Petrographic composition of coal within the Benue Trough, Nigeria and a consideration of the paleodepositional setting

Abstract

The petrographic composition of Cretaceous-age coals hosted in the Benue Trough, Nigeria is presented and discussed in terms of the paleodepositional settings that influenced the coal-bearing formations. The Benue Trough is a failed arm of the triple junction of an inland sedimentary basin that extends in a NE-SW direction from the Gulf of Guinea in the south, to the Chad Basin in the north. A total of twenty-nine (29) coal samples were obtained from nineteen coal localities in the Upper (UBT), Middle (MBT), and Lower Benue Trough (LBT). The high average volatile matter yield, low average ash yield, high calorific value (24.82 MJ/kg, on average), and low sulphur values indicate good quality coal deposits. The organic matter is dominated by vitrinite, reported at an average of 59.3% by volume (mineral-matter free). Variation was noted in the inertinite content across three sub-regions. Liptinite macerals were not commonly observed in the studied samples and were absent in the MBT samples. Coal facies studies decipher the paleoenvironmental conditions under which the vegetation accumulated. Indices commonly used are the gelification index (GI), tissue preservation index (TPI), ground water index (GWI and variations), vegetation index (VI), and wood index (WI). Comparing the array of coal facies models applied, the MBT samples differ from the UBT and LBT samples, concurring with the coal quality data. The UBT and LBT coals formed in an upper deltaic to drier piedmont plane depositional environment, while the MBT coal formed in a lower deltaic marsh to wet forest swamp depositional environment. All samples indicate an ombrotrophic paleomire. In view of the modified equations and the plots used, interpreting depositional environments from just a single model is not reliable.

Metalliferous Coals of Cretaceous Age: A Review

“…In the Late Cretaceous, as the Tethys Sea experienced a southward transgression and Atlantic waters moved northwards linking to the Tethys Sea, coal facies developed along the epicontinental seaways and coastal plains across North Africa [138]. The Nigerian Cretaceous-aged coal deposits are lignitic to sub-bituminous to bituminous in rank [143,144], and the coal seams are usually interbedded with bands of shales and mudstones, some of which are oil-bearing. Chukwa et al [145] reported Nigerian coal The Nigerian Cretaceous-aged coal deposits are lignitic to sub-bituminous to bituminous in rank [143,144], and the coal seams are usually interbedded with bands of shales and mudstones, some of which are oil-bearing.…”

“…The Nigerian Cretaceous-aged coal deposits are lignitic to sub-bituminous to bituminous in rank [143,144], and the coal seams are usually interbedded with bands of shales and mudstones, some of which are oil-bearing. Chukwa et al [145] reported Nigerian coal The Nigerian Cretaceous-aged coal deposits are lignitic to sub-bituminous to bituminous in rank [143,144], and the coal seams are usually interbedded with bands of shales and mudstones, some of which are oil-bearing. Chukwa et al [145] reported Nigerian coal deposits as 39% bituminous, 49% sub-bituminous, and 12% as lignite.…”

“…The Benue Trough is geographically subdivided into the Upper Benue Trough (UBT), the Middle Benue Trough (MBT), and the Lower Benue Trough (LBT) (Figure 21). Mangs et al [144] concluded that the geological history and variable depositional environments of the Benue Trough impacted the coal quality in the three sub-basins, with the MBT differing from the UBT and LBT. Liptinite appears to be absent in the vitrinite-rich MBT samples, and ash values, on average, are higher compared to the UBT and LBT samples [144].…”

“…Mangs et al [144] concluded that the geological history and variable depositional environments of the Benue Trough impacted the coal quality in the three sub-basins, with the MBT differing from the UBT and LBT. Liptinite appears to be absent in the vitrinite-rich MBT samples, and ash values, on average, are higher compared to the UBT and LBT samples [144]. Obaje et al [149] undertook Rock Eval pyrolysis on samples across the Anambra Basin (LBT) and UBT Benue Trough and concluded that the coal deposits are Type III kerogens, thus capable of generating oil and gas in the deeper subsurface.…”

“…The total sulphur values for the LBT coals are generally low (below 1%), with higher values reported for MBT samples (Table 7). Ayinla et al [151], Akinyemi et al [30,143,146], Mangs et al [144], and Yandoka et al [152] provided recent geochemical, petrological, and mineralogical data for Cretaceous coals of Nigeria (Tables 7 and 8), while Ogala et al [153] provided data for some elements from the Anambra Basin (LBT). It is difficult to draw accurate correlations between the published data in Tables 7 and 8 as there is some degree of inconsistency in the reporting of the sample location, formation and seam, analytical equipment, and even analytical reporting basis.…”