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Having studied the biomarker composition and maturity of dissolved hydrocarbons from Ordovician formation waters, the authors presented molecular geochemical evidence for the controversial origin of natural gases in central Ordos Basin. The dissolved hydrocarbons in Well Shan 12 and Well Shan 78 are relatively high in abundance of tricylic terpane, pregnane series and dibenzothiophene series and low in Pr/Ph (<2) and hopane/sterane ratios, indicating the source input of marine carbonates. In contrast, the dissolved hydrocarbons in Well Shan 81 are free from tricyelic terpane and pregnane series, with trace dibenzothiophene series and high Pr/Ph (3.27) and higher hopane/sterane ratios, which are the typical features of terrestrial organic matter. Furthermore, Well Shan 37 and Well Shan 34 are between the two situations, having a mixed source of marine carbonate and terrestrial organic matter. The maturity of biomarkers also supports the above suggestions. These results are consistent with the geological background and source rock distribution in this region. [ABSTRACT FROM AUTHOR]