PURPOSE: To recover and move a highly viscous petroleum derivative in the form of an aqueous dispersion by using a sulfonate dispersant prepared by sulfonating a steam-cracked fuel oil with a molecular weight increased with a catalyst such as BF3 and neutralizing the product of sulfonation.

CONSTITUTION: A high-boiling steam-cracked fuel oil (FOK) produced by cracking naphtha or the like is oligomerized with the aid of a catalyst being BF3 or a BF3/strong acid complex to increase its molecular weight. The amount of the catalyst used is such that 0.01-0.2 mol, desirably, 0.02-0.06 mol of boron is present per 100 g of FOK. The obtained compound is sulfonated with SO3 or the like to form a sulfonate, and the sulfonate is neutralized with a hydroxide of an alkali metal, an alkaline earth metal or ammonium to prepare a sulfonate dispersant. The weight ratio of the SO3 to the oligomerized FOK is 0.7-1.7, desirably, 0.8-1.5. This dispersant is used in an amount of 0.2-2.5 wt.% based on the dispersion to convert a heavy petroleum product into an aqueous dispersion (of a water content of, desirably, 15-40 wt.%), which is then moved and stored.