A drilling fluid formulation is provided, which includes a fluid phase, an organophilic phyllosilicate as an anti-sagging additive, and a weighting agent (e.g. barite). The organophilic phyllosilicate contains an ion-exchange reaction product of a clay material (e.g. smectite) and quaternary ammonium cations. The organophilic phyllosilicate is effective in preventing barite sagging as demonstrated by low sag factor when drilling at elevated temperatures. Rheology properties of the drilling fluid including gel strength, yield point, plastic viscosity, and storage modulus are also specified.