ASPECTS OF ORGANIC PETROGRAPHY AND GEOCHEMISTRY OF MAASTRICHTIAN COALS AND SHALES AT GOMBE, GONGOLA BASIN, NIGERIA
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Date
2019-10
Authors
JIMOH, YUSUF AYOOLA
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Publisher
UNIVERSITY OF ILORIN
Abstract
The investigated source rock interval of the Maastrichtian Gombe Formation is located at the Maiganga coal mine, near Gombe, Gongola Basin, Nigeria. Samples were collected from a borehole and the exposed part of the mine (Phase I and II). The sections at Maiganga coarsen upward with the basal part made up of coal and shale interbeds while the upper part consists of siltstone, ironstone and sandstone. The coals and interbedded shales appear most prospective source rock facies in the formation and were evaluated with respect to their hydrocarbon source rock potential including the kinetics, paleodepositional environments, source, type and maturity of the organic matter in the organofacies. Geochemical techniques employed includes TOC, Rock-Eval Pyrolysis, Soxhlet Extraction, Gas Chromatography, Medium Performance Liquid Chromatography, Gas Chromatography - Mass Spectroscopy, Isotope Ratio – Mass Spectroscopy, Palynological technique, Maceral and Visual kerogen analysis were employed in this study. Inorganic study includes Elemental and Proximate analysis.
The log motif of the sections at Maiganga mine and other outcrop sections at Zange, Liji, Mallam Inna, Maiduku and Garin Magaji revealed coarsening upward sequence. Field observations in these locations indicate fluvial depositional settings. The poorly sorted sandstones with sub angular to angular grains observed in the locations represent channel facies while the fine grained clayey siltstones and shale represent overbank deposits reflecting low energy condition. The occurrence of coal at Maiganga suggests development of incoaling condition in a deltaic setting.
The studied samples (coal, shale and coaly shale) were rich in organic carbon, the TOC of the coals and shale range between (32.40-65.29wt %) and (0.64-4.84 wt %) with an average of 38.03wt% and 3.14 wt% respectively while the coaly shale has an average of 12.1wt%. The average Genetic potential (GP) value for the coals, shales and coaly shales are 70.7mg/g, 5.04mg/g and 12.6mg/g respectively indicating an excellent source potential. HIs is less than 200mgHC/gTOC for all the samples. The organic matter in the coaly source rocks suggests Type III kerogen and plant contributions from terrestrial sources probably from plant (Vicia villossa), peat vegetation (Sphagnum L) or mangrove. This is corroborated by broad activation energy (Ea) distribution (41-65kcal/mol) reflecting heterogeneous composition. Consequently, gaseous hydrocarbon potential is exhibited in the source beds.
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The major phase and peak of hydrocarbon generation occurs at 460 – 465oC for the coal and shale facies corresponding to Ea (54-55kcal/mol) defining up to 70% of the petroleum potential.
The coals and the shales were characterized by high molecular weight n-alkanes which range from C10-C41 for the coal facies and C9- C41 in the shaly facies. The n-alkanes were the dominant component of the aliphatic hydrocarbon fraction and they show variation in quantity and distribution. The extracts were characterized by predominance of polar compounds indicating low maturity level of kerogen cracking. Saturated fractions comprises of tricyclic and pentacyclic terpanes, steranes, diasteranes and tetracyclic polyprenoids while aromatic fractions comprises mono, triaromatic steroids and triaromatic methylsteroids.
The n-alkanes distributions in the coaly source rocks maximizes at C29. This pattern indicates organic matter derived mainly from terrestrial organic matter while the shaly facies maximizes at C16 and C29 indicating mixed source. Pr/Ph ratio for the coaly facies ranges from 2.15-3.51 (avg. 2.79) while the shaly facies ranges between 0.95-2.37 (avg. 1.82) indicating fluvio-deltaic depositional environment in sub-oxic to oxic setting. The predominance of Pristane (n-C17) over Phytane (n-C18) suggests humic origin of the organic matter for the coaly source rocks in a mixed/transitional depositional environment.
The coals were rich in C29 steranes (> 50%) and (~40%) calculated from isosterane αββS and αααR configuration. This is an indication of significant land plant contribution as evidenced from the very low C27 steranes. This is corroborated by the long chain length and isotopic value of -27.7 δ13‰ PDB and -31.0 δ13‰ PDB for the coal and shale respectively. The shale also have high C29 steranes but with appreciable quantity of C27 and C28 αββS sterane (26% and 29%) and (34% and 24%) in C27, C28αααR sterane confirming probable marine contribution for the organic matter. The sterane/hopane ratio values range from 0.06-0.35 (<0.6) suggesting incorporation of high level of bacterial inputs commonly associated with terrigenous organic matter in coals. The dibenzothiophene/phenanthrene (DBT/PH) ratio range from 0.03 to 0.19 for the coal and 0.08-0.14 for the shaly source rocks respectively indicating appreciable quantity of terrestrial organic matter.
The average Tmax for the coal and shaly facies were less than 435oC, and the vitrinite reflectance less than 0.55Ro. The carbon preference index (CPI) is greater than unity and in conjunctions with other saturate and aromatic maturity indices indicates that the samples were immature to low maturity status. Petrographic examination revealed vitrinite is the most abundant maceral followed by inertinite and liptinite, although there were absence of alginate and amorphous organic matter. The organic matter in the organic facies consists of opaque to non opaque biostructured phytoclasts and cuticles associated with common amounts of spores and pollen such as Arecidites sp., Ephedripites sp., Monocolpites marginatus, Cyathidites sp., Foveotriletes margaritae, Longapertites marginatus, Longapertites vaneendenburgi, Proxapertites cursus, Proxapertites operculatus, Cingulatisporites ornatus Constructipollenites ineffectus, Retibrevitricolpites triangulatus, Echitriporites trianguliformis, Nyssapollenites sp., Araucariadites australis, and Fovoetriletes margaritae. The coal and shales facies were dated Early- Late Maastrichtian and assemblage zone of Arecipites – Dictyophyllidites harrisii were assigned.
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The proximate analysis of the coals revealed sub-bituminous rank, non-coking but suitable for fuelling, combustion and power generation due to high calorific value, high volatiles and low ash content. The average concentrations of the trace elements are lower when compared to world coals and indicate lesser negative impact to the environments, plants, animals and humans. The positive correlations of the trace elements such (Na, Ba, Th, and Co) with SiO2 and Al2O3 in the coals suggests primarily aluminosilicate affinities.
In conclusion, the Maastrichtian coal and shale of Gombe Formation, Gongola Basin constitute good to excellent source rocks with potential to generate mainly gas and will generate oil if higher thermal maturity level is attained. Also, the coal facies were formed from organic matter derived from mainly terrestrial/terrigenous organic matter under oxidizing condition and deposited in the fluvial/deltaic environments, while the shaly facies were from mixed sources deposited under reducing lacustrine environments.
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Keywords
ORGANIC PETROGRAPHY, GEOCHEMISTRY, MAASTRICHTIAN COALS, SHALES, GOMBE, GONGOLA BASIN