Assuming DEC means decatherm.
A therm is 100,000 BTU's (look at your home gas bill, it is usually in therms)
Deca is times 10
Decatherm is 1,000,000 BTU's
At a typical heating value of 1000 BTU's per cubic foot of natural gas, 1 Decatherm = 1,000 Cubic Feet or 1 MCF