Abstract: Based on the geological condition analysis of surface geochemical prospecting for oil and gas in Ejin Banner and its vicinities, the authors systematically analyzed such indexes as concentrations of methane and heavy hydrocarbon, dry coefficients and methane carbon isotope. There exist high hydrocarbon background values and strong hydrocarbon anomalies in the area, and the anomalies are characterized by relatively heavy methane carbon isotope and high dry coefficient. Combined with an analysis of typical oil and gas reservoirs and geochemical exploration anomalies in different geological backgrounds, it is considered that there are two sets of petroleum systems with different evolution degrees of hydrocarbon source rocks, and that the oil and gas geochemical exploration anomalies are mainly hydrocarbon geochemical anomalies of highly mature-overmature hydrocarbon source rocks or mixed-source oil and gas reservoirs (or compound oil and gas reservoirs).