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Surface geochemical haloes are a well-documented phenomenon associated with oil and gas reservoirs. They are they consequence of upward-migrating fluids and geochemically redistributed components. Migration of these geochemical components is controlled primarily by microseepage, diffusion/effusion, and chemical dissolution and precipitation of variously soluble mineral and elemental species derived from hydrocarbons and brines.

Radon comes directly from radioactive decay of radium, a daughter of uranium. In reservoir systems, uranium tends to remain in residual hydrocarbon fluids, whereas radium tends to associate with brines. Upward microseepage of brines transports and precipitates radium within reduced geochemical columns. Positive radon anomalies over reservoirs are associated with transported subsurface and near-surface radium.

Extremely low detectability (10^-12ppb) of radon, specific data acquisition methods, and compensation for background (equilibrium) soil radon produces quite specific radon anomalies. Comparison of radon data indicates that radon signatures are strikingly similar over known producing fields in several regions of the United States. Radon anomalies have been detected over reservoirs at depths greater than 6,000 ft, and relatively high-resolution traverses appear to sharply define reservoir boundaries.

Used alone, or (preferably) in conjunction with other geochemical and geophysical methods, radon appears to be an exploitable tool that can be used to prioritize prospect targeting and subsequent exploration drilling.

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