中下扬子地区二叠纪缺氧环境研究

在中下扬子地区二叠纪尤其是孤峰期和大隆期,缺氧环境十分发育,形成了＂黑色页岩-薄层硅岩-纹层状石灰岩＂的缺氧沉积组合?本区二叠纪缺氧环境的形成与当时的气候?水温?水体能量?海底地形和上升洋流有着密切的关系?根据放射虫生态?相序?沉积特征及与现代缺氧环境的对比分析,探讨了本区二叠纪缺氧环境的古水深?缺氧沉积可成为有利的生油气源岩,而不同的缺氧沉积类型可形成不同类别的生油气源岩;研究表明,本区二叠系具有良好的生油气条件?

Study on Anoxic Environments of Permian in the Middle and Lower Yangtze Region

Anoxic environments developed best in Permian,especially in Gufeng and Dalong Stages in the Middle and Lower Yangtze region. Special complex of anoxic sediments of "black shale-chert- limestone "was formed in this kind of environments. According to palaeoecology and sedimentary petrology,anoxic sediments can be divided into two kinds, dysaerobic and anaerobic sediments. Anaerobic sediments are characterized by black and dark-grey, thin-beded or laminated,organicrich, and phosphoric-rich chert, shale and mud-bearing limestone, which are devoid of benthic fossils and biodisturbance,but bear plankton fossils, e. g. radiolaria and Ammonoids. Dysaerobic sediments are characterized by black-grey and dark-grey, thin or middle-beded,organic-rich and phosphoric-rich shale, mud-bearing limestone and chert, which are devoid of epifaunal fossils, but bear infauna,plankton fossils and biodisturbance. The distribution of anoxic sediments of Permian in the study area,is closely related to the framework of lithofaces and palaeogeography. Anoxic sediments of Gufeng Formation are distributed in the north and south of Lower Yangtze region,and those of Dalong Formation in the north of Lower and Middle Yangtze region,both of which are closely related to deeper slope and basin. There are two main origins of anoxic environments of Permian in the region. One is the euxinic orgin. Under the palaeogeographic conditions of low paleolatitude of Permian,high temperature of marine water,low energy of marine water and subaqueous relief of "twofold platform and basin"result in the euxinic condition of deeper water layers. The other orgin is the influence of upwelling.The active upwelling coming from the northward ocean promotes high biologic productivity,and in turn the recycling of dead organic matter in the water column creates a very high oxygen demand which can trigger anoxic conditions in deeper water layers under the upwelling. The distribution of anoxic and aerobic environments is determined by the depth of marine water. The sedimentary model of anoxic environments of Permian in the region is established on the basis of comprehensive analysis. The depth of sea floor of anoxic environments is analyzed from the viewpoint of palaeogeography, petrology,and comparative study of modern anoxic environments. It is estimated that the sea floor of anoxic envirionments of Permian in the study area is not very deep. The depth of dysaerobic environments is less than 300m,and anaerobic environments less than carbonate compensation depth(CCD). Abundant organic matter in the anoxic sediments may result in.the formation of potential hydrocarbon source beds. According to the difference of various anoxic sediments, We classified three kinds of potential hydrocarbon source beds, and estimated the distribution of those beds in the study area. Research shows that there are excellent conditions of the formation of oil and gas in Permian of the Lower and Middle Yangtze region.