Research paperDepositional environments of Late Cretaceous Gongila and Fika formations, Chad (Bornu) Basin, Northeast Nigeria

•The Gongila and Fika formations were deposited in a normal marine environment.•Organic rich upper part of the Fika Formation was deposited under anoxic conditions.•Level of oxygenation influenced by water depth controlled source rock development.•No evidence of deposition under anoxic conditions in mature samples.

Organic geochemical and palynofacies studies of 172 ditch cuttings samples of possible source rock shales from the Late Cretaceous Gongila and Fika formations in the Chad Basin of NE Nigeria were carried out to determine their paleoenvironments of deposition. Although dominated by amorphous organic matter, C/S ratios and molecular parameters suggest the mostly organic lean shales (TOC contents typically below 1.5%) were deposited in a normal marine environment. Levels of oxygenation influenced by water depth in the depositional environment appear to control organic richness and quality of the dark grey shales.The organic rich (TOC > 2.0%) upper part of the Fika Formation was deposited under anoxic conditions during the Late Cretaceous and could represent an Oceanic Anoxic Event. Mature intervals where such conditions prevailed would have generated liquid hydrocarbon, although none were sampled here.A trend of increasing organic richness towards the central part of the larger Chad Basin observed in this and other studies supports the development of organic rich marine shales (average TOC contents of 2-3%) of equivalent age in the Termit Basin where water depth would have been deeper and oxygen conditions at levels that permitted preservation of marine organic matter.