Palynofacies and predictions for hydrocarbon generation potential of the Upper Cretaceous lignite from Maiganga Coalfield, Northern Benue Trough, Nigeria.

Abstract

The Upper Cretaceous Coalfield of Northern Benue Trough, represent excellent
hydrocarbon source rock potential within the Nigeria inland basins. The source rock
potential of coal and shale whole rock samples from Maiganga and Yaya-Ngari areas in
the Northern Benue Trough was investigated using palynofacies and organic
geochemical methods for hydrocarbon generation predictions. Palynofacies is integrated
in this study as a useful tool for kerogen typing, paleoenvironment indicator, thermal
maturity determination and hydrocarbon generation potential of source rock which
equally relies on the relative abundance of the three kerogen groups (AMO, Phytoclast
and palynomorphs). Current result shows that identified palynomorphs (P), phytoclast
(Ph) and amorphous organic matter (AOM) for the studied Yaya-Ngari shale show
significant amount of dark-brown to orange colour phytoclast compared to brown
heterogenous AOM and brown to orange palynomorphs. The palynomorphs are often
<10 μm, although few ones are up to 80 μm sizes. In the case of Maiganga facies, they
are characterized by mainly terrestrial kerogen group. This is evidently shown by the
presence of highly oxidized black, opaque tissues of higher plants often associated with
some brown degraded tissue of higher plants, light brown AOM (probably resin and
algae material) and trilete spore and pollens of plants. Thus, it is an indication of a more
gas-prone Type III kerogen than the associated liptinitic materials of oil-prone Type II
kerogen as observed under the UV-light. The GC-MS result of saturated hydrocarbon as
corroborated by the aromatic hydrocarbon fraction indicates low concentration of
dibenzothiophenes compared to aromatic phenanthrenes (m/z 184 and 178) which
resulted in DBT/P ratio of <1. This indicates a marginal marine environment mainly
deltaic setting (Maiganga) associated marine shale at Yaya-Ngari for the analysed
sediments. Similarly, It is further corroborated by high concentration of hopane
compared to sterane resulting in hopane/(hopane+sterane) ratio > 0.7. Generally,
biomarker studies suggest deposition of Type III/II kerogen within suboxic to oxic
environment for the Maiganga, while Yaya-Ngari samples are characterized by mixed
organic matter deposited in a suboxic conditions.