Source-Rocks of Giant Hydrocarbon Deposits in Deep Offshore, Niger Delta

Chukwuemeka M. Ekweozor
Getamme Geochem, Port Harcourt, Nigeria

The occurrence of giant hydrocarbon fields in the deep/ultra-deep offshore of the Niger delta implies propitious convergence of factors that favored development and operation of excellent petroleum systems. Here we report geochemical investigations inferring that the oil and gas deposits originated from carbon-rich Cretaceous rocks related to, respectively, the Lokpanta facies of the Eze-Aku Shale (Upper Cenomanian-Lower Turonian) and the Nkporo Shale (Campanian-Maestrichtian) of southeastern Nigeria sedimentary basins.

The bituminous Lokpanta facies contains oil-prone Type II kerogen and its deposition in the Abakaliki Basin coincided with the Cenomanian-Turonian Boundary Anoxic Event that heralded formation of prolific source-beds elsewhere in the world's oceans and seas. In the mainland, the sediments entered the oil window just before or during the Santonian tectonism. But the corresponding strata underlying the Niger delta in the deep offshore where the Tertiary overburden is considerably thinned down ought to have generated huge amounts of liquid petroleum more recently to contribute to the giant oil pools. The Nkporo Shale of the Anambra Basin is also associated with great quantities of sedimentary organic matter but the kerogen is typically terrigenous/gas-prone (Type III), hence the equivalent sediments below the Tertiary Niger delta should have formed large abundance of hydrocarbons for the gas giants. We therefore conclude that the Lokpanta facies of the Eze-Aku Shale and Nkporo Shale formations represent the two end members of source-rock types corresponding to the principal petroleum systems of the Niger Delta Deep/Ultra-Deep Offshore.