The fundamental linkage between the two is that they are in some ways replacement products for each other. If, for example, heating oil prices rise too high it will encourage people to switch to natural gas heating. Unlike heating oil and gasoline, however, nat gas is not directly tied to crude oil as a distillate. Also, because oil is used in many more ways than gas (think lubricants, plastics, etc.) the factors effecting the supply/demand aspects of the two markets can be considerably different.