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Reconstructing syn-rift paleoenvironmental systems requires an understanding of the complex interplay among tectonics, climate, and lacustrine sedimentation, which is crucial for petroleum exploration. The Piçarras Formation, deposited during the Barremian in the Santos Basin, offshore Brazil, constitutes a significant example of an alkaline lacustrine environment shaped by tectonic subsidence and climatic variability. This study presents the first integrated petrographic, mineralogical, lithogeochemical, and geophysical analysis of the formation, based on cutting samples and well logs from the 1-BRSA-1363-RJS well. Eight lithologies were classified into Mg-clay-bearing and Mg-clay-free assemblages, each with distinct mineralogical and geophysical signatures. A key insight of this study is the identification of saponite, a trioctahedral smectite occurring exclusively at depths below 6000 m, which serves as a diagnostic Mg-clay mineral for the Piçarras Formation. Its presence is associated with high alkalinity, brackish lacustrine waters, and low-energy depositional settings, correlating with elevated MgO contents (up to 16.1%). Saponite influences geophysical responses, notably by increasing clay-bound water in NMR logs, lowering resistivity, and altering neutron and sonic log readings. The Piçarras Formation comprises laminated siliciclastic mudstones, lime mudstones, laminites, fragmented bioclastic packstones, and hybrid rocks, recording cycles between more arid periods characterized by carbonate deposition and less arid intervals marked by increased siliciclastic input. A sharp lithological and mineralogical transition defines the contact with the Itapema Formation, evidenced by the disappearance of saponite and shifts in geochemical trends, such as SiO2 and MgO. Operationally, saponite's swelling properties pose significant challenges for drilling with water-based fluids. This work refines the paleoenvironmental understanding of the Piçarras Formation and offers insights critical for pre-salt exploration and drilling strategies.