西藏乌郁盆地烃源岩有机地球化学特征研究

Organic geochemical characteristics of source rocks in Wuyu Basin,Tibet

乌郁盆地为西藏高原新生代山间盆地,渐新世以来自下而上发育渐新统日贡拉组、中新统芒乡组、上新统嘎扎村组、上新统宗当村组地层,这些地层中均有暗色烃源岩出露。地化分析表明,芒乡组烃源岩有机质类型好、丰度高、生烃潜量大,处于生油高峰期阶段;nC21-/nC22+均值为1.2,表明有机质主要来源于水生低等生物;Pr/Ph均值为1.2,表明有机质沉积环境为深湖—半深湖弱还原环境;γ-蜡烷指数较高,均值达到了3.32,说明沉积时期水体出现分层并具有较高的盐度,有利于有机质的保存;芒乡组烃源岩是乌郁盆地的主力烃源岩。芒乡组煤层样品有机质丰度较高,类型以Ⅲ型为主,进入成熟阶段,是良好的气源岩。

The Wuyu Basin is a Cenozoic intermontane basin in the Tibetan Plateau. Since Oligocene, it has been deposited in the basin from base to top the formations as followed： the Oligocene Rigongla Formation, the Miocene Mangxiang Formation, the Pliocene Gazhacun Formation and the Pliocene Zongdangcun Formation. Dark source rocks have been found during field explorations in these strata. Geochemical studies have indicated that source rocks from the Mangxiang Formation are featured by good organic type, high abundance and superior hydrocarbon generation potential. They are during the peak period of hydrocarbon generation. The nC2/nC：2. value is 1.2 in average, showing the lower aquatic organism sources for organic matters. The Pr/Ph value is 1.2 in average, showing the deep or semi-deep lacustrine weak reduction environment for organic matter deposition. The average value of γ-acerance index has reached 3.32, indicating the stratification pattern and high salinity situation of water during deposition, which are favorable for organic matter preservation. The Mangxiang source rocks are the major source rocks in the Wuyu Basin. Coal samples from the Mangxiang Formation are good gas source rocks which have reached the mature stage and are abundant in organic materials, type III primarily.