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The relationship between oil and gas fields and high concentrations of iodine in water is well known. Iodine is a good, indirect surface indicator of areas favorable for oil and gas accumulations. Humic substances are the main initial source of iodine in subsurface waters. At the high temperatures achieved during burial, structural degradation of large molecules of unsaponified organic matter and of insoluble residues and bitumens also provide a source of iodine. Although the iodine background content in formation waters of different geologic ages varies widely, it has no apparent effect on the use of iodine as a pathfinder for potential oil and gas prospects.
Samples are taken from the top 2 to 4 in. (5 to 10 cm) of the soil and are analyzed for total iodine content. In the analysis for iodine, the total iodine content--iodine that is firmly retained by humic soils and iodine that is contained in soluble iodide form--is expressed in weight percent. Depth appears to influence the extent of iodide fixation by clay soils. For example, in arid and humid-temperate soil clays, the extent of iodide fixation increases with the decrease of soil depth. Clays obtained from different depths of soils located within the water table do not record any difference in iodide fixation; however, identification of the sand/clay ratio in the 170 mesh soil sample is important.
High iodine concentrations occur about the perimeter of a surface geochemical anomaly. This surface expression of the reduction chimney, the so-called halo effect, is associated with oil and gas anomalies. A typical anomaly exhibits values greater than two standard deviations above the statistical mean.
Iodine is an effective pathfinder in surface prospecting because of: (1) the simplicity of taking and non-critical handling required of the surface samples, (2) the ability to integrate a detailed survey into an earlier reconnaissance survey, and (3) relatively low-cost analysis permits a greater sampling density, which provides better identification and definition of anomalies.
Iodine, as all geochemical parameters, should not be used by itself, but rather in combination with other geochemical techniques, and the results should be cross-correlated for an optimum confidence level. Iodine analyses are a good cross-check on the validity of radiometric anomalies or magnetic anomalies. They are also a good geochemical tool to use in the reconnaissance mode prior to using more expensive hydrocarbon analyses in the detailed phase of an exploration program.
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