STRATIGRAPHY, SEDIMENTOLOGY AND GAS SHALE EVALUATIONS OF PART OF THE EZEAKU GROUP, ABAKALIKI ANTICLINORIUM, SOUTHERN BENUE TROUGH, NIGERIA

SOURCE:

Faculty: Physical Sciences
Department: Applied Geophysics

CONTRIBUTORS:

Ede T. A. David
Okoro A. U.

ABSTRACT:

Stratigraphic, sedimentologic and geochemical studies were carried out on the calcareous shale and marlstones of the Eze-Aku Group exposed on the western limb of the Abakaliki Anticlinorium in the Southern Benue Trough, with the aim of evaluating the stratigraphy, paleogeography and the gas shale potential of the rocks. The specific objectives of this work are to produce a geologic map, reconstruct the paleoceanographic conditions, determine environment of deposition, ascertain its biostratigraphy as well as undertake the inorganic and organic geochemical analyses of the rocks. The methods adopted in this study include geological field mapping, petrographic analysis, Rock-Eval pyrolysis, X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF), Atomic Absorption Spectrometry (AAS) and palynological studies. Result of geological field mapping identified a carbonaceous shale unit comprising interbedded calcareous shale and marlstone lithofacies with sideritic concretions. Its type section and type locality are along the Nmavu River channel in Onoli-Awgu. The lithostratigraphic unit is named the Lokpanta Shale. Lithofacies characteristics and the structures are deduced as Anoxic shallow marine association. Evidence from the biogenic structures, palynology, and micropaleontology; mostly planktic and rare benthic microfossils indicate that deposition in anoxic, inner-middle neritic paleoenvironments. Palynological evidence indicates that the rocks are Cenomanian - Turonian in age. Petrographic analysis revealed that the marlstones are biomicritic, with abundant bivalve fragments and tests of Hedbergellids, Heterohelicids and rare benthic foraminera. X-ray diffraction results reveal that calcite, kaolinite and quartz are the dominant minerals in the rocks. Cross plots of elemental oxides reveal that the sediments were deposited in a passive margin tectonic regime. Plots of major oxides against Al2O3 and CaO reveal sediment supply was from mixed sources. Plot of TiO2 vs Al2O3 as well as presence of kaolinite indicates granitic to intermediate rocks provenance in a wet and warm tropical climate. Chemical Index of Alteration (CIA) values reveal that the source area of these rocks was strongly to very weakly weathered. Elemental ratios of Sr/Ba, V/V+Ni, Ni/Co and V/Cr revealed eunixic-anoxic paleoredox conditions and hypersalinity for the Cenomanian - Turonian sea. The abundance of calcareous microfossils as revealed by petrographic studies indicate high biogenic productivity (carbonate factory) existed in the ancient sea at the time of deposition of the rocks. The high biogenic productivity further indicates a temperature range of 20-250C. High Cd and Ba values reveal that organic paleoproductivity in the environment of deposition was high. Hydrocarbon source rock properties evaluation using Rock-Eval pyrolysis shows that Total Organic Carbon (TOC) values vary from 0.83 to 4.56wt%, genetic potential 2.34 to 24.33mgHC/g, S1values 0.15 to 2.91mgHC/g and S2 values 2.06 to 21.42mgHC/g. These are all indicative of very good to excellent source rock. Organic matter evaluation using HI vs OI and S2/S3 ratio indicates that they are predominantly Type II kerogen. A plot of HI vs Tmax, S2 vs TOC and PI vs Tmax reveal predominantly Type II and subordinate Type II-III kerogen, capable of high hydrocarbon (oil and gas) generation. With regards to its gas shale status, these rocks are potential sources of shale gas play with Ezzaegu area being the most promising.