The amount of Energy in Crude Oil is 5,800,000 BTUs per barrel (42gal).
BTU stands for British Thermal Unit, and it is the amount of energy needed to heat one pound of water by one degree Fahrenheit.
Many common energy sources are used to generate energy measured in BTUs. The following is an excerpt from a U.S. Department of Energy guide listing the BTUs generated by energy products:
BTU Content of Common Energy Sources