ISOTOPIC REVERSAL IN NATURAL GAS: AN EXAMPLE OF DEEP-STRATA GASES FROM XUJIAWEIZI DEPRESSION, SONGLIAO BASIN

The isotopic composition of the natural gas is affected by such factors as the organic type of the source rock, the thermal evolutionary growth, and the secondary change after the pool formation. The isotopic distribution pattern of the unit hydrocarbon is used to differentiate between geochemical fingerprints affected by various factors. The isotope in the unit hydrocarbon of the natural gas often turns heavier with the increase of the number of the carbon. However, the isotopic reversal in the unit hydrocarbon of the natural gas is relatively universal in the deep strata of the Xujiaweizi depression, Songliao basin. The high geothermal gradient and bacterial biodegradation have a small effect on the formation of the isotopic reversal in this region. No sufficient geological evidence is available for the consequence of the mixing process of the oil type gas in the shallow layer or of the inorganic gas. The mixing of different types of gases in the organic materials in the same layer and the evaporative fractionation via the micro leaking of the gas in the cap rock may have been the major causes for the isotopic reversal of the natural gas in the deep strata of the Xujiaweizi depression, Songliao basin.