PURPOSE: To prepare a composition for enhanced oil recovery comprising sea water containing a small amount of a high molecular weight poly(vinylamine) and demonstrating a stability even under the condition of an elevated temperature and the presence of Ca-salts because of its improved viscosity retentiveness in the sea water.

CONSTITUTION: This invention relates to a composition for enhanced oil recovery comprising sea water containing 500 to 2,500 ppm poly(vinylamine) of which average molecular weight is at least 106 and ≥50% of which is hydrolyzed. The composition shows 5 to 25 cps Brookfield viscosity at 7.9 sec-1 and at ≤90°C. The viscosity retentiveness is improved when the composition is used for the enhanced oil recovery because viscosity stability at 90°C is improved. This vinylamine polymer with an extremely high molecular weight demonstrates an improved stability under the condition of an elevated temperature and the presence of Ca-salts (i.e., the conditions when it is used as a thickener for enhanced oil recovery at an elevated temperature).