Where does petroleum occur?

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: