SOURCE ROCK EVALUATION AND DEPOSITIONAL ENVIRONMENT OF MIDDLE EOCENE – EARLY MIOCENE SEDIMENTS IN UMUAHIA AND IT’S EVIRONS, NIGER DELTA BASIN, SOUTH EASTERN NIGERIA.
ABSTRACT
The study area is bounded by latitudes 5027’N and 5034’N, and longitudes 7025’E and 7035’E. It falls within the Northern Niger delta Basin. The aim of this present study is to evaluate the hydrocarbon potentials of the sediments in the area, determine their age and environment of deposition. To achieve this aim, geochemical analysis (TOC and Rock-Eval) and Palynological studies were carried out on fifteen (15) samples. Results from the analysis indicates that the total organic carbon (TOC) values of the lignites range from 15.65 – 48.61wt.%, averaging (34.23wt.%), while TOC of shales range from 1.07 – 2.61wt.%, averaging (2.11wt.%). The genetic potential (GP) of the lignites range from 0.54 to 31.08 mg HC/g rock, averaging (9.13 mg HC/g TOC), and the shales, range from 0.42 to 2.33 mg HC/g TOC, averaging (1.07 mg HC/g TOC). Hydrogen Index (HI) of the lignites range from 01 to 75 mg HC/g TOC averaging (23.4 mg HC/g TOC), and the shale range from 12 to 85 mg HC/g TOC averaging (42.8 mg HC/g TOC). The above values indicate that the sediments have moderate to high organic matter concentration with potential to generate gas. Hydrogen Index (HI) versus Oxygen Index (OI) diagram classifies the organic matter in the samples as mainly Type IV with minor Type III kerogen. HI versus Tmax plot also showed that the shale and lignite fall within the immature and rpostmature zone. This indicates that the sediments have potential to generate gas at appropriate maturity while the lignites that fall within the postmature level are recycled, and/or contaminated materials. Tmax and Production Index (PI) of the analysed samples also fall within the thermally immature and post mature organic matter. From Palynofacies studies, the major sporomorphs identified in the samples were pollen grains and spores. These are terrestrial sporomorphs which support type III kerogen. An age range from middle Eocene – Early Miocene is assigned due to some age diagnostic pollen and spore markers species in the samples such as Pachydermites diederixi, Verrrucatosporites usmensis, Inaperturopollenites hiatus, Psilatriporites rotundus, Magnastriatites howardi in the samples. These species are typical of fresh water swamp forest.