轮南低凸起凝析气藏的蒸发分馏作用机制

Mechanism of evaporative fractionation in condensate gas reservoirs in Lunnan low salient

在蒸发分馏作用控制下,凝析气藏的形成将不受烃源岩热演化程度的制约,这是对经典生油理论中凝析气藏成因机制的一种重要补充。轮南低凸起奥陶系及石炭系发育大规模的凝析气藏,地球化学的研究结果表明,这些凝析气藏中凝析油Rc在0.8%～1.0%(远低于热裂解成因Ro=1.3%的门限),凝析气为过成熟干气(回归Ro>2.0%);同时,区内原油与干气的充注期相异,表明轮南地区的凝析气藏并非热裂解成因的产物。通过分析该区各层系凝析气藏流体性质及相态特征发现,轮南地区凝析气藏具有典型的蒸发分馏特征:下部(奥陶系)层系中凝析气藏的地露压差较小,带有典型的油环;上部(石炭系)层系中凝析气藏的地露压差较大,以纯气相形式存在。石炭系的凝析油相对于下伏的奥陶系残余油具有密度较轻、饱和烃含量偏高的特征。结合该区烃源岩热演化、油气成藏时间、断裂活动史等成藏要素的四维耦合性剖析,结论认为:喜山期断裂泄压作用以及大量他源干气的侵入诱发了该区蒸发分馏作用的产生。

Under the control of evaporative fractionation, formation of condensate gas reservoirs is no longer constrained by thermal evolution of source rocks. This is an important supplement to condensate gas pooling mechanisms of typical hydrocarbon generation theories. Large condensate gas pools are developed in the Ordovician and Carboniferous in the Lunnan low salient. Geochemical study shows that the Ro of gas condensate in these gas pools is in the range of 0.8% - 1.0% (far below the threshold of Ro=1.3% for thermal cracking), and the condensate gas is overmature dry gas (with Ro>2.0%). Moreover, the crude oil and dry gas in the study area were charged in different stages, indicating that the genesis of the condensate gas was not thermal cracking. Analyses of fluid properties and phase behaviors of the condensate gas in each layer reveal that the characteristics of typical evaporative fractionation. The condensate gas reservoirs in the lower interval (the Ordovician) are characterized by relatively low difference between formation pressure and dew point pressure and typical oil rims, while that in the upper interval (the Carboniferous) feature in relatively large of the difference between formation pressure and dew point pressure and pure gas. In comparison with the residual oil in the underlying Ordovician, the gas condensate in the overlying Carboniferous is lighter in density and higher in saturated hydrocarbon content. Comprehensive analyses of hydrocarbon pooling elements such as thermal evolution of source rocks, the time of pooling and the history of fracturing show that evaporate fractionation was induced by the Himalayan decompression through fractures and the invasion of large volume of allochthonous dry gas.