【作者】 王立成；

【导师】 王成善； 李亚林；

【作者基本信息】 中国地质大学（北京） ， 矿产普查与勘探， 2008， 硕士

【摘要】 白垩纪C-T(Cenomanian-Turonian)界线大洋缺氧事件(OAE2)及其所形成的黑色页岩的成因是国际上的热点问题,藏南地区记录了此次事件。本文根据藏南岗巴、定日和江孜地区四条经典剖面的沉积学和有机地球化学研究表明,浅海陆棚相的岗巴地区冷青热组和察且拉组黑色页岩、浅黑色钙质页岩,碳酸盐缓坡相的定日地区岗巴东山组黑色页岩以及下斜坡相的江孜加不拉组黑色页岩有机碳含量一般大于0.8%,少数最高可超过2%。生油母质来源主要是以海相浮游生物和菌藻类为主,生烃潜力一般,但黑色页岩的有机碳含量要比西特提斯明显偏低。有机碳堆积速率AR-org和碳酸钙堆积速率AR-CaCO3的研究表明,在岗巴地区二者变化基本相同,而且基本上都在C-T界线时达到最大值,而江孜地区的不一致则可能是由于增加的AR-CaCO3稀释了有机质组分。据此计算了岗巴、定日地区C-T界线时沉积物的古生产力为77.96gC /m2·a和11gC /m2·a,明显偏低,这与碳同位素正偏、高沉积速率和可能的上升洋流所反映的高古生产力不符;通过碳酸钙堆积速率与古生产力之间的转换函数,得到江孜地区的C-T界线黑色页岩的古生产力在加不拉和床得剖面分别为46.0 gC /m2·a和17.3 gC /m2·a。由于该地区曾发生过大洋缺氧事件,这种缺氧滞留环境(“黑海模式”)的存在也极有可能存在早期形成富有机碳的黑色页岩。有机碳与沉积速率关系图解也反映了C-T界线时,藏南地区都应存在着高有机碳的沉积物。通过不同沉积环境下古生产力、沉积速率、岩性和有机质显微组分等特征的分析,表明高古生产力、高沉积速率和细粒沉积物更易形成富有机碳沉积,而陆棚、斜坡和盆地相的干酪根有更好的生油潜力。更多还原

【Abstract】 The transition of late Cenomanian to early Turonian is characterized by a well known Oceanic Anoxic Events (OAE2) associated with widespread bituminous black shales. The cause of enrichment of organic carbon in the black shales has been under vigorous discussion. The Cenomanian-Turonian black shales, which were regarded as the sediments of OAE2, were found in Gangba, Tingri, and Gyangze areas in southern Tibet. A combined research from 4 well documented sections sedimentary and organic geochemistry in these areas indicates that black shale and darkish calcareous shale of Lengqingre Formation depositioned on shallow shelf at Gamba, black shale of Gangbadongshan Formation depositioned on carbonate ramp at Tingri and black shale of Gyabula Formation depositioned on lower slope at Gyangze provide moderate source rock potential which is characterized by an average TOC more than 0.8%, only some more than 2%, and is originated from marine plankton, bacteria and algea. However, the TOC value of the black shale is apparently lower than those in the western Tethys. The plots of mass accumulation rates of TOC and calcic carbonate present a same trend as a certain section at Gamba, and reach their maximum value at C-T boundary (CTB) ,but show a different trend in Gyangze possibly due to increased carbonate mass accumulation rate has diluted the organic matter content . We calculated the paleoproductivity of sediments during CTB based on the mass accumulation rates, an average values 77.96gC /m2·a and 11gC /m2·a at Gamba and Tingri respectively, which is disagree with the result come out from positive excursion ofδ13 C ,high sedimentation rate and possible upwelling. It certainly be the result of weathering. The paleoproductivity of CTB black shale shows average values of 46.0 gC /m2·a and 17.3 gC /m2·a from Gyabula section and Chuangde section respectively based on a transfer function between carbonate mass accumulation rates and paleoproductivity which is suit for open ocean environments. It also likely resulted in organic-rich black shale deposition during the CTB for its restricted deep-water circulation just like Black Sea. Moreover, the correlation between TOC and sedimentation rate may be used as a paleoenvironmental indicator, indicates that organic carbon-rich sediments should be deposited and the source rock potential of black shale should be much more than expected in southern Tibet. Detailed analysis on paleoproductivity, sedimentation rate, lithology and compositon of organic matter in the different depositional environment indicated that high paleoproductivity, high sedimentation rate and fine-grained sediments is more likely to organic carbon-rich sediments, and kerogen deposited at shelf, slope and abyssal basin has a good oil source rock potential.更多还原