Mineralogical and chemical studies have been made of shale cuttings from the Akiri‐1 well in Oligocene — Miocene sediments of the Niger Delta. The<2μm fraction from the 2,183 to 3,720m stratigraphic interval was analyzed by X‐ray diffraction. Kaolinite and chlorite are more abundant than other clay minerals. The illiteh mectite composition changes from a phase with 32% illite layers, to one with 82% illite layers over the 3,250 to 3,720m stratigraphic interval. Kaolinite decreased in shallow shales from 22 to 16% and increases to 29% in deep shales. Quartz increases in abundance with depth, while chlorite and plagioclase feldspar show no systematic trend with depth. Calcite is present in small amounts in all the samples.Variations in the bulk chemical composition of true shale with depth only show significant changes in the K2O and Fe2O3content: other major chemical constituents show no systematic variations. The potassium appears to be derived from the pore water. The loss of smectite and abundance of Fe coincided with hydrocarbon occurrence. On the basis of the apparent systematic transformation of smectites to illites through mixed layers, the illite/smectite reference interval in the subsurface has been established: this interval coincided with hydrocarbon occurrence.Mineralogical and chemical changes observed are dependent on the pore system chemistry, the temperature, and possibly compaction e