UPPER CRETACEOUS COALS FROM THE ANAMBRA BASIN, NIGERIA : PALEOENVIRONMENTS AND HYDROCARBON GENERATION POTENTIALS

The Lower Maastrichtian Mamu Formation within the Anambra Basin of South Eastern Nigeria consists of a cyclic succession of coals, carbonaceous shales, siltstones and sandstones of a deltaic environment. Sub-bituminous coals in this formation are distributed in a N-S trending belt from Onyeama, Owukpa to Okaba areas in the northern part of the basin.We have carried out a sedimentological, organic petrology , rock-eval and hydrous pyrolysis studies to evaluate the depositional environments, hydrocarbon generation and expulsion potentials of the coals. Maceral analyses revealed that the coals are dominated by huminites with lesser amounts of liptinites and inertinites. Vitrinite reflectance (%Ro of 0.44 to 0.6) and Tmax values (417 to 429^oC) indicate that the coals are thermally immature to marginally mature with respect to petroleum generation. Rock-Eval data with Hydrogen Index (HI) values of 203 -266mg HC/g TOC, and S1+S2 yields from 139.97 – 199.28mg HC/g rock ; together with the GC–MS traces derived from Ruthenium tetroxide oxidised samples suggest an oil and gas generation potential for the coals. Stepwise artificial maturation by hydrous pyrolysis from 270 to 345^oC / 72 hours of selected coals, indicate a significant increase in the S1 yields, Production Index (PI) and the Bitumen Index (BI) with corresponding decrease in HI during maturation ; thus confirming petroleum generation with increasing maturity. A decrease in the Bitumen Index at a Tmax of 453^oC suggests efficient oil expulsion; at an approximated vitrinite reflectance of 1%Ro.

The new results suggests that coals of the Anambra Basin possess the ability to generate and expel liquid hydrocarbons at maturity as their age equivalents and younger coals in the Taranaki Basin of New Zealand and the basins of Indonesia where commercial oil discoveries can be correlated to coals.The Anambra Basin coals could have generated a presently unknown amount of oil and gas as part of the Cretaceous petroleum system in the subsurface.