<p>This study presents the first documented occurrence of the Fe-rich clay minerals berthierine and chamosite in continental siliciclastic sandstones of the Oligocene Mugrosa Formation, within the Nuevo Mundo Syncline of the Middle Magdalena Basin (MMB), Colombia. These hydrocarbon-saturated sandstones, comprising over 70% of the rock volume, were analyzed using an integrated geochemical and mineralogical approach that included optical petrography, X-ray diffraction (clay fraction), Raman spectroscopy, and SEM-EDS (scanning electron microscopy with an energy-dispersive X-ray spectrometer). Berthierine was identified as an early authigenic phase formed under reducing conditions, which then transformed into chamosite during mesogenetic burial. This mineralogical change indicates a complex interaction of diagenetic and catagenetic processes, including compaction, cementation, dissolution, replacement, hydrocarbon biodegradation, and secondary migration. Additional factors, such as redox reaction-controlled fluid-rock interactions, mixing with meteoric and chemically undersaturated waters, and regional burial-exhumation events in the study area, further supported the formation and transformation of these Fe-rich phyllosilicates. These features are observable at both macroscopic and microscopic scales. The berthierine-to-chamosite ratio shows potential as a quantitative indicator for paleotemperature reconstruction and reservoir diagenetic evolution in petroleum systems, supported by geochemical trends and 1D thermal modeling. These findings improve understanding of authigenic clay mineralization in continental depositional systems and its impact on reservoir quality in petroleum basins.</p>