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Lithofacies and Paleoenvironmental Reconstruction of Wells A & B in Marginal Offshore Niger Delta Using Wireline Gamma Ray Log and Microscopic Grain Analysis

Abstract

Varying sedimentary environments have led to diverse diagenetic pathways in Niger Delta basin. Integrating wireline gamma log and microscopic grain data analyses to reconstruct the lithofacies and paleoenvironmental conditions allows for a more comprehensive assessment of Well A and Well B, located in the marginal offshore Niger Delta. The gamma ray log provides log signatures of the well's for the identification of distinct lithofacies and depositional environments. Microscopic grain analysis, on the other hand, offers a detailed understanding of the sedimentary textures, composition, and paleoenvironment. By combining these two methods, we aim to: Identify and characterize the lithofacies present in Wells A & B, reconstruct the paleoenvironmental conditions and depositional settings of the wells, gain insights into the geological history and evolution of the Niger Delta. A total number of sixty (60) ditch cuttings from intervals 11140 – 12580 ft of well A and 11620 – 13570 ft of well B and gammar ray log data set were utilized for this study. Schlumberger’s Petrel 2010 version software was used for processing the data. From the lithosections and the detailed lithostratigraphic description, the lithology shows siltstone with alternation of sand and shale. The shale is brown to grey in colour and moderately hard. The sand is also grey and whitish in colour. The shift from silt at 12,300ft to sandyshale (90% shale and 10% sand) at 11,800ft and back to sandstone at 11,400ft implies a transition from a low energy level such as lake where fine particles settle slowly out of suspension through a more slightly more energy environment such as marine or deltaic environment leading to the deposition of organic or marine material. Between 13,100ft to 12,400ft, there was an alternation of sand and silt and eventual deposition of sandyshale with 60% shale and 40% sand which also correspond with the gamma ray response having a serrated shape. The lithofacies and environment of deposition of Wells A & B revealed deltaic distributaries, lacustrine, fluvial flood plains and tidal channels which took place in a range of energy zones, from low to high. This demonstrates a wide range of deposition environments, from deltaic to shallow marine to fluvio-marine.

Keywords: Lithosections, well logs, lithofacies, paleodepositional environment