The second member of the Paleogene Funing Formation (E1f2) in the Gaoyou Sag, Subei Basin, is a promising shale oil target, yet its organic matter (OM) enrichment mechanisms remain poorly understood. This study integrates petrological and multi-proxy geochemical analyses to investigate lithofacies, paleoenvironmental evolution, and OM enrichment of the E1f2 shale. Seven lithofacies types transition upward from laminated (submembers III to V) to blocky structures (submembers I to II). TOC and hydrocarbon potential increase stepwise from bottom to top, with Type II OM dominant. The paleoenvironment evolved from arid, saline, and semideep lacustrine with strong terrigenous input (V); through semiarid to arid, brackish to saline, and semideep to deep lacustrine with peak productivity (III to IV); to humid to semiarid, fresh to brackish, and deep lacustrine with minimal terrigenous input (I to II). Anoxia persisted throughout. OM enrichment is jointly controlled by paleoclimate, water depth, paleosalinity, and terrigenous input, with paleoproductivity subordinate and redox conditions insignificant. Critically, terrigenous input exerts a non-linear dual control, defining an optimal window where nutrient supply, dilution, and oxidation are balanced. The highest OM enrichment in submembers I to II results precisely from terrigenous input falling within this window. This challenges productivity/preservation-dominant paradigms and provides a new framework for shale oil sweet-spot prediction in saline lacustrine basins.