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Aromatic Hydrocarbon Distribution and Calculation of Oil and Gas Volumes in Post-Santonian Shale and Coal, Anambra Basin, Nigeria

By
I.M. Akaegbobi
I.M. Akaegbobi
Department of Geology University of Ibadan Ibadan, Nigeria
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J.I. Nwachukwu
J.I. Nwachukwu
Department of Geology Obafemi Awolowo University Ile-Ife, Nigeria
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M. Schmitt
M. Schmitt
Geochemische Analysen Sehnde, Federal Republic of Germany
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Published:
January 01, 2000

Abstract

The generation and distribution of C11+ aromatic hydrocarbons has been investigated in 24 rock samples from the Campanian–Maastrichtian Nkporo shales and coals of the Anambra Basin in the lower Benue Trough. The study focused on dicyclic and tricyclic aromatic hydrocarbons identified on gas chromatograms. The distribution of these compounds and their alkyl homologs are strongly controlled by thermal maturation of organic matter. Compositional variation in the organic macerals as indicated by optical observation seemed to be of minor significance.

A gradual depth trend was observed for compound ratios calculated from the relative variations of the methyl aromatics in each family. The methylphenanthrene index (MPI 1) in particular, as well as other indices derived from dicyclic aromatics, demonstrated a good correlation with maturity. Theoretically calculated vitrinite reflectance values from MPI 1 range from 0.60 to 0.89% Rc. These values are consistent with measured reflectance values (0.61–0.83% Ro) and Tmax values (430–471°C) for type III kerogen within the corresponding depth intervals. These results confirm that the Nkporo Shale and the Lower Coal Measures (Mamu Formation) are mature within the depth range of 1555–2389 m.

Volumetric estimates of the hydrocarbons generated by these shales and coals show that about 779 million bbl of oil or 4.57 × 1013 ft3 of gas may have been generated and probably entrapped in the basin. The basin's low source potential index (4.02) indicates an undercharged system and hence low exploration potential for the basin.

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Contents

AAPG Memoir

Petroleum Systems of South Atlantic Margins

Marcio Rocha Mello
Marcio Rocha Mello
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Barry J. Katz
Barry J. Katz
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American Association of Petroleum Geologists
Volume
73
ISBN electronic:
9781629810706
Publication date:
January 01, 2000

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