Abstract: Recent petroleum exploration has revealed that the vertical zoning in the hydrogen chemistry of oil field water in petroleum bearing formations may appear reversed. The main manifestations are that the degree of mineralization reduces when the depth increases, the type of hydrochemistry is a Na-HCO₃-type water with rich sodium ions and poor calcium ions,and there is clearly a negatyve correlation between the contents of Ca²⁺ and HCO₃ ions. These characteristics are more obviously near petroleum gathering areas. So a thorough study on them will provide beneficial information for petroleum exploration.