Crude oil is a naturally occurring, complex mixture composed primarily of hydrocarbon molecules, which are chains of hydrogen and carbon atoms. While this substance is not perishable in the traditional sense, it is chemically and biologically dynamic, meaning its quality and composition can change over time. Exposure to the environment, particularly during storage and transport, leads to degradation. These alterations affect its commercial value and make the subsequent refining process more difficult.
Stability vs. Spoilage: Defining “Bad”
Unlike organic materials such as produce or dairy, which undergo rapid biological spoilage, crude oil is chemically stable. Its hydrocarbon structure is relatively inert, giving it a shelf life that can span millennia when trapped underground. The concept of crude oil “going bad” refers not to decomposition, but rather to a reduction in its quality specifications. This degradation impacts the oil’s American Petroleum Institute (API) gravity and viscosity, which are the main determinants of its market price. The loss of lighter, more desirable components or the formation of heavy sludge represents a financial loss rather than a conventional spoilage event.
Chemical Alterations During Storage
One of the most significant non-biological changes is weathering, dominated by the physical process of evaporation. Lighter, more volatile organic compounds (VOCs), which are often the most valuable for refining, escape into the atmosphere when the oil is stored in tanks that are not completely sealed. This loss increases the average molecular weight of the remaining oil, making it denser and more viscous. The oil essentially becomes heavier and less fluid, reducing its overall API gravity.
Another major chemical pathway for degradation is oxidation, which occurs when crude oil is exposed to oxygen. This process is accelerated by heat and involves reactions with the non-hydrocarbon impurities present in the oil, such as sulfur and nitrogen compounds. Oxidation leads to the formation of polar, high-molecular-weight molecules known as resins and asphaltenes. These heavy compounds can drop out of suspension over time, leading to sedimentation.
The accumulation of these heavier components results in the formation of tank bottom sludge, a thick, highly viscous material. This sedimentation decreases the usable volume within storage facilities and necessitates costly cleaning operations. The presence of water accelerates these chemical reactions and physical separation processes. These changes collectively lower the quality of the crude oil, making it harder to pump, transport, and ultimately refine into high-value products.
The Impact of Microbial Activity
Biological factors play a distinct role in crude oil degradation, especially where water is present in storage systems. Hydrocarbon-utilizing microorganisms (HUMs) and sulfate-reducing bacteria (SRBs) thrive at the oil-water interface, using hydrocarbons as a food source. These microbes preferentially consume the straight-chain alkanes, which are the most desirable components, leaving behind less valuable, complex cyclic and branched hydrocarbons.
The activity of SRBs is particularly problematic, as they respire by reducing sulfate ions found in the water layer. Their metabolism produces metabolic byproducts, most notably hydrogen sulfide (\(\text{H}_2\text{S}\)) gas. The generation of \(\text{H}_2\text{S}\) is known as “souring” the oil, which dramatically lowers its commercial grade. This gas is highly toxic, corrosive, and requires expensive removal processes before refining.
Microbial growth also results in the formation of biofilms and thick, slimy biomass. This contributes to sludge formation and can clog pipelines and filters. The presence of these organisms also accelerates the corrosion of metal storage tanks and equipment. Consequently, these microbial communities pose a significant operational challenge and a direct threat to the financial value of the stored crude oil.