Abstract: The formation mechanism of strong carbonate-cement sand caprock of Yingnan-2 condensate gas reservoir in the eastern Tarim Basin was discussed.There are two luminescences of calcareous cement in tight sandstone exhibited under the cathodoluminescence microscope.The earlier phase calcite shows nacarat color,and the later phase calcite shows saffron.The earlier phase calcite was replaced by the later one.The carbon isotopic compositions of carbonate cement by using laser micro-pyrolysts show a strong negative isotopic excursion (δ¹³C value ranges from-7.17‰ to-9.06‰),which relates to microbe participating.The δ¹⁸O values ranged from-12.44‰ to-21.85‰ of carbonate cement show that the diagenesis of carbonates was formed by freshwater seepage.The carbonate cement was principally formed by oxidation of thermogenic migrating hydrocarbon.Bacterial oxidation of the migrating hydrocarbon within the lower Jurassic sands was probably by sulphate reduction and liberated carbon dioxide for subsequent carbonate precipitation.Therefore,the tight sandstone is the product of "hydrocarbon-related diagenetic zones"(HRDZs).The formation of HRDZs is a slowly biodegraded process of hydrocarbon.During formation of HRDZs,gas composition and δ13C values of gas reservoir in Yingnan-2 reservoir were fractionated.Yingnan-2 reservoir is a special condensate gas reservoir,in which source rock is under layer,and container rock lies on super stratum.The formation of gas reservoir is a process of earlier phase diffusion and later phase accumulation.Consequently,the formation process of Yingnan-2 gas reservoir is prominently related to the organic-inorganic interaction.

Key words: Tarim Basin, Yingnan-2 gas reservoir, cement, isotope analysis, gas reservoir formation mechanism

摘要： 探讨了塔里木盆地东部英南2凝析气藏的非常规盖层———致密钙质胶结砂岩的形成机理。这种致密砂岩孔隙中钙质胶结物共有两期,早期方解石阴极发光下发出桔红色光,晚期发出桔黄色光,早期方解石被晚期交代。碳酸盐岩胶结物微区和全岩碳、氧同位素分析表明,钙质胶结物碳同位素组成表现出与生物成因有关的负值,δ¹³C分布在-7.17‰～-9.06‰,氧同位素组成具有淡水渗流成岩的特点,δ¹⁸O分布在-12.44‰～-21.85‰,这说明致密砂岩是"与烃类相关的成岩作用带(HRDZs)"的产物。HRDZs的形成是烃类缓慢渗漏遭受生物降解的过程,它使气藏的天然气组分的成分和同位素组成产生了分异。该气藏是特殊的下生上储、早期散失、晚期聚集成藏的次生凝析气藏,成藏过程伴随着圈闭致密性的逐渐加强,气藏的成藏过程是有机无机相互作用的结果。

关键词: 塔里木盆地, 英南2气藏, 胶结物, 同位素分析, 气藏成藏机理