It is usually natural gas or methane that is dissolved in the reservoir. Natural gas which is produced along with crude oil is known as associated gas while natural gas which is produced alone is known as dry gas.
Associated gas is also known as Associated Petroleum Gas (APG) as it is produced along with the crude oil.
In a reservoir, natural gas being the lightest of all, is present freely on top of crude oil or dissolved in the crude oil and thus, referred to as associated free gas.
Associated gas reserve is responsible for producing associated gas. This is the associated free natural gas which is produced as an alternate substitute along with crude oil production.
In the 19th century when natural gas was not that predominant, associated natural gas was considered as a byproduct of crude oil. It was flared and vented into the atmosphere or injected back into the reservoir.
However, with increased demand of natural gas these days, E&P organizations make sure that they separate the associated gas from crude so that it can be processed and sold to natural gas marketers.
In a non-associated gas reservoir, only natural gas is produced which is called dry gas.