<P> Crude oil is a mixture of many varieties of hydrocarbons and most usually have many sulfur - containing compounds . The oil refining process commonly includes hydrodesulfurization which converts most of that sulfur into gaseous hydrogen sulfide . Raw natural gas also may contain gaseous hydrogen sulfide and sulfur - containing mercaptans, which are removed in natural - gas processing plants before the gas is distributed to consumers . </P> <P> The hydrogen sulfide removed in the refining and processing of crude oil and natural gas is subsequently converted into byproduct elemental sulfur . In fact, the vast majority of the 64,000,000 metric tons of sulfur produced worldwide in 2005 was byproduct sulfur from refineries and natural - gas processing plants . </P> <P> In ISO 20815 define downstream in definition section as: 3.1. 8 downstream business process, most commonly in petroleum industry, associated with post-production activities . </P> <Dl> <Dd> <Dl> <Dd> Example: refining, transportation and marketing of petroleum products . </Dd> </Dl> </Dd> </Dl>

Upstream and downstream process in oil and gas