Depositional Facies Analysis and Controls of the Organic – Rich Cretaceous Enugu Shales/Mamu Formation in Anambra Basin, South – Eastern Nigeria 

BOORE, OLUWOLE, University of Ibadan, Department of Geology, Ibadan, Oyo State, Nigeria, [email protected]           

 

The outcrop sections of the Enugu Shales and Mamu Formation at the selected localities have allowed sedimentary logging and facies scheme to be carried out.

Lithologically, they consist of dark grey shales, siltstone, sandstones, heteroliths and coal beds, which were subjected to sedimentological, geochemical and petrological studies.

Sedimentologically, the sandstone facies are fine-grained moderately well sorted, very fine skewed leptokurtic.

Scattergrams among the textural parameters indicate the sandstones to be fluvial origin.

Heavy minerals assemblage includes; ultrastable suite, staurolite, epidote, garnet and apatite. The framework elements comprise of quartz (97%), feldspar (1%) and rock fragment (2%).

Major element geochemistry illustrates an enrichment of quartz and a depletion of chemically unstable grains.

Total organic carbon results show that the samples are fairly to moderately rich in organic matter (0.12-2.92 wt.%TOC)The maceral composition is mainly vitrinitic and liptinitic of terrestrial origin. The Rock-Eval pyrolysis data classify the kerogens to type III/II organic matter at low to marginal thermal maturity.

Integrated information as provenance indicator suggests that the sample sets were derived from cannibalization of the adjacent pre-Santonian Abakaliki trough and the basement complex Oban Massive.

Framework composition and major element data indicate a highly recycled nature and /or extreme chemical weathering under humid and warm climate. Sedimentary attributes of the sedimentary products suggest deposition in shallow marine where operational wave energy was low to moderate.

Organic geochemical and organic petrographical parameters employed indicate that these Formations have fair to moderate, gas to minor oil-gas-prone source rock potential at low thermal maturity.