塔里木盆地前陆区和台盆区天然气的地球化学特征及成因

Geochemistry and Genesis of Natural Gas in the Foreland and Platform of the Tarim Basin

通过对塔里木盆地天然气碳氢同位素分析,主要存在两种类型天然气,即油型气与煤型气。油型气烷烃气碳同位素组成较轻(δ13C2<-28‰,δ13C3<-25‰),氢同位素组成偏重,成烃母质主要为海相沉积环境形成的寒武系—下奥陶统或中下奥陶统烃源岩,分布区域主要为台盆区;而煤型气烷烃气碳同位素组成较重(δ13C2>-28‰,δ13C3>-25‰),氢同位素组成偏轻,成烃母质主要为陆相沉积环境形成的三叠系—侏罗系烃源岩,分布区域主要为前陆区。在塔里木盆地,烷烃气同位素组成局部倒转主要与烃源岩热演化程度差异有关;同时,在局部地区硫酸盐热还原(TSR)也可引起碳同位素组成的局部倒转。塔里木盆地天然气中3He/4He值偏高可能与残留在岩石中的少量深部气体混入气藏有关。

Based on carbon and hydrogen isotopic compositions of gaseous alkanes in the Tarim Basin, two type gases were recognized： oil-type and coal-type. The former with lighter carbon （δ^13 C2〈-28‰, δ^13 C3〈-25‰） and heavier hydrogen was mainly derived from the marine sources in Cambrian--Lower Ordovician or Middle--Lower Ordovician, distributing mainly in the platform of the basin; the latter with more negative δ^13C （δ^13C2〉-28‰,δ^13 C3 〉-25‰） and less negative 3D originated from continental humic in Triassic-Jurassic, distributing in the foreland of the basin. The partial reversal trend of carbon isotopes for gaseous alkanes in the Tarim Basin may be related to thermal maturity degrees of sources. TSR is also a reason for the partial reversal trend of carbon isotopes in some regions. The relatively higher ^3 He/^4He ratio of the natural gas from the Tarim Basin may be caused through the mixing of a little deep gas remained in rocks into the gas reservoirs.