SOURCE:

Faculty: Physical Sciences
Department: Geology

CONTRIBUTORS:

Chiadikobi, K. C.
Orajaka, I.P.
Okoro, A.U.

ABSTRACT:

The Nkporo Group of the Anambra Basin consists mostly of dark-grey to black shales, mudstones, sandstone and siltstone. Samples were collected on a traverse from the base at Oduduma Stream in Abia Town through Oji- Nkwo Stream along Ugbonabo to Achi Road in Abia State to the top at Ojume Stream in Ikem-Uruaram area in Enugu State. The aim of this study is to evaluate the source rocks and paleoenvironments of the Nkporo Group in the Anambra Basin. The objectives are to: determine the organic richness of the source rock and the type of organic matter (kerogen type), determine their thermal maturity, evaluate the hydrocarbon generation potential, interpret the age and depositional environments of the source rocks of the Nkporo Group (Nkporo Formation and Enugu Shale). The study is undertaken in three stages: desk work, field and laboratory studies. Twenty-five samples from the Nkporo Formation and Enugu Shale are analyzed using Leco CS 125 and HAWK pyrolyser for the organic geochemical parameters, Gas Chromatograph- Mass Spectrometer (GC-MS) for biomarkers and optical microscope for visual kerogen and palynological data. These parameters are used to assess the source rock quantity and quality, type of organic matter, maturity, generating potential and paleoenvironments of deposition of the source rocks (shales). From the results of the geochemical analysis, the Total Organic Carbon (TOC) contents of the Nkporo Formation and Enugu Shale varied from 0.41 to 2.42wt.% and 0.53 to 3.15 wt.% respectively. This shows that Nkporo Formation and Enugu Shale has fair to very good organic matter contents. The organic matter richness varies from 0.08 to 4.40mgHC/g for the Nkporo Formation and 0.36 to 3.97mgHC/g for the Enugu Shale. Hydrogen Index versus Oxygen Index plots and visual kerogen shows that the Nkporo Formation was of Type III/IV while Enugu Shale was of Type II/III kerogen. The Thermal maturity index (Tmax) of the Nkporo Formation and Enugu Shale varies from 418 to 443°C and 430 to 443°C respectively. These suggest that the Nkporo Formation is marginally mature to mature while Enugu Shale is mature. From the results of biomarker analysis, the isoprenoids (Pr/Ph) for the Nkporo Formation and Enugu Shale varies from 0.33 to 0.78 and 0.65 to 0.75 respectively; the two formations were therefore deposited in marine and marginal marine, reducing environments respectively. The presence of hopane, homohopane (C31-C35) in all the samples shows that bacteriohopanetetrol and other polyfunctional C35 hopanoids common in prokaryotic microorganisms have significant contributions to the organic matter. The occurrence of oleanene isomers, gammacerane and C29 Steranes and diasteranes in Nkporo Formation and Enugu Shale samples indicate marine input of the organic matter. From the results of palynological analysis, the dynocysts; Dinogymnium acuminatum, Achomosphaera sp. and Paleocystodinium austrialinium are used to date the Nkporo Formation and Enugu Shale are within Late Campanian to Early Maastrichtian. The results of this study has shown that the source rocks of the Nkporo Formation is immature and has not generated hydrocarbon while that of the Enugu Shale is mature and has generated hydrocarbon.