Ogbe Ovie B, Osokpor Jerry
Department of Earth Sciences, Federal University of Petroleum Resources, PMB 1221, Effurun, Delta State, Nigeria.
Heliyon. 2021 Jan 19;7(1):e05846. doi: 10.1016/j.heliyon.2020.e05846. eCollection 2021 Jan.
A sedimentological investigation was carried out in Agu-Awka and Umunya localities to interpret the facies, depositional environment, sequence stratigraphy, reservoir potential, and architecture of the outcropping Eocene Nanka Formation of the Ameki Group in southeast Nigeria. Petrographic analysis reveals that the sediments are composed of predominantly subangular to subrounded recycled quartz grains with a minor amount of rock fragments. It indicates that the sediments are texturally submature and mineralogically mature. Lithofacies analysis indicates that the formation is composed of fine to coarse-grained, trough and planar cross-bedded sandstone with clay-drape units, interpreted to be a subtidal bar facies, and a minor mudstone interval with sand to muddy heterolith interpreted to be a tidal mudflat facies. This suggests that the formation is of mixed environments, such as a tidally influenced high-energy fluvial and low-energy marsh to lagoonal settings. The lower sand unit of the subtidal bar facies (fine to coarse-grained cross-bedded sandstone with clay-drape) represents a highstand systems tract. The sharp base sandy heterolith lithofacies that forms the upper unit of the subtidal facies, and the overlying tidal mudflat facies that comprises a bedded mudstone and overlying wave rippled sandy heterolith intervals represent a transgressive systems tract. The reservoir quality of the sandstone facies is predicted to be moderate to good based on textural statistical analysis. The various relationships of the sedimentological parameters of the outcrops revealed by this study may assist and contribute to the prediction and understanding of the reservoir stratigraphic heterogeneity and properties of the subsurface depositional facies of the Nanka Formation.
在阿古-奥卡和乌穆尼亚地区开展了一项沉积学调查，以解读尼日利亚东南部阿梅基群露头的始新世南卡组的岩相、沉积环境、层序地层学、储层潜力和结构。岩石学分析表明，沉积物主要由次棱角状至次圆状的再循环石英颗粒组成，含有少量岩石碎屑。这表明沉积物在结构上为亚成熟，在矿物学上为成熟。岩相分析表明，该地层由细粒至粗粒、具槽状和平行层理的砂岩以及粘土披盖单元组成，被解释为潮下砂坝相，还有一个较小的泥岩间隔，含砂质至泥质的混杂岩，被解释为潮坪相。这表明该地层具有混合环境，如受潮汐影响的高能河流环境以及低能沼泽至泻湖环境。潮下砂坝相的下部砂单元（细粒至粗粒平行层理砂岩与粘土披盖）代表高位体系域。构成潮下相上部单元的底部尖锐的砂质混杂岩岩相，以及上覆的潮坪相（包括层状泥岩和上覆的波状起伏砂质混杂岩间隔）代表海侵体系域。基于结构统计分析，预测砂岩相的储层质量为中等至良好。本研究揭示的露头沉积学参数的各种关系，可能有助于预测和理解南卡组地下沉积相的储层地层非均质性和性质。
