Mineralogical studies of Tertiary subsurface sediments in the Niger Delta have shown that smectite, kaolinite, mixed-layer illite/smectite and illite ave the principal clay minerals, with subordinate quantities of chlorite. Clays from the Recent delta have similar mineralogical compositions. Sediments in the eastern Niger Delta are contributed by rivers which drain the Oban and Bamenda Massifs, and the Tertiary volcanics and Cretaceous sediments of the Benue Trough. Sediments in the central and western delta are derived by weathering of the North, Central and SW Nigerian Basement, and also from Cretaceous sediments which are drained by the Niger/Benue river systems. The clay-mineral assemblages of the western, central and eastern Niger Delta sequences differ These differences are thought to be related to geothermal gradients, environments of deposition, source materials, pore-waters chemistry and basin tectonics. Clay-mineral assemblages from nine wells in the Niger Delta are used in this study as a possible means of correlation. A relationship can be established between clay mineralogy and the occurrence of hydrocarbons in the wells investigated.