Chemostratigraphy and paleoenvironmental analysis of sand-shale formations using core geochemical spectroscopy tools

Modern investigations into spectral gamma-ray (SGR) records enhance understanding of sedimentary environments and paleoenvironmental conditions. This study examines a 111 m sand and shale interval in the Tak-1 well, Niger Delta Basin, to characterize clay mineral types and interpret depositional settings. The evaluation of lithology, mineral composition, and geochemical properties through core slabs investigation and spectroscopic analysis revealed information about depositional settings. Spectroscopic analysis of spectral gamma-ray data identified key elements—potassium (K), uranium (U), and thorium (Th)—to differentiate clay minerals, correlate chemostratigraphy, and reconstruct paleoenvironments. Results show that the lithology consists predominantly of sandstone, with occasional interbedding of shaly sand and sandy shale. Uranium and thorium are abundant in mudrocks, while feldspars, micas, and glauconite dominate sandstones. Th/K ratio cross-plots identified smectite/mixed-layer clays, with low Th/K values indicating potassium-rich minerals such as illite, mica, or feldspar. Sandstones contained potassium-bearing minerals like glauconite and evaporites. High uranium values (10–15 ppm) and low Th/U ratios signified organic-rich source rocks formed under anoxic, reducing conditions. Extremely low potassium values (<2%) indicated terrestrial sandstone deposits rich in feldspar. These geochemical and mineralogical characteristics suggest a transitional depositional environment influenced by both terrestrial and marginal marine settings. Fluctuating sea levels shaped sedimentation, with the study interval primarily reflecting a marine paralic anoxic setting with intermittent deltaic influence.