NOVELTY - Preparing a rheological agent, comprises polymerizing monomers comprising (a) at least one styrene-based monomer, (b) optionally at least one (meth)acrylate-based monomer or butadiene, and (c) optionally a copolymerizable surfactant containing an optionally substituted vinyl function and moieties including propylene oxide and/or ethylene oxide, preferably an ionic and non-ionic surfactants, optionally in the presence of a non polymerizable surfactant containing moieties, and optionally in the presence of another surfactant.    USE - The rheological agent is useful as a thixotropic agent in an oil-based drilling fluid (claimed), which is useful for drilling oil, gas and geothermal subterranean wells.    ADVANTAGE - The rheological agent exhibits improved dissolution rate in oil-based drilling, improved chemical and physical stability on a large range of temperatures, improved thixotropic properties i.e. rheological agent forms a gel upon standing, has a low viscosity under high shear, allows control of the rheology of the drilling fluid during transport and storage phase, and provides better control of the rheological profile of the drilling fluid, such as a lower plastic viscosity.    DETAILED DESCRIPTION - Preparing a rheological agent, comprises polymerizing monomers comprising (a) at least one styrene-based monomer, (b) optionally at least one (meth)acrylate-based monomer or butadiene, and (c) optionally a copolymerizable surfactant containing an optionally substituted vinyl function and moieties including propylene oxide and/or ethylene oxide, preferably an ionic and non-ionic surfactants, optionally in the presence of a non polymerizable surfactant containing moieties, and optionally in the presence of another surfactant, where the copolymerizable surfactant or the non polymerizable surfactant is present during the polymerization. INDEPENDENT CLAIMS are included for:    (1) the rheological agent obtainable by the method; and    (2) an oil-based drilling fluid comprising a rheological agent.