What was earth like in the past where oil was more abundant

That source of hydrocarbons seeps up through the earth's layers and slowly replenishes oil sources.

Explanation:

radiative zone

explanation    

radiative zone is the layer of the sun which energy is transferred away from the core by radiation. although, the core is the only part of the sun that produces an appreciable amount of heat through fusion. in fact, 99% of the energy produced by the sun takes place within 24% of the sun's radius.