珠江口盆地原油中新二环、四环萜烷的检出及其地球化学意义

Identification and geochemical significances of novel bicyclic and tetracyclic terpanes in crude oils from the Pearl River Mouth Basin

珠江口盆地原油富含多种类型的特殊萜烷,经GC-MS分析,在二环倍半萜烷系列中检出2,2,4a,7,8-五甲基-十氢化萘(新C15二环萜烷);三、四环萜烷中鉴定出降A-奥利烷及相关化合物,并检测到一个C24未知结构的四环萜烷。新C15二环萜烷和降A-奥利烷源于奥利烷类先质物在成岩作用过程中的降解作用,可指示被子植物有机源的输入。这些奥利烷降解物的含量和分布在研究区原油中呈区域性变化。新C15二环萜烷A/补身烷和降A-奥利烷/C24未知四环萜烷比值可作为生源和指相参数,区分各坳陷或凹陷之间及内部的不同类型原油。盆地南部的珠二坳陷白云凹陷及番禺低隆起原油中这些陆源标志物很丰富,这两个参数值大都分别高于1.0和9.0,表明其烃源岩有机质生源中被子植物输入比例较高,且沉积环境的氧化性较强。盆地东部的珠一坳陷各凹陷原油中,这些奥利烷类降解物均相对较少,两个比值分别多在0.5和3.0之下,指示成油母质中陆源有机质较少。盆地西部的珠三坳陷文昌凹陷及邻区原油中,这些被子植物源标志物的含量变化在前两个坳陷原油之间,其中文昌A凹陷原油相对较高。

The crude oils from the Pearl River Mouth Basin are rich in various kinds of uncommon terpanes. Based on GC-MS analytical data, 2, 2, 4a, 7, 8-pentamethyl-decalin （novel C15 sesquiterpane）, de-A-oleanane and related tricyclic and tetracyclic terpanes, as well as an unknown C24 tetracyclic terpane, are identified in this study. The novel C15 sesquiterpane and de-A-oleanane are probably derived from degradation of oleanoids during diagenesis, so it can be useful as markers of angiosperm input. In addition, these compounds in the studied oils exhibit a regional change in relative abundance and distribution. Novel C15 sesquiterpane/drimane and de-A-oleanane/C24 unknown tetracyclic terpane ratios can be used as indicators of biological source and depositional environment, to recognize different kinds of the oils in various depressions of the Pearl River Mouth Basin. For example, the oils from Baiyun sag and Panyu low uplift in the Zhu-Ⅱ depression, southern region of this basin present abundant terrigenous-derived biomarkers. Their novel C15 sesquiterpane/drimane and de-A-oleanane/C24 unknown tetracyclic terpane ratios are higher than 1.0 and 9.0, respectively, indicating a significant input of angiosperm for organic source and an oxic depositional condition. In contrast, the oils from the various sags of Zhu- Ⅰ depression in the eastern basin contain an obviously lower concentration of the oleanane-degraded alkanes, and their C15 sesquiterpane/drimane and de-A-oleanane/C24 unknown tetracyclic terpane ratios are below 0.5 and 3.0, respectively, suggesting a less terrestrial input in biological source. The angiosperm-derived indicators in the oils from Wenchang sag and adjacent area in Zhu-Ⅲ depression of the western basin display middle abundances ranging between the oils in previous two depressions, with a relatively higher amount in the oils of Wenchang sub-sag A.