According to the methane

heavy hydrocarbons and mean ΔC measured with acidolytic hydrocarbon method

and variation coefficient

the geochemical fields in China's major petroliferous basins can be classified as high

medium and low ambient fields

as well as highly non-homogeneous

non-homogeneous and homogeneous fields.As a whole

the geochemical fields of the petroliferous basins in western China(to the west of Ordos and Sichuan basins) are higher than that of the petroliferous basins in eastern China

and that of petroliferous basins in northeastern China is lower than that of the petroliferous basins in central China but higher than that of the petroliferous basins in southern China.In addition to the petroleum geologic conditions

the main causes are also related with the properties of near surface sediments and the geographical and geomorphologic landscape conditions.Based on the results of nation-wide geochemical reconnaissance surveys

and combined with the composite features of geochemical prospecting anomalies in the proven oil and gas reservoirs

areas with strong local composite anomaly and good consistence of various indexes in relatively under-explored regions are selected as prospective areas

including Changling fault depression in the southern end of central depression

western slope

and Qianjiadian fault depression in southwestern uplift of Songliao basin

Tanzhuang-Shenqiu depression in Zhoukou basin

Xiaman-Manrima-Awancang anomaly zone in Songpan-Aba region

and Wuwei-Hexian and Wangjiang anomaly zones in Lower Yangtze region.Twenty five plays have further been subdivided based on the results of geochemical prospecting in 7 basins(or regions).