A total of 63 sidewall and drill-cutting shale samples obtained from a depth slice of 800 to 3,650 m in two onshore and two offshore wells located respectively in the eastern and western Niger delta basin, Nigeria, were analyzed for organic-matter type, concentration, and thermal maturity.

Dominant sedimentary kerogens were the humic and mixed types. Organic carbon contents of the sediments ranged from 0.4 to 4.4% and the degree of thermal evolution as evidenced by vitrinite reflectance (R_o%), odd-even ratios [OER, 2C₂₉/(C₂₈ + C₃₀)], and variations of the ratios of the concentration of soluble organic matter to total organic carbon content (SOM/TOC) were within the ranges 0.2 to 0.7, 4.0 to 1.0, and 10 to 240 respectively.

On the basis of the variations of the maturation parameters, the onset of mature source beds in the onshore and offshore delta were delimited at approximately 3,375 and 2,900 m respectively. The inferred threshold temperature was about 95°C.

The results of the present study support the hypothesis that the main source beds of the Niger delta basin are the deeply buried paralic or paralic-marine facies.