Estimation of shale volume (Vsh) of lithofacies of Agbada Formation, offshore western Niger Delta, Nigeria: gamma ray (GR) logs perspectives

Abstract

This paper presents the results of a comprehensive analysis of five gamma ray logs from the offshore western Niger Delta, Nigeria. The investigation was aimed at using gamma ray logs to estimate the shale volume, identify lithology, correlate between formations, and establish the comparative rate of accretion of deposits of the five studied wells. Five lithologies were recognized, namely, sandy mudstone, shale/mudstone, fine-medium grained sandstone, argillaceous sandstone, and coarse-grained sandstone. The lithology thicknesses varied down the profile of the studied wells due to the discrepancy in the subsidence with erosion and onlap of sediments. The shale volume (Vsh) investigation revealed low Vsh values for argillaceous, fine-medium, and coarse-grained sandstones and high values for shale/mudstone and sandy mudstone lithologies. However, lithologies are discriminated into three types of formation, namely, clean formation (Vsh < 10%), shaly formation (Vsh ranged from 10 to 30%), and shale formation (Vsh is more than 33%). These differentiated lithologies consist of laminated, structural, and dispersed shale, respectively. It is shown that when the Vsh values increase, the Vsh/lithological thickness ratios increase, and when the Vsh/lithology thickness ratios increase, the lithology thicknesses decrease. The graphic correlation revealed that for 1 m of sediment accretion in MOX4 well, 0.10 m, 0.30 m, 0.31 m, and 0.42 m of rocks were accumulated in MOX1, MOX2, MOX3, and MOX5 wells, correspondingly. These comparative rates of sedimentary particle accumulation proved that there were either fewer erosion occurrences or extra accommodation gaps within MOX4 than in MOX1, MOX2, MOX3, and MOX5 wells.