INTERPRETATION OF GAMMA-RAY LOGS OF THE STRATIFIED OIL SHALE SEAM IN THE ATTARAT UM GHUDRAN DEPOSIT, CENTRAL JORDAN

The study analyses gamma-ray logging data (NGAM) collected by the Attarat Power Company (APCO; a subsidiary of Enefit) about 16 coring boreholes drilled in its exploration area in the Attarat Um Ghudran oil shale (OS) deposit, central Jordan during 2008-2013. Data are compared with lithostratigraphic information that is based on the visual and chemical description of cores. The original natural gamma radiation data were not corrected for the (unspecified) drill hole diameter or casing, thus, the amplitudes of signals were incomparable. The study analyses the raw data in order to produce a master log that could be used for stratigraphic purposes within the Attarat Um Ghudran and, possibly, the neighbouring deposits in future. The creation of a master log is based on the data about 16 individual logs and stratigraphic descriptions. It was possible as the deposit is monoclinally layered, and there exist radiogenic anomalies of primary sedimentation origin. A master log was produced by comparing the averaged signals (with radiuses of 1 and 2.5 m) with each other and producing a binary (bar code-like) log based on their relationship. Such a method describes wavelengths of natural gamma radiation intensity for preference, without considering its absolute values. Interpretation was tested comparing gamma-ray logs and stratigraphic data about three drill holes. The results confirmed geologically determined stratigraphic boundaries of layers with reasonable accuracy (the standard error of predicted depths less than or equal to 0.76 m). The method enables one to partly replace the coring-sampling-laboratory testing complex with a more cost-effective percussion drilling followed by gammaray logging.