鄂尔多斯盆地西缘中-上奥陶统烃源岩的形成环境、成烃生物及岩矿特征

The formation environment,hydrocarbon generating organisms,rock and mineral characteristics of the Middle-Upper Ordovician source rock layers in west margin of Ordos Basin

鄂尔多斯盆地西南缘奥陶系发育与盆地本部奥陶系沉积特征截然不同的沉积层,其中西缘的中-上奥陶统烃源岩发育较好的海相烃源岩层。近期,通过对该套烃源岩形成的构造—沉积环境、成烃生物特征及烃源岩层岩矿特征等方面的分析,表明有利烃源岩的发育受多种因素的控制,其分布具有一定的规律性:1)盆地中-上奥陶统主力烃源岩主要形成于沉积水体不断加深的构造沉积背景,其中沉积水体能量较弱,陆源碎屑注入相对较慢的半深水斜坡环境最有利于富有机质烃源岩发育;2)西南缘奥陶纪虽然生物类型多样,种群繁盛,但能形成重要烃源岩层的母源生物(生烃母质)却多以浮游生物、疑源类(微体藻类)及蓝细菌为主,相对还原的介质环境和适宜的沉积埋藏速率对有机质的有效保存也起重要的控制作用;3)X-衍射物相分析及扫描电镜等微观研究显示,主力烃源层段所谓“泥页岩层”的造岩矿物类型仍以石英、长石类为主,次为伊利石类黏土矿物与碳酸盐矿物,少量黄铁矿等;黏土矿物及黄铁矿含量与有机质丰度关系密切。

The Ordovician in the southwest margin of Ordos Basin has developed sedimentary layers with completely different sedimentary characteristics from central-east part of the basin,among them,the west margin developed marine source rocks in Middle-Upper Ordovician.Recent analysis on tectonic sedimentary evolution background of hydrocarbon source rock formation,hydrocarbon generating biological characteristics and rock-mineral characteristics of hydrocarbon layers,show that the development of favorable source rocks is controlled by many factors,and the distribution has certain regularity:1)the main hydrocarbon source rocks of Middle-Upper Ordovician are mainly formed under the structural background of deepening sedimentary water body.The semi deep-water slope environment,where the energy of sedimentary water body is weak and the terrigenous clastic injection is relatively slow,is the most favorable for the development of organic-rich hydrocarbon source beds.2)Although the Ordovician in the southwest margin of the Ordos Basin has diverse biological types and prosperous populations,the parent organisms(hydrocarbon generating parent materials)that can form important hydrocarbon source rocks are dominated by plankton,agnostic species(microalgae)and cyanobacteria,because the relatively reduced medium environment and appropriate deposition burial rate also play an important role in controlling the effective preservation of organic matter.3)Through X-ray diffraction phase analysis and scanning electron microscope analysis,it shows that rock forming minerals of“mud shale”is mainly quartz and feldspar particles,followed by illite clay minerals,carbonate minerals,and a small amount of pyrite,and so on.The content of clay minerals and pyrite is closely related to the abundance of organic matter.