ABSTRACT

This paper reports a test of the hypothesis that radioelements are either enriched or depleted over petroleum accumulations. Total count surveys of gamma-ray emissions are subject to signal variations due to differences in soil composition and soil moisture. Methods are presented to reduce the variations due to soil composition and eliminate variations due to soil moisture. We conclude however, that even if hydrocarbon-related anomalies are present they would be overwhelmed by signal variations related to soil composition.

Differences in soil composition can cause: (1) a four-fold change in total count and U, (2) a two-fold change in K, and (3) a thirteen-fold change in Th. Modified U/K, Th/K, and U/Th ratios reduce the variations due to soil composition to ±50% of normal conditions. This permits detection of anomalies as low as 1.0 ppm U, 2.0 ppm Th, or 0.7% K. Statistical detection limit for the surveys is ±0.24 ppm U at 2.5 ppm U concentration.

Despite the sensivity of the detector, it may be difficult or impossible to distinguish the actual source of an emission anomaly. The amplitude and frequency of detected anomalies is no greater over oil fields than elsewhere. This observation and the lack of anomaly repeatability from line to line precludes recognition of a hydrocarbon source. The majority of anomalies are attributed to soil or lithologic sources. Others are responses to uranium mineralization.