Geochemical exploration for oil and gas is based on the vertical micro-migration theory of hydrocarbons. Based on a comprehensive discussion of the indexes of oil and gas geochemical exploration, the authors hold that hydrocarbon, pyrolysis-Hg and methane isotope in the soil are effective indexes for oil and gas geochemical exploration. The model of anomaly is also discussed. According to the surface geochemical anomaly characteristics and combined modeling, the property and preservation situation of a deep oil and gas pool can be judged, which provides a dependable geochemical basis for oil and gas exploration.