Abstract: Hydrocarbon vertical microseepage is the theoretical foundation of surface geochemistry exploration.Surface geochemical anomalies are dominated by oil-and-gas distribution,cap rock,faults and fractures system as well as reservoir fluid property.In order to learn the dominating factors of surface expressions from hydrocarbon microseepage,surface geochemical explorations with free gas and headspace gas methods have been made over the Zhujiadun Gas Field,the Yancheng Sag,the northern Jiangsu Basin.It has been found out that,hydrocarbon index anomalies are in good response to favorable migration passages over gas fields,proving hydrocarbon vertical microseepage to surface.Analyzing sealing conditions and migration pathways of known gas accumulations,it has been concluded that fracture system in faults and cap rocks and microfracture system in strata are the controlling factors for near surface anomalies of hydrocarbon vertical microseepage.The results are good evidences for further studies of anomaly pattern and mechanism over oil and gas accumulations.