Here's why:

* Formation: Both oil and natural gas are formed from the decomposition of organic matter (primarily ancient marine organisms) over millions of years under intense heat and pressure deep within the Earth's crust.

* Composition: Natural gas is primarily composed of methane, but it also contains other hydrocarbons like ethane, propane, and butane. Oil, on the other hand, is a complex mixture of hydrocarbons with longer chains.

* Reservoir Coexistence: Oil and natural gas often occur together in the same geological formations. They are typically found in porous and permeable rock layers (like sandstone or limestone) called reservoirs.

While natural gas is the most common, other gases can also be found alongside oil, such as:

* Carbon dioxide

* Hydrogen sulfide

* Nitrogen

These gases are typically present in smaller quantities than natural gas.