The space distribution of the hydrocarbon geochemical field changes with time evolution and geological processes such as geothermal field, stress field and hydrodynamic field. The hydrocarbon field source is the underground hydrocarbon congeries, and its evolution is in the exchange of matter and the transformation of energy between hydrocarbon and its surrounding medium. So changes of the terrane and the lithology around the hydrocarbon take place because of their interaction, and they will impose geochemical effects on near surface. The abnormality of geochemical indexes over a reservoir is regarded as one of the most direct effects incurred by the geochemical field. The factors influencing geochemical exploration include the attribute of the reservoir, the attribute of geochemical index (the character of geochemical index), the transportation system (terrane and the fractured formation), and the environment of the earth surface. The successful examples of geochemical exploration display that the combination use of data smoothing, multi-methods and multi-index do well in reducing the interferences of shallow hydrocarbon factors to the geochemical exploration abnormality, increasing the reliability of abnormal signals for targeting deep oil and gas reservoirs.