青藏高原伦坡拉盆地沉积岩中烷烃氢同位素与藿烷甾烷类成熟度指标的关系

Relationship between hydrogen isotopic values of alkanes and maturity parameters in sedimentary rocks of the Lunpola Basin, Qingzang Plateau

沉积岩中烷烃的氢同位素有潜在的古降水同位素意义，但是成岩作用有可能改变原始同位素信息，从而难以用于恢复古降水同位素。然而，判断烷烃氢同位素值（δD）是否受到了成岩改造作用在学术界尚无统一标准。本文以青藏高原中部伦坡拉盆地渐新世末期到中新世早期的沉积岩为研究对象，对样品中正构烷烃和植烷（Ph）δD值以及藿烷、甾烷类的成熟度指标进行了测定。Ph与nC18正构烷烃的δD值分别为–105‰-–139‰和–267‰-–324‰，两者间平均相差–170‰，暗示正构烷烃原始δD值基本处于未改变的状态。藿烷、甾烷类的成熟度指标显示样品在未成熟到低成熟范围内有较大变化，但各成熟度指标与 Ph、nC18正构烷烃的δD值之间的相关关系较弱，表明这些成熟度指标并不能很好地反映烷烃δD值是否遭受了改变。

N-alkaneδD of sedimentary rock is a potential indicator of paleo precipitationδD. However, it could be affected by the diagenesis leading to n-alkane δD unapplicability for reconstructing paleo precipitation δD. Till now, no unified standard is suggested to judge whether or not alkaneδD has been altered during sedimentation. In this paper, we investigate the relationship between alkane δD values and hopane- and sterane-based maturity indices of the Upper Oligocene to Lower Miocene sedimentary rocks in the Lunpola Basin, central Qingzang Plateau. The δD values of C18 n-alkane and phytane range from –105‰ to –139‰ and from –267‰ to –324‰, respectively. The large δD differences between C18 n-alkane and phytane indicate that primary information of alkane δD are well preserved. Hopane- and sterane-based maturity parameters show that maturity of the studied samples varies from immature to low mature. However, no good correlation is observed between maturity parameters and δD values of alkanes, suggesting that hopane and sterane-based maturity indices may not be suitable to determine whetherδD values of alkanes have been diagenetically altered or not.