In physics, energy is a scalar physical quantity that describes the amount of work that can be performed by a force, an attribute of objects and systems that is subject to a conservation law. Different forms of energy include kinetic, potential, thermal, gravitational, sound, light, elastic, and electromagnetic energy. The forms of energy are often named after a related force. (Wikipedia) Energy units are often used in relation to environmental protection, energy reserves and geopolitics.
You are currently converting energy units from billion ft3 natural gas to megatonne TNT