松辽盆地徐家围子断陷深层天然气同位素倒转现象研究

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

天然气的同位素组成受源岩有机质类型、热演化程度和成藏后的次生变化等多种因素的影响 ,其单体烃同位素分布特征是区分各种因素影响程度的地球化学指纹 .天然气单体烃同位素通常有随碳数增加而变重的趋势 ,而徐家围子深层天然气单体烃同位素倒转现象却比较普遍 ,高地温和细菌氧化对于造成该区同位素倒转的可能性不大 ,浅层油型气或无机气的混合作用没有充分的地质证据 ,同层有机质中不同类型气的混合和盖层微渗漏造成的蒸发分馏作用可能是导致徐家围子断陷深层天然气同位素倒转的主要原因 .

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.