Pyrolysis analysis, organic petrology, and thermal effects of volcanics on Bima Formation shales, Yola Sub-basin, Northern Benue Trough, Nigeria

Abstract

The shale sediments of Bima Formation from the Yola Sub-basin of the Northern Benue 
Trough, northeastern Nigeria were studied based on pyrolysis analysis (rock-eval pyroly
sis) and organic petrology (vitrinite reflectance and palynofacies) to determine the ther
mal effects of Tertiary (Neogene) volcanics on the organic matter thermal maturity. The 
study revealed that shale samples are of Type III and Type IV kerogens. This indicates high 
thermal maturity level and classified the samples as late-oil window to post-mature stage 
of hydrocarbon generation. This high thermal maturity could be attributed due to the 
Tertiary volcanic intrusion in most part of the basin. The samples, however, appeared as a 
good source of oil- and gas-prone hydrocarbons. This study, therefore, suggests that the 
initial oil and gas-prone of the Type II and II-III kerogens in the shales have been cracked 
into thermogenic gas probably due to the high thermal effects of Tertiary volcanic intru
sion in the basin. This has yield the present-day Types III and IV kerogens. The volcanic 
plugs, however, intruded the Cretaceous sedimentary successions of the Northern Benue 
Trough during the Tertiary period and as such, they enhanced thermal maturity of the 
organic matters within the shale samples.