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A 1950s radiometric survey of the giant Redwater oil field in Alberta, Canada, showed that such surveys, if properly conducted, yielded anomaly maps that correlated well with the Redwater’s underlying hydrocarbon reservoirs. Gamma-ray spectrometry (GRS) provides a nondestructive method for measuring the proportion of naturally occurring radioactive elements within a material. Successful application of airborne gamma-ray spectrometry (AGRS) has been demonstrated in a variety of geologic disciplines. Although applications to hydrocarbon exploration have also been reported in the literature for decades, the effectiveness of the technique has been the subject of controversy. Little effort has been made by the North American oil industry to utilize the potential benefits of GRS in the exploration of onshore hydrocarbon deposits. GRS can assist hydrocarbon exploration in a variety of structural settings by mapping potassium, uranium, and thorium anomalies associated with hydrocarbon deposits. Many of the potassium anomalies observed over oil fields and mineral deposits are related to formation of illite and hydromicas (muscovite).

In 1956, a ground radiometric survey was undertaken to evaluate radiation anomalies detected by airborne surveys over the giant Leduc (Upper Devonian) hydrocarbon pool at Redwater, Alberta, Canada. On the basis of the ground results, soil correction factors were developed and applied to the airborne total-count and radium-count data. The resulting residual anomalies correlated well with the Redwater hydrocarbon pool. Radon, uranium, and zinc halos were also observed across the Redwater pool and were explained by the presence of annular fracture zones resulting from compaction, which promoted vertical migration of hydrocarbon microbubbles, other gases, water, salts, and ions. Carbon dioxide could act as a carrier gas facilitating these migrations.

The Redwater reservoir is a coral reef. During basin sedimentation, internal competency contrasts produced differential compaction, which resulted in development of fractures. Similarly, at Ten Section oil field in California, radiometric-low anomalies coincide with the limits of the original gas cap, probably because of the presence of annular fractures along the flanks of the Ten Section anticline, which may have provided avenues of migration. The zones of fractures (“rabbit ears” in profile view) are inclined at the Redwater field and are vertical at the Ten Section field. On the surface, the points of emergence of these fractures are marked by the accumulation of hydrocarbons, water, salts, and ions that form the observed radiometric and geochemical anomalies. Hydrocarbons and H2S leaking to the surface produce a dramatic decrease in Eh (oxidation potential), resulting in the precipitation of uranium and other multivalent elements (Ni, V, Co, Mn, and others).

Various geochemical anomalies result from diverse physicochemical conditions involving an interchange of hydrocarbons, salts, water, and gases migrating toward the surface and focused along macrofractures and microfractures. The pattern of migration depends on the nature of the trap (reef, anticlinal, stratigraphic, or fault) and will influence anomaly shape. A carrier gas can support long-distance transport of radon, other atoms, molecules, or small aggregates of matter, via microbubble streams that migrate vertically or along fractures, faults, and sedimentary features. Trace Au anomalies have been observed in geogas and in waters overlying deep faults. Several elements may be transported as halides, arsenides, organo-metallic complexes, or colloids. At the surface, these materials may be adsorbed on organic matter or oxide coatings on mineral grains. Halogen halos (Br, Cl, I) have been observed over oil and gas pools. During increased diastrophic activity (earth tides, earthquakes, and storms), the hydrocarbons, gases, and water would migrate rapidly. During quiescent periods, migration would be slower.

Although work was conducted more than 40 years ago at the Redwater oil field and at the Ten Section field in California and the L’Assomption prospect in Quebec, their case histories provide a valuable scientific basis for evaluating many factors that affect the interpretation of radiometric surveys as well as most other geochemically based surveys. A method is discussed that compensates for variations in soil types, vegetation, and drainage, to improve the application of radiometric surveys.

A mechanism is proposed, on the basis of these case histories and supported by current work by other researchers, to explain the observed geometric shapes and positions of radiometric and other geochemical anomalies relative to the causative hydrocarbon reservoir. This mechanism is presented in Appendix A and appears to be applicable for interpretation of most, if not all, data acquired by surface exploration methods.

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