Radon and Hydrocarbon Prospecting in Basins with Shallow Black Shale Deposits

Darioush T. Ghahremani

Surface geochemical prospecting for hydrocarbon deposits provides a unique method that can be used along with other geological and geophysical data in order to delineate commercial target areas for exploration and development drilling.

Because black shales are found to be not only the source for hydrocarbons, but also for radioactive radon due to decay of uranium present in the shales, any gas migrating upward from deeper sources will carry radon with it to the surface, creating an anomaly. The magnitude and density of such anomalies will be greater where a network of microfractured bedrock enhances the migration of gases to the surface.

Various types of radon gas detectors and gas collection techniques have been used along with TCD and FID gas chromatographs to separate and measure soil gas hydrocarbons and their associated gases. Gas components considered to be important in this regard are the light hydrocarbons (C₁-C₄), H₂, and He.

Laboratory analysis of the soil samples, coupled with comparison between scintillometer readings and radon values in the same sites, suggested that soil gas radon and hydrocarbon anomalies detected at the surface had a deeper origin mainly due to occurrences of (C₂-C₄) hydrocarbons. Soil gas hydrocarbon compositions, as measure by ratios of C₁/C₂, C₁/C_n, and C₃/C₁ × 1,000, were found to be consistent with known samples of petrogenic gases present in the area.

This compositional relationship offers strong support for the viability of radon and hydrocarbon soil gas prospecting techniques in basins with shallow black shale deposits, such as the Michigan, Illinois, and Appalachian basins.

AAPG Search and Discovery Article #91041©1987 AAPG Eastern Section Meeting, Columbus, Ohio, October 7-10, 1987.