Uncovering the Late Cretaceous paleoenvironment and paleoclimate of the Agbaja Plateau, Bida Basin, Nigeria: An inorganic geochemical analysis of shales and implications for organic matter enrichment

Abstract

• Geochemical proxies indicates that the shales were deposited in an oxic condition. • Environments of deposition ranges from fresh water to marine. • Moderate enrichments of Mo and U suggest moderate to high bioproductivity. • Warm and humid paleoclimatic conditions were inferred for the studied shales. • The paleogeographic condition was favorable for source beds with gas potential. The bulk geochemical composition of the shale samples of the Maastrichtian Patti Formation exposed at the Agbaja Plateau, southern Bida Basin, Nigeria was studied using X-ray fluorescence (XRF) and inductive coupled plasma mass spectrophotometry (ICPMS) techniques. The objectives of this study are to decipher their paleoenvironment, palaeoclimate and examine the influence of the paleogeographic conditions on the hydrocarbon source rock potential. The geochemical proxies indicate that the shales were deposited under oxic and warm and humid conditions. The plots of V/(V + Ni) vs. U/Th and Ni/Co vs. U/Th support the oxic setting for the shales. The slight Ce anomalies (0.93–1.26), ΣREE (39.40–1607.37 ppm) and (La/Yb) N ratio (4.11–10.78 ppm) are in support of the sub-oxic to oxic condition. The studied shales show Th/U values from 1.98 to 4.38 which indicates prevalence of high primary paleoproductivity. The clustering of the analyzed samples near the A-K line in the A-CN-K (Al 2 O 3 -CaO + Na 2 O-K 2 O) ternary diagram depicts an intense chemical weathering in the source region. In conclusion, the obtained paleogeographic scenario in the study area (warm temperature, high precipitation, oxic to dysoxic condition, and moderate to high primary paleoproductivity) typical of the west Africa upper Cretaceous and freshwater to brackish water environment would have supported the development of moderate organic matter rich source beds with potential for gas and oil in the Bida Basin. This study has underscored the significance of understanding the interplay between sedimentary environment, organic productivity and organic matter enrichment in predicting and exploring hydrocarbon source rocks.