Abstract
The Maastrichtian Gombe Formation is located within one of the inland basins of Nigeria (Gongola Basin). Surface and borehole samples of coal and interbedded shale of this formation were evaluated to determine the organic matter richness, quality, maturity, source and depositional environment through organic geochemical techniques such as total organic carbon (TOC) determination, Rock–Eval pyrolysis, chromatography and organic petrography. The genetic potential and average TOC value of the coals, shales and coaly shales are 38.03 wt%, 3.14 wt% and 12.1 wt%, respectively, indicating moderate-to-good source potential. HIs are less than 200 mgHC/gTOC for all the samples indicating Type III kerogen for most of the samples (gas prone) and plant contributions from terrestrial sources. The source rocks were characterized by high molecular weight n-alkanes which range from C10–C41 in the coals and C9–C41 in the shales. The n-alkanes distributions in the coals maximize at C29 indicating terrestrial sourced organic matter, while the shales maximize at C16 and C29 indicating mixed sources. Pr/Ph of the coals and shales ranges from (2.15–3.51) to (0.95–2.37), respectively, suggesting fluvio-deltaic deltaic and coastal swamp depositional environments. The predominance of pristane over phytane suggests humic origin of the organic matter. The sterane/hopane values (< 0.6) indicate incorporation of bacterial inputs commonly associated with terrigenous organic matter in the coals. The CPI is greater than unity, Tmax is generally < 435 °C and vitrinite reflectance less than 0.55 Ro for most of the samples and suggest the samples are of low maturity status to expel liquid hydrocarbon.
Highlights
The assessments of hydrocarbon accumulation in a petroleum system are subject to accurate characterization of oil source potential through various analytical techniques such as pyrolysis, elemental, chromatography and spectroscopy
The total organic carbon (TOC) values in the coal, shale and shaly coal samples exceeded the acceptable minimum values required for a potential source rock (Hunt 1996a, b) and indicate good source potential
This is corroborated by the average genetic potential (GP) value for the coals, shales and coaly shales which are 70.7 mg/g, 5.04 mg/g and 12.6 mg/g, respectively
Summary
The assessments of hydrocarbon accumulation in a petroleum system are subject to accurate characterization of oil source potential through various analytical techniques such as pyrolysis, elemental, chromatography and spectroscopy. The investigated source rock interval of the Maastrichtian Gombe Formation is located at the Maiganga coal mine, near Gombe, Gongola Basin, Nigeria. Previous workers in the study area (Onoduku 2013; Jimoh and Ojo 2016) have applied preliminary organic geochemical screening methods such as TOC and Rock–Eval pyrolysis to characterize the source rock potential in the study area, and due to limitation of these techniques, a further probe was considered necessary. There are no detailed organic geochemical investigations on the coal and shaly source rocks in respect of biomarker characterization. The palaeodepositional environments, source, type and maturity of the organic matter in the source beds were investigated in this study based on Rock–Eval pyrolysis and biomarker techniques
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