Petrophysical Variability Across Gamma‐Ray Log Motifs: An Interval‐Scale Multivariate Assessment of the Central Swamp Depobelt, Niger Delta Basin

ABSTRACT

Gamma‐ray log motifs provide useful descriptors of vertical grain‐size trends and stacking architecture, but their ability to discriminate reservoir quality is nonunique and requires quantitative testing. This study evaluated whether bell, cylindrical, funnel, serrated, and symmetrical motifs are associated with systematic differences in the petrophysical quality of Agbada Formation reservoirs in six wells from the Central Swamp Depobelt, Niger Delta Basin. Conventional wireline logs from wells UB‐14 to UB‐19 were quality‐controlled, interpreted, and divided into 28 motif intervals. Shale volume, effective porosity, water saturation, net‐to‐gross ratio, hydrocarbon pore‐volume fraction, and resistivity were evaluated using consistent operational cutoffs of VCL ≤ 0.40, PHIE ≥ 0.15, and SWT ≤ 0.60. Motif distributions were assessed using median–interquartile‐range boxplots, Kruskal–Wallis tests, epsilon‐squared effect sizes, false‐discovery‐rate correction, Spearman correlation, and standardized principal component analysis. Bell intervals recorded the highest median PHIE (0.258), NTG (0.980), and HCPV (0.195), whereas symmetrical intervals showed the highest median SWT (0.334). Cylindrical motifs contributed the greatest cumulative net pay (126.186 m) and hydrocarbon pore thickness (22.387 pore‐m), reflecting their greater frequency and aggregate thickness rather than perinterval superiority. Kruskal–Wallis tests detected no statistically significant motif differences after false‐discovery‐rate correction; SWT produced the largest effect size ( ε ² = 0.083). PHIE correlated strongly with NTG ( ρ = 0.88) and inversely with VCL ( ρ = −0.67), while SWT was inversely associated with log10(Rt) ( ρ = −0.57). The first two principal components explained 84.8% of total variance. Reservoir quality varied substantially within individual motifs, demonstrating that log shape alone does not provide statistically supported reservoir‐quality discrimination in this dataset. Gamma‐ray motifs are therefore best treated as architectural descriptors that should be integrated with quantitative petrophysics, borehole‐quality control, and multivariate statistics. The scope is limited to log‐derived interval‐scale relationships in six wells; no interval‐matched core, seismic structural framework, pressure, or production calibration is used to claim unique facies or structural compartmentalization.