GEOLOGY AND GEOCHEMISTRY OF THE LOKPANTA FORMATION OIL SHALE, NIGERIA

Geochemical and mineralogical study was carried out on the Lokpanta oil shale (OS), Nigeria to reconstruct tectonic and depositional settings as well as paleoredox conditions. The geochemical and paleoenvironmental features of the Lokpanta oil shale were compared with those of the graptolite argillite of Estonia.

Samples from three drill cores and several outcrops were utilized for this study. The mineral phases in OS include calcite, quartz, dolomite, feldspar, illite, kaolinite, halloysite, pyrite and gypsum, as well as a trace amount of anatase. The Lokpanta oil shale shows little variation in geochemistry. It is depleted in trace elements Ba and Rb and major compounds except CaO, and is enriched in trace elements Mo, Sb, As, V, Zn, Ni and U with reference to the Post-Archean Australian Shale (PAAS). These enrichments are, however, in most cases lower than those in the Estonian graptolite argillite (GA). Trace element ratios (U/Th, Ni/Co, V/Ni, V/(V + Ni) V/(V + Cr)) indicate that the Lokpanta oil shale was deposited in an anoxic environment. Discriminant diagrams also suggest its deposition in an active continental margin setting and a transitional to marine environment.