The present invention relates to a method to prepare a rheological agent comprising the polymerization of the following monomers: (a) at least one styrene-based monomer, (b) at least one (meth)acrylate-based monomer or butadiene, and (c) optionally a copolymerisable surfactant containing an optionally substituted vinyl function and moieties derived from propylene oxide and/or ethylene oxide, optionally in the presence of a non polymerisable surfactant containing moieties derived from propylene oxide and/or ethylene oxide, and optionally in the presence of another surfactant, wherein at least the copolymerisable surfactant or the non polymerisable surfactant is present during the said polymerization, as well as a rheological agent obtainable by said method, an oil-based drilling fluid containing such a rheological agent, the use of such a rheological agent as a thixotropic agent in an oil-based drilling fluid and a method of drilling using an oil-based drilling fluid containing such a rheological agent.