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 pyrolysis) and organic petrology (vitrinite reflectance and palynofacies) to determine the thermal 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 intrusion 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.