Based on a statistical analysis of the main oil-gas geochemical indices and variation characteristics of 14 major oil-and gas-bearing basins or areas in China, this paper puts forward the criteria for classifying regional geochemical fields of the major oil-and gas-bearing basins. The variation regularity governing regional geochemical background fields of the major oil-and gas-bearing basins in China has been studied. It is pointed out that from west to east the geochemical background fields tend to decrease in intensity. The distribution and variation regularity of the regional geochemical background fields is as follows: The basins in western China are of the high regional background field, the basins in central and eastern China as well as the offshore basins belong to the intermediate regional background field, and the basins in northwestern China are of the low regional background field. Their variation also tends to increase i.e., from west to east, the fields change from uniform fields to nonuniform fields. The moisture coefficients of hydrocarbons tend to change from dry in the west to wet in the east. The zonal field characteristics of such basins as Liupanshan are discussed.