Drilling fluid compositions include an emulsifier having a generic structure A, structure B, or a combination thereof. Structure A includes an amide (e.g., amic acid group) and structure B includes a cyclic imide. The emulsifier of the emulsifier system is formed by reacting a fatty oil amine (e.g., oleyl amine), with a cyclic anhydride (e.g., succinic anhydride) in the absence of diluent or in a diluent that does not react with the starting materials. The reaction takes place via application of a stepwise increase in temperature. An emulsifier based on structure A is formed when the reaction temperature is maintained at 50 to 100° C. for 1 to 3 hours. A further increase in reaction temperature (e.g., up to 200° C.) can include water elimination which results predominately in the formation of a molecule represented by structure B.