Heating value The average number of British thermal units per cubic foot of natural gas as determined from tests of fuel samples. Heavy gas oil Petroleum distillates with an approximate boiling range from 651degrees Fahrenheit to 1000 degrees Fahrenheit. Heavy Oil The fuel oils remaining after the lighter oils have been distilled off during the refining process. Except for start-up and flame stabilization, virtually all petroleum used in steam plants is heavy oil. Includes fuel oil numbers 4, 5, and 6; crude; and topped crude. HT/HP High Temperature/High Pressure. Hydrocarbon A chemical compound that contains a carbon backbone with hydrogen atoms attached to that backbone. What we refer to as petroleum is actually liquid, geologically extracted hydrocarbons, and gaseous geologic hydrocarbons are what we know as natural gas.