Paleodepositional reconstruction of Lokpanta Shale, Anambra Basin, Nigeria, using organic geochemistry, scanning electron microscopy, and trace elemental analysis

Abstract

The organic-rich Lokpanta shale in the Anambra Basin is considered a potential unconventional hydrocarbon resource in Nigeria. Exploration of unconventional shales requires the paleoenvironmental reconstruction of organic matter productivity and preservation for sweet spot mapping. However, limited studies have been conducted on the paleodepositional factors governing organic matter accumulation in the Lokpanta Shale. This study used scanning electron microscopic and geochemical analyses to reconstruct the paleodepositional settings. Total organic carbon (TOC) content averages 4.34%, indicating very good hydrocarbon generative potential. Lokpanta shale contains Type II organic matter and planktonic foraminifera without benthos, suggesting a marine but limitedly oxygenated setting. The Sr/Ba (1.28–9.27) and Rb/K (37.18–51.93) ratios indicate high paleosalinity, aiding organic matter preservation. Certain enriched trace elements (Mo, Cd, As, Cu, Ti, Ni, and Fe) are similar to signatures in modern hypoxic (e.g., Namibian shelf) and anoxic-euxinic (e.g., Mediterranean sapropels and Black Sea) marine environments, which are both associated with sulfidation for organic matter preservation. Observed pyrite exhibits a primarily spheroidal framboid morphology with an average size of <5.2 μm and a standard deviation of ∼2, indicating a syngenetic origin of an euxinic depositional environment. The paleoredox proxies (Mo/TOC <15 ppm/gTOC, Th/U > 2, V/Sc > 16, V/(V + Ni) > 0.70, and Ni/Co > 6) indicate deposition in highly anoxic and intensely sulfidic waters within a highly restricted sill basin exhibits characteristics of an euxinic setting. In addition, Rb/Sr (0.031–0.132) and Sr/Cu (13.77–28.45) and C-value (<0.1) ratios suggest hot and arid paleoclimate, which aided carbonate productivity. The enrichment of CaO, Si, Ti, Zr, and depletion of Th and rare earth elements (REE) contents suggests low clastic influx aiding in carbonate productivity and organic matter preservation. However, the ratios of Ba/Al (18.5–46.7), Baxs (56.63–489.14), Fe/Ti (4.85–11.29), and Al/Al + Fe (0.58–0.79) suggest that organic matter accumulation was governed by neither primary paleoproductivity nor hydrothermal activities. This euxinic setting may not have extended to coeval shales in other coastal basins of West Africa, even though anoxic conditions existed in those areas.