Name two products obtained from petroleum which can be used as a fuel.

Coal with the highest carbon content is obtained from the organic matter through a number of steps under some atmospheric factors as follows:

Wood  $\underset{\mathrm{Y}}{\overset{\mathrm{X}}{\to }}$ Peat $\underset{\mathrm{Y}}{\overset{\mathrm{X}}{\to }}$ Lignite $\underset{\mathrm{Y}}{\overset{\mathrm{X}}{\to }}$ Bituminous coal $\underset{\mathrm{Y}}{\overset{\mathrm{X}}{\to }}$ Anthracite

$\mathrm{X}$ and $\mathrm{Y}$ refers to:

The image shows petroleum and natural gas deposits.

Why is the layer of oil formed between natural gas and water?