A Cross-Plot–Based Petrophysical Workflow for Lithology Discrimination and Shale–Clay Typing

Abstract

Petrophysical evaluation of sandstone reservoirs is often complicated by shale distribution and clay mineral variability, which significantly affect porosity and fluid saturation estimates. This study presents the implementation of integrated petrophysical cross-plot methods for lithology identification, shale characterization, and clay type analysis in the Messla sandstone reservoir, Sirte Basin, Libya using conventional well log data. Neutron–density, MID Matrix Identification Density Log, density–photoelectric factor (PEF), thorium–potassium (Th–K) cross-plots and Thomas-Stieber cross-plots were employed to discriminate sandstone lithology, evaluate shale distribution, and infer dominant clay minerals. The results demonstrate that cross-plot techniques provide reliable lithological separation and shale typing using log data only, offering a cost-effective and robust interpretation approach in the absence of core measurements. The proposed workflow improves reservoir characterization and reduces uncertainty in petrophysical interpretation of sandstone formations. The results indicate that the Messla reservoir is dominated by quartz-rich sandstone with variable shale content, mainly occurring as laminated and dispersed shale. Clay typing analysis suggests the predominance plot of a kaolinite-dominated clay assemblage with minor mixed-layer contributions in cleaner sandstone intervals. The proposed workflow demonstrates the effectiveness of cross-plot methods as a rapid and reliable approach for petrophysical characterization in data-limited environments and provides valuable input for reservoir quality assessment and completion planning in Libyan sandstone reservoirs.