Geochemical characteristics of shale gas in Xiasiwan area, Ordos Basin

Well head gas samples from nine shale gas wells in Xiasiwan area were collected to examine their geochemical characteristics through analyzing their composition, carbon and hydrogen isotopes, and light hydrocarbon. The study shows the shale gas from Yanchang Formation is wet gas from primary cracking of sapropelic kerogen, and the shale system, still a closed system, has expelled only small amount of gas, and thermal maturity is the paramount factor restricting shale gas exploration; the shale gas from Shanxi Formation is dry coal-derived gas from secondary cracking, characterized by isotope reversal and delta C-13(2) of less than -29%, which proves that the ethane's carbon isotope of coal-derived gas can be very low in high or over mature stage. A similar isotope "rollover" phenomenon in shale gas has also been observed in conventional gas of the Ordos Basin, so it is concluded that the gas in Lower Paleozoic is coal-derived gas and the C-13 light ethane is the result of secondary cracking instead of mixing with oil-associated gas. Analysis of some indexes (such as delta C-13(1), delta C-13(2)-delta C-13(1) and light hydrocarbon) shows that the recoverable resources of shale gas in the mature stage are overestimated.