北部湾盆地始新统湖相富有机质页岩特征及成因机制  

Geochemical characteristics and formation mechanism of Eocene lacustrine organic-rich shales in the Beibuwan Basin

作　　者：黄保家 黄合庭 吴国瑄[2] 游君君 

机构地区：[1]中海石油(中国)有限公司湛江分公司,广东湛江524057 [2]同济大学,上海200092

出　　处：《石油学报》2012年第1期25-31,共7页Acta Petrolei Sinica

基　　金：国家重大科技专项(2011ZX05025-002);油气资源与探测国家重点实验室开放课题(KFKT2010-01)资助

摘　　要：始新统流沙港组二段发育的中深湖相富有机质页岩是北部湾盆地涠西南凹陷重要的优质烃源岩,这些富有机质页岩呈深灰色或棕褐色,页理发育,富含非海相沟鞭藻、葡萄藻、光面球藻等藻类化石。有机碳含量在3.07%～10.35%,有机质以偏腐泥的混合型为主;Pr/Ph为0.65～2.5,富含五环三萜、三环二萜及四环萜烷,而指征陆源高等植物输入的奥利烷含量甚微;C27、C28、C29规则甾烷呈"V"型分布,源于低等藻类的C304-甲基甾烷十分丰富,表明其有机质来源主要为低等水生生物。丰富的藻类化石,尤其是较多非海相沟鞭藻的出现,和相对高丰度的无定形有机显微组成反映流二段富有机质页岩形成于湿润气候条件下的微咸水且水体分层的中深湖缺氧环境中。流二段顶部富有机质页岩发育于高水位体系域的顶面;流二段底部富有机质页岩分布于水进体系域的底面。该区流沙港组二段沉积时湖水较深,沉积速率较慢,湖盆处于欠补偿状态;由于水体分层结构,在表层藻类等水生生物勃发乃至死亡导致湖水有机质输送量增多,而古湖底层缺氧环境使沉积有机质得到有效保存,从而造就了这套湖相富有机质页岩。Mid-deep lacustrine organic-rich shales in Member 2 of the Liushagang Formation（LS2） are the most important high-quality source rock in the Beibuwan Basin.These dark-grey or-brown shales with developed lamellation contain abundant non-marine dinoflagellate,rugasphaera and leiosphaeridia fossils,they are rich in organic matter,with TOC content ranging between 3.07%~10.35%,type II1 and I kerogen and Pr/Ph ratios varying from 0.6 to 2.5.The shales are characterized by a high abundance of 4-methyl steranes that are probably related to certain dinoflagellates thriving in lakes,relatively abundant pentacyclic triterpanes,tricyclic diterpanes and tetracyclic terpanes but a very tiny amount of oleanane indicative of terrigenous higher-plant input,and a ＂V＂-shaped distribution of C27,C28 and C29 regular steranes.All these characteristics indicated that the organic matter in those shales was mainly of algal origin.The data of various algal fossils,especially plenty of non-marine dinoflagellates and amorphous organic macerals suggested that they were formed in an anoxic and stratified environment of mid-deep brackish lakes in humid climate,where organic-rich shales of the upper LS2 occurred at the top of a high-stand system tract and those of the lower LS2 were distributed at the bottom of a transgressive system tract.In this area the sedimentation rate for Member 2 of the Liushagang Formation（LS2） was relatively low and the lake remained as a starved basin,meanwhile the bloom and death of algae near surface supplied a great deal of organic matter to lacustrine sediments,and the oxygen-depleted bottom water of palaeo-lakes,due to the water-column stratification,favored the accumulation and preservation of sedimentary organic matter,resulting in the formation of organic-rich shales,a high-quality source rock.

关 键 词：富有机质页岩 地球化学特征 生物标志物 藻类化石 中深湖相 北部湾盆地 

分 类 号：TE125.2[石油与天然气工程—油气勘探]