Research Document - 2002/129
Biogeochemical Benchmarks for Source Identification of Contaminants from an Offshore Oil and Gas Industry.
By Cretney, W., Yunker, M., and Yeats, P.
Abstract
Crude oils are complex mixtures of tens of thousands of compounds. Because numerous structures are possible, most compounds can only be characterised by their elemental composition, if they are known at all. Fortunately, compounds occur in classes or groups of similar compounds. Hence, a classification tree can be produced for crude oils and refined products that permits preliminary assessments to be made of their toxicity and physicochemical properties. For example, oils can be chemically separated into an aromatic fraction, which contains most of the toxic components, and a non-aromatic fraction, which is much less toxic. These two fractions may be further separated into boiling point ranges and thence into groups of compounds of similar structure. The order in which the classification is done is arbitrary and can be adjusted to fit the needs of the assessor.
Groups of compounds having similar structure often may be traced to common, naturally occurring precursors. Some members of such groups are known as biomarkers because of their biological origins. Useful biomarkers are biologically and chemically robust compounds that survive degradation when released into aquatic environments. These chemical biomarkers thus can be used to track petroleum products released into the ocean. The presence or absence of particular compounds usually cannot be used to distinguish among crude oils or their refined products, although such distinctions can occur. Rather, it is the compositional differences in biomarker compounds that allow one oil sample to be distinguished from another. Various analytical tools are available to produce biomarker "fingerprints," which like human fingerprints, can be visually compared. Nowadays, however, sophisticated computer-based statistical methods often are used to make comparisons for oil identification purposes.
A major complication in the source identification of oils through their biomarker fingerprints is the presence of a background of the same and similar compounds in the environment. The biological precursors of biomarkers in petroleum are produced continuously in the environment and undergo diagenic transformations in soils or sediments to give compounds nearly indistinguishable from those in petroleum. Eroding coal and shale oil deposits and underwater oil seeps also can introduce many of the same petroleum biomarkers into sediments, biota and seawater.
Having a measure or benchmark of the background biomarker profiles may be crucial for assessing the impact of a catastrophic spill or unequivocally assigning blame for the cumulative effect of many little spills. The Exxon Valdez oil spill has been the showcase for modern source identification studies using biomarkers and statistical methods. The source of petroleum in the spill zone and surroundings, however, is the subject of continuing debate and litigation. Papers are still being published on the impact of the spilled oil, some because no petroleum benchmark for the area was established prior to the spill event. This mistake can be avoided in the case of the Queen Charlotte basin.
During exploration and production drilling, waste muds and cuttings can be released into the marine environment. These wastes contain metals, petroleum and a number of additives whose fingerprint may be superimposed upon that of the natural background. Although volume and density considerations as well as experience indicate that the influence of these compounds would be limited and local, establishing a fingerprint for key compartments such as bottom sediments and surface microlayer would help to resolve future environmental questions.
Petroleum spills, big and little, and wastes from combustion processes and drilling are not the only sources of organic compounds and metals entering marine environments from an offshore oil and gas industry. Over the lifetime of an oil field, produced water may exceed recovered oil by as much as ten-fold. If produced water is not re-injected into oil depleted formations and instead is released at sea, it can represent a major source of organic and inorganic constituents. Source identification of these constituents is necessary to assess the environmental impact of produced water. As in the case of oil itself, the importance of establishing a benchmark for constituents found in produced water should not be underrated. In this regard, widespread alteration of the composition of the sea-surface microlayer is of particular concern.
Accessibility Notice
This document is available in PDF format. If the document is not accessible to you, please contact the Secretariat to obtain another appropriate format, such as regular print, large print, Braille or audio version.
- Date modified: