A data-driven workflow for constructing depth-dependent rock-physics templates for the Oligocene D Formation in the Cuu Long Basin, offshore Vietnam

Abstract

Rock physics templates are widely used to link seismic elastic attributes with lithology, porosity, and pore-fluid effects. However, their application becomes uncertain where elastic responses vary systematically with burial depth, especially where laboratory-derived rock-frame parameters are unavailable. This study presents a data-driven workflow for constructing depth-dependent rock physics templates for the Oligocene D Formation reservoir interval in the Cuu Long Basin, offshore Vietnam. The workflow integrates well-log-derived elastic properties, petrophysical interpretation, core and composite-log information, and lithology interpretation from 15 wells. Measured logs, including Gamma Ray (GR), compressional wave slowness (DTC), shear wave slowness (DTS), bulk density (RHOB), water saturation, total porosity, and effective porosity, were used to calculate P-impedance (AI), compressional wave to shear wave velocity ratio (Vp/Vs), and saturated elastic moduli. Instead of relying on fixed laboratory-derived rock-physics parameters, matrix and dry-frame elastic moduli were estimated through a nonlinear optimization scheme by minimizing the mismatch between modeled and logged saturated bulk and shear moduli. The calibrated parameters were then used to generate rock physics templates in the P-impedance versus Vp/Vs domain for different depth intervals. Three key findings obtained from representative wells show that: (i) A single template cannot adequately describe the full D Formation interval because compaction, lithological variation, and shale volume shift the elastic trends with depth; (ii) Depth-calibrated templates improve separation of clean sand, shaly sand, and shale, and better capture porosity variations compared with a single generalized template; and (iii) A preliminary geostatistical interpolation of optimized elastic parameters suggests that depth-dependent templates can be extended beyond wells and enhance lithology and porosity prediction in seismic inversion workflows.