In the depsits, why is the layer containing petroleum oil and gas above that of water?

Petroleum oil and water are insoluble with each other. Therefore when we mix them both, petroleum oil form layer above water. So, in the deposits, above water there is layer containing petroleum oil and gas. This is based on the principle of " Like dissolves like" . Petroleum and gas are non polar in nature while water is a polar solvent.
Petroleum oil and gases have lesser density  as compare to water due to which they are lighter and float over the surface of water