Pub Date : 1900-01-01DOI: 10.3997/2214-4609.201902811
Lei Cao, Zhihuan Zhang, Zhong Ningning, Guangli Wang, H. Xia, R. Chang
Introduction The Beibuwan Basin, located in the Beibu Gulf of the South China Sea, is a Cenozoic extensional sedimentary basin (Zhang et al., 2013). The 2nd member of the Eocene Liushagang Formation is the main source rock consisting of oil shales, mudstone and dark shales. This suite of source rocks has high organic matter abundance and good organic matter type (Fu et al., 2017), but the paleoenvironment, mechanism of the enrichment of organic matter and development model of source rock remain unclear (Huang et al., 2012). It is of great significance to define the enrichment mechanism of organic matter and control factors of this suite of source rocks, and to establish the development model of oil shale in the freshwater lake basin, which is also of guiding value to deepen the oil and gas exploration in the study area. In this work, the enrichment mechanism of organic matter and development model of source rock were studied by comprehensively applying seismic, drilling, logging and geochemical data, adopting geological, geochemical and geophysical research ideas. The data of conventional geochemistry, biomarker, organic maceral and palynology were analysed comprehensively. Based on the distribution characteristics of some nutrition elements(such as P, Fe and Mo) and geochemical parameters(such as 4-methyl sterance index, gammacerane index, S, Sr/Cu, Sr/Ca, V/(V+Ni), δCe, (La/Yb)N, VS-MnO (Sugisaki et al,1984)), combined with the La/Yb-∑REE chart (Allegre C J et al,1978), discrimination function chart of major elements and rare earth element partition curve chart, the geochemical characteristics and distribution features of oil shale, mudstone and dark shale of the 2nd member of Liushagang Formation were summarized. Meanwhile, the paleoenvironment of oil shale, mudstone and dark shale were reconstructed, source composition of organic matter was determined. Furthermore, the controlling effects of the provenance properties, paleoproductivity of lake, redox conditions and sedimentation rate on the accumulation of organic matter in oil shale and mudstone shale were analysed. Finally, the eutrophication mechanism of freshwater lake basin water was discussed, and the development model of oil shale in the study area was established.
北部湾盆地位于南海北部湾，是一个新生代伸展沉积盆地(Zhang et al.， 2013)。始新统流沙岗组二段为主要烃源岩，由油页岩、泥岩和暗色页岩组成。该套烃源岩有机质丰度高，有机质类型好(Fu et al.， 2017)，但对烃源岩的古环境、有机质富集机制和发育模式尚不清楚(Huang et al.， 2012)。明确该套烃源岩有机质富集机制及控制因素，建立淡水湖盆地油页岩发育模式，对研究区油气勘探的深化具有重要的指导意义。综合运用地震、钻井、测井、地球化学等资料，采用地质、地球化学、地球物理等研究思路，研究了烃源岩有机质富集机理和发育模式。综合分析了常规地球化学、生物标志物、有机显微组分和孢粉学资料。根据部分营养元素(如P、Fe、Mo)的分布特征和地球化学参数(如4-甲基浓度指数、伽马烷指数、S、Sr/Cu、Sr/Ca、V/(V+Ni)、δCe、(La/Yb)N、VS-MnO (Sugisaki et al,1984))，结合La/Yb-∑REE图(Allegre C J et al,1978)、主要元素判别函数图和稀土元素分配曲线图，分析了油页岩的地球化学特征和分布特征。总结了六沙港组二段泥岩和暗色页岩。同时重建了油页岩、泥岩和暗页岩的古环境，确定了有机质的来源组成。分析了物源性质、湖泊古生产力、氧化还原条件和沉积速率对油页岩和泥岩页岩有机质富集的控制作用。最后，探讨了淡水湖流域水体富营养化机理，建立了研究区油页岩发育模式。
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