Petroleum Generation Potentials and Expulsion Efficiency of Shales from Pindiga Formation, Northern Benue Trough

Abstract

This study integrates lithofacies analysis, Rock-Eval pyrolysis and mass-balance expulsion 
modelling to evaluate the petroleum generation potential of Cretaceous shales of the 
Pindiga Formation around Pindiga village, Gongola Sub-basin, Northern Benue Trough, 
Nigeria. Seven lithofacies were recognized from the measured succession: black 
carbonaceous shale, dark grey laminated shale, bioclastic limestone, calcareous shale, silty 
shale, ripple-laminated sandstone and cross-bedded sandstone. Their vertical association 
records offshore shelf to delta-front and shoreface sedimentation during a Cenomanian
Santonian marine transgressive-regressive cycle. Six representative shale samples were 
analysed for total organic carbon and pyrolysis parameters. TOC values range from 1.2 to 
2.8 wt.% with a mean of 2.02 wt.%, indicating good to very good organic richness. Hydrogen 
Index values of 180-340 mg HC/g TOC and Oxygen Index values of 30-50 mg CO2/g TOC 
indicate mixed Type II/III kerogen, reflecting combined marine algal and terrestrially 
derived organic inputs. Tmax values of 430-439 °C and Production Index values of 0.072
0.115 place the studied shales from immature/early mature levels into the early oil window. 
Generated petroleum potential ranges from 2.36 to 10.42 mg HC/g rock. Material-balance 
calculations, using an initial HI baseline of 420 mg HC/g TOC, indicate expulsion 
efficiencies of 16.7-23.6% in the mature samples. The results demonstrate that the 
Kanawa Member shales constitute an effective but incompletely expelled source-rock 
interval, capable of charging adjacent sandstone reservoirs while retaining significant 
hydrocarbons within the shale matrix, thereby suggesting additional unconventional tight
shale resource potential in the Pindiga Formation and wider Gongola Sub-basin petroleum 
system under favourable burial conditions locally and regionally.