The second law of thermodynamics states that when energy is converted from one form to another, it goes from a more useful form to a less useful form. The measure of this change in energy is known as entropy. When the second law of thermodynamics is in effect, entropy increases. An example of this is when someone gets into a hot tub of water. After a while the water gets cold. Where did all the hot water go? The air in the room is warm and your body is warm because the heat energy in the tub transformed to a less-quality energy.
According to the second law of thermodynamics, a barrel of oil is a high-quality energy resource and can only be used once as a fuel source and cannot be recycled. Based on the second law of thermodynamics, when a high-quality energy, or heat, is dispersed into a large amount of matter, it is converted into a less-quality energy. Based on the second law of thermodynamics, after the barrel of oil has been used it no longer retains the same energy, therefore it would not be a useful heating source.