Why Is Sulphur in Fuel?

Represented by the atomic symbol S, sulphur is one of 118 known chemical elements on the periodic table. As well as being a naturally occurring component in crude oil, sulphur is the 10th most common element in the world, found everywhere from bubbling hot springs to smoking volcanoes.

Sulphur is a key parameter used to classify petroleum products, with the terms “sweet” and “sour” used to indicate how much sulphur a particular blend contains. According to categories enforced by the New York Mercantile Exchange, oil containing less than 0.5% sulphur is classed as “sweet” and oil with higher sulphur concentrations is classed as sour.

Why sulphur concentrations matter

Concentrations are used to determine the market value of the oil, as well as suitability for different refineries and its expected environmental impact. Sweet oil is generally more desirable than sour alternatives. This is because when sulphur is released into the atmosphere it interacts with oxygen to form sulphur dioxide, a dangerous pollutant linked to acid rain and other environmental issues. High sulphur levels can also damage machinery, fast-track corrosion and compromise the efficacy of catalysts used during the refining process.

The average oil and gas reservoir contains sulphur concentrations ranging from between 1 to 5%, making sulphur extraction methods fundamental to the oil and gas sector. So, why is sulphur in fuel? Below, we explore why sulphur exists in fuel and how it got there in the first place.

A naturally occurring chemical element

Despite its environmental impact, sulphur is a naturally occurring chemical element found around the world. It’s produced when organic, sulphur-containing substances decay and release the chemical element into the sedimentary environment.

In an article published in the journal Energy Fuels, the authors assert “most sulfur compounds are generated in the early diagenetic period through the reaction of unsaturated organic matter with H2S or elemental sulfur released from sulfate via bacterial sulfate reduction.”

The role of the early diagenetic period

The early diagenetic period is a geological term used to describes the physical and chemical changes that take place in sediments after they have been deposited and are buried deep enough to avoid being affected by surface processes. These changes are usually caused by microbial activity, compaction and interactions between water and rocks.

The authors add that concentrations of sulphur and sulphur-containing compounds could also be influenced by thermal maturation. It’s these early diagenetic period processes that determine sulphur concentrations in crude.

Find out more about sulphur, including where benchmark blends such as WTI and Brent fall on the sweet/sour scale, in 'A Complete Guide to Sulphur in Fuel.'