<p>Cementing is a critical operation in petroleum well drilling. It is important to have high-quality cement in the annulus to ensure casing protection and prevent fluid flow into the well. The cement quality in the annulus is often evaluated using the bonding index, derived from the cement bond log/variable density log (CBL/VDL) and the segmented bond tools log (SBT) logs. However, the reliability of these logs in assessing cement slurry quality remains uncertain due to influencing factors such as formation lithology. This study interprets SBT logs from eight wells. Log interpretation reveals that washouts are more frequent in sandstone formations than limestone, negatively impacting the bonding index between cement and casing. The aim of this paper is to investigate the reasons behind the weak bonding index. The hypothesis that sandstone cuttings contaminate cement, weakening the bond, was tested through experiments, including crush tests, ultrasonic cement analyzer (UCA), and CT scans. Three samples were prepared: pure cement, limestone-contaminated cement, and sandstone-contaminated cement. Surprisingly, sandstone-contaminated cement exhibited 523 psi higher compressive strength than limestone-contaminated samples. CT scans quantified impurity volumes, showing 14.6% for sandstone and 17.6% for limestone, correlating with stronger bonds in sandstone-contaminated cement. In contrast, UCA results indicated weaker bonding in sandstone samples, aligning with SBT log observations. UCA’s compressive strength results showed that ultrasonic waves encounter more difficulties in sandstone formations. These findings conclude that weak bonding indices through sandstone formations are more due to the sandstone’s nature, which affects ultrasonic and sonic waves, than the cement slurry quality.</p>