In chemistry terms , cracking is a process by which complicated organic molecules ( such as kerogens or heavy hydrocarbons ) are broken down into simpler molecules ( e.g. light hydrocarbons ) by breaking their carbon-carbon bonds . In petroleum geology and chemistry , cracking is the process whereby complex organic molecules such as kerogens or heavy hydrocarbons are broken down into simpler molecules such as light hydrocarbons , by the breaking of carbon-carbon bonds in the precursors .