 lbm/ft3 =
12 790 536 lbm of natural gas.
This is an example, find the pressure and temperature of the gas and you will have a more precise response.