Bacteria and the cost of oil

Oil is considered sour if it contains reduced sulfur compounds (sulfides) at concentrations of 1% or greater. High concentrations of sulfides in oil are problematic for a variety of reasons including:

• Hydrogen sulfide is extremely corrosive and can cause damage to the pipes used to transport oil.
  
• When complexed with other metals such as iron, the sulfides can form precipitates that restrict the flow of oil in the pipes.
  
• Sulfides are toxic and cause environmental and health problems in areas where sour oil is produced, processed or burned.

Removal of sulfides is costly and so refining costs for sour oil is significantly greater than for sweet oil.

Souring of oil is exacerbated by the common practice of pumping water into older oil fields to increase the pressure in the fields as a way to increase oil recovery. Depending on the source of the water used, this practice can introduce large quantities of sulfate (SO₄) into the oil/water mixture. Any oxygen present in the water when it is first pumped underground is rapidly consumed by microbial activity. Once the oxygen is gone, anaerobic microbes can contiune to extract energy from the organic mater present by using compounds other than oxygen as terminal electron acceptors. Sulfate reducing bacteria or SRBs are anaerobes that are able to use sulfate as an electron acceptor. The process results in the production of oxidized carbon compounds and reduced sulfur (sulfides). Biocides are often added to the water to inhibit microbial activity but this process is not efficient requiring enormous amounts of toxic compounds to be added to the water to have a lasting impact.

Thus, microbial activity in oil fields contributes the the cost of oil production. Microbial sulfide production is not limited to oil reservoirs. SRBs are also responsible for the 'rotten egg' smell associated with other anaerobic environments such as swamps and septic systems.