A decatherm is approximately equal to one million British Thermal Units (MMBTU).
In a perfect world, one million cubic feet of natural gas has the same energy (heating capacity) of a MMBTU.
However, the world is not perfect. Some gas has more value than others. Yours does, since the gas energy is approximately 1.2MMBTU/MMCF, instead of 1 BTU for each MCF. So, that is good news. It is about 20% richer than the standard.
Gas is produced by the cubic foot at standard temperature and altitude. It is sold by its energy content. BTU or decatherm.
PS. A btu has about the heating or energy equivalent of a burning kitchen match. At least it was over 45 years ago in high school.
Buddy Cotten