SEDIMENTARY FACIES AND DEPOSITIONAL ENVIRONMENT OF SIGMA WELL AND THETA WELL, NIGER DELTA, NIGERIA
Abstract
ABSTRACT
Understanding facies and depositional environments aids basin reconstruction, reservoir investigation, and hydrocarbon prospecting, as local-scale heterogeneity in the Niger Delta remains underexplored. This study investigates lithofacies types and depositional environments in Sigma (2929.78–2959.48 m) and Theta (3041–3096.21 m) wells, using integrated sedimentological, core photography, and spectral gamma ray (SGR) log analysis. Cores were described based on grain size, sedimentary structures, and lithology. SGR data from Theta Well, using uranium (U), thorium (Th), and potassium (K) concentrations, provided additional insights into organic content and lithological variation. Sigma Well is dominated by fine- to medium-grained sandstones, laminated shales, and heterolithic facies with ripple lamination, wavy bedding, and cross-bedding—features typical of tidal flats and lower shoreface under mixed wave-tide influence. A coarsening-upward pattern suggests progradational deposition. In contrast, Theta Well exhibits a broader facies spectrum, including cross-bedded coarse-grained sandstones, hummocky cross-stratified beds, laminated shales, and bioturbated units. These indicate higher energy conditions typical of fluvial channels, storm-dominated delta fronts, and tidal flats. SGR analysis in Theta Well reveals minor uranium enrichment and the dominant lithology is clean sandstone, suggesting an oxic, high-energy setting. The higher intensity of bioturbation in Theta implies greater marine influence or longer depositional stability. Comparative analysis shows lateral facies variation driven by changes in sediment supply, energy conditions, and topography. Sigma reflects a more distal tidal to delta front setting, while Theta represents a proximal, fluvially dominated environment with stronger hydrocarbon potential, as indicated by visible oil staining and fluorescence in sandstone reservoirs. The depositional environment of the study area is interpreted as deltaic to marginal marine, reflecting the transitional settings typical of the Agbada Formation in the Niger Delta Basin.
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