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Petrographic and Geochemical Characterization of the Post – Santonian sediments of the Anambra Basin, Southeastern Nigeria.

Ajayi Oluwakemisola

University of Ibadan, Department of Geology

Ibadan, Oyo State, Nigeria.

[email protected]

A total of twenty- six samples comprising shales and sandstones from the Anambra Basin were subjected to petrographic and geochemical analytical methods. This was with a view to deduce their tectonic setting, provenance, degree of source area weathering, quantity of organic matter and sediment maturity. To achieve these, the samples were analysed for their lemental geochemistry, TOC and SOM content and petrographic properties.

Based on some discriminating parameters such as Fe2O3+MgO and SiO2, the sediments showed significant variation in their major and trace element geochemistry. The active continental margin sands of the Ajali and Nanka Sandstones are characterized by low Fe2O3+MgO and TiO2 , while the island arc shales are characterized by higher Fe2O3+MgO and TiO2. Discriminant function diagrams have identified quartzose sedimentary provenance and marginally, a mafic igneous provenance for the sands and shales respectively, while average TOC values have classified the shale as good source rocks. SOM values increased as the TOC increased, and ranged from 2015ppm - 5330ppm for the shales. The ZTR index has suggested that the sands are supermature sands while the (CIA) value indicated an intense weathering of the source area under humid condition.

It follows that these sediments are reworked and matured sediments sourced from the highly weathered surrounding complexes of Oban Massif and Abakaliki uplift. Geochemical data showed that the sediments of the Post – Santonian shales possess adequate criteria for hydrocarbon source rocks, while the clean and supermature sands of the Ajali Sandstone, can act as a good reservoir.

AAPG Search and Discovery Article #90060©2006 AAPG Foundation Grants-in-Aid