Comparison between downhole and core gamma measurements for lithology logging and stratigraphic correlation.

Drilling is one of the best ways of acquiring reliable geological, geophysical and geochemical information of an area. Geoscientists are unable to get enough information from the drill core with their naked eyes, and as a result they resort to using tools that can measure the physical properties and the chemical compositions of core to get a clear understanding of the geology. Natural gamma is a geophysical dataset that is acquired because of its usefulness for lithological logging and stratigraphic correlation. The REFLEX EZ-GAMMATM survey tool from IMDEX was used to acquire downhole gamma data on a select few boreholes at First Quantum Mineral's Kasempa and EROI projects in Zambia. Core gamma measurements using handheld scintillometers were also acquired for comparison and to compliment the downhole data. High quality downhole gamma data were taken every 10 cm in the boreholes, with data being made available in real time for lithological logging and correlation. The downhole gamma dataset was exceptional at picking subtle geological features and was thus used for stratigraphic correlation of deeper boreholes. Stratigraphic correlation was only done on the deep EROI boreholes because of the multiple stratigraphic sequences they intersect, unlike the Kasempa boreholes which are short and all drilled in the same basin. Gamma logs work well at complementing the geologist's lithology logs by fine tuning the contacts and picking thin beds which could not be seen by the geologist.