Rock-Eval pyrolysis and organic petrographic analysis of the Maastrichtian coals and shales at Gombe, Gongola Basin, Northeastern Nigeria
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Date
2016-05-19
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Arabian Journal of Geosciences: Springer
Abstract
The Gongola Basin forms one of the inland
Cretaceous to tertiary sedimentary basins in Nigeria with relatively
unknown petroleum system. In this study, the investigated
source rock intervals of the Maastrichtian Gombe
Formation are located at the Maiganga coal mine, near
Gombe, Gongola Basin, Nigeria. The exposed part of the
mine consists of about 35-m thick coarsening upward section
with the basal part made up of coal and shale interbeds while
the upper part consists of siltstone and sandstone and probably
deposited in fluvio-deltaic environment. The coal and interbedded
shale appear most prospective source rock facies in the
formation and were evaluated with respect to their hydrocarbon
source rock potential. The organic geochemical results
showed high TOC for the coals (37.71–65.29 wt%) and moderate
organic carbon concentration (1.19–4.81 wt%) for the
shales. S2 values ranges from 57.96 to 103.21 mgHC/g rock
and 0.51–6.22 mgHC/g rock for the coal and shales, respectively.
The HI is less than 200 mgHC/gTOC in all the coal and
shale samples suggesting Type III kerogen and predominant
plant contributions from terrestrial sources. Consequently,
gaseous hydrocarbon potential is exhibited in the source beds.
The organic petrography shows vitrinite as dominant maceral
followed by inertinites and liptinites with inorganic minerals
like pyrite and kaolinite. The Tmax is less than 435 °C in all
the samples indicating pre-oil window stage; this is corroborated
by the Romax value ranging from 0.45 to 0.55 %. Plot of
PI against Tmax indicates low level organic matter conversion
while the vitrinite reflectance values suggest a sub-bituminous
coal. The study suggests that the investigated coal and shale
samples constitute good source rock and have potential for
gas. The generally low HI in the coals and the associated
shales indicate more allochthonous and hydrogen poor organic
matters in the source beds. Maceral composition of the coals
suggests wet swamp environment and predominance of arborescent
vegetation type. At present level, the organic matters
are immature to marginally mature deeper level equivalents
might have generated gas.
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Jimoh and Ojo, 2016