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

IF 2.2 4区 地球科学 Q2 GEOSCIENCES, MULTIDISCIPLINARY Journal of African Earth Sciences Pub Date : 2024-11-23 DOI:10.1016/j.jafrearsci.2024.105491
Gabriel C. Unomah , Manika Prasad , Michael A. Oladunjoye , Idowu A. Olayinka
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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.
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利用有机地球化学、扫描电子显微镜和痕量元素分析重建尼日利亚阿南布拉盆地洛克潘塔页岩的古沉积过程
阿南布拉盆地富含有机质的 Lokpanta 页岩被认为是尼日利亚潜在的非常规油气资源。非常规页岩的勘探需要对有机质的生产力和保存情况进行古环境重建,以便绘制甜点图。然而,有关 Lokpanta 页岩中有机质积累的古沉积因素的研究十分有限。本研究利用扫描电子显微镜和地球化学分析来重建古沉积环境。总有机碳(TOC)含量平均为 4.34%,表明具有很好的碳氢化合物生成潜力。Lokpanta 页岩含有第二类有机物和浮游有孔虫,但不含底栖动物,这表明这里是海洋环境,但含氧量有限。Sr/Ba(1.28-9.27)和 Rb/K(37.18-51.93)比率表明古盐度较高,有助于有机物的保存。某些富集的微量元素(钼、镉、砷、铜、钛、镍和铁)与现代缺氧(如纳米比亚大陆架)和缺氧-缺氧(如地中海边坡和黑海)海洋环境中的特征相似,这两种环境都与硫化作用有关,有利于有机物的保存。观察到的黄铁矿主要呈球状框架形态,平均尺寸为 5.2 微米,标准偏差为 2,表明黄铁矿起源于缺氧沉积环境。古氧化代用指标(Mo/TOC <15 ppm/gTOC、Th/U >2、V/Sc >16、V/(V + Ni) >0.70和Ni/Co >6)表明,在一个高度受限的闪长岩盆地内的高缺氧和强硫酸盐水体中的沉积显示了优新环境的特征。此外,Rb/Sr(0.031-0.132)和 Sr/Cu(13.77-28.45)以及 C 值(0.1)比值表明古气候炎热干旱,有利于碳酸盐的生产。CaO、Si、Ti、Zr的富集以及Th和稀土元素(REE)含量的减少表明,碎屑流入量低有助于碳酸盐生产力和有机物质的保存。然而,Ba/Al(18.5-46.7)、Baxs(56.63-489.14)、Fe/Ti(4.85-11.29)和Al/Al + Fe(0.58-0.79)的比率表明,有机质的积累既不是受原生古生产活动的影响,也不是受热液活动的影响。这种缺氧环境可能没有延伸到西非其他沿海盆地的同时期页岩,即使这些地区存在缺氧条件。
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来源期刊
Journal of African Earth Sciences
Journal of African Earth Sciences 地学-地球科学综合
CiteScore
4.70
自引率
4.30%
发文量
240
审稿时长
12 months
期刊介绍: The Journal of African Earth Sciences sees itself as the prime geological journal for all aspects of the Earth Sciences about the African plate. Papers dealing with peripheral areas are welcome if they demonstrate a tight link with Africa. The Journal publishes high quality, peer-reviewed scientific papers. It is devoted primarily to research papers but short communications relating to new developments of broad interest, reviews and book reviews will also be considered. Papers must have international appeal and should present work of more regional than local significance and dealing with well identified and justified scientific questions. Specialised technical papers, analytical or exploration reports must be avoided. Papers on applied geology should preferably be linked to such core disciplines and must be addressed to a more general geoscientific audience.
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Hybrid turbidite-contourite system on the upper-slope continental margin of the offshore southern Tanzania Lithogeochemistry and origin of the komatiites from Mundonguara mine in the Manica greenstone belt, Mozambique Combined geophysical approach as a tool to identify spatial groundwater aquifer distribution in structurally complex area. Case study of Kasserine aquifer system (central Tunisia) Paleodepositional reconstruction of Lokpanta Shale, Anambra Basin, Nigeria, using organic geochemistry, scanning electron microscopy, and trace elemental analysis Imaging upper lithospheric structures of the Benue Trough and adjoining basement areas in Nigeria and Cameroon from satellite gravity data
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