Geochemical Characteristics of Radioactive Reservoir Rocks in the Niger Delta Basin: Implications for Source of Radioactivity and Environment of Deposition

Abstract

The major, trace and rare-earth elements composition of core samples of shale and reservoir sandstone litho-units in a section of a sedimentary succession penetrated by Well (X) in the Niger Delta basin of Nigeria have been determined and compared with the gamma log of the section towards establishing the source(s) of the radioactivity in the sandstone units with anomalously high radiation signature referred to as ’hot sand’. The hot sand is characterized by high amount of Th, U, Ba, Hf, Zr and the heavy rare earth elements. This is attributed to higher amounts of heavy minerals in the hot sand. A very significant correlation of the radioactive elements with Zr and TiO₂ suggests that the radioactive elements are hosted mainly by zircon and rutile. Calculated gamma activities contributed by the radioactive elements indicate that the bulk of the radioactivity is due to Th and not U as once suggested by Weber (1971). Similarity in chondrite-normalized abundance patterns in the hot sand and the other sandstones implies common source rock for all the sandstone types. However, marked enrichment in the amounts of the heavy elements, including the radioactive elements U and Th as well as the heavy rare earth elements in the hot sand is indicative of different sedimentary processes in the transportation and deposition of the hot sands. Sedimentary process indicators such as Th/Sc and Zr/Sc suggest that hydraulic sorting was very important during the transportation and deposition of the hot sand. A possible impact of the high amount of Ba, probably from corresponding high amount of barite (barium sulphate) in the hot sand is the reaction of injection fluids with the barite (barium sulphate) in the reservoir rocks. This may cause a decrease in the permeability of the reservoir by precipitating salts in the pore spaces.