【摘要】 有机质丰度是页岩油富集的重要物质基础,是页岩油甜点预测和资源评价的关键指标.与海相沉积相比,陆相湖盆页岩层系有机质丰度变化大,非均质性强,其形成和保存受水体盐度影响更明显.因此,揭示水体性质及其影响因素是揭示陆相有机质差异富集的关键科学问题.鉴于此,本文通过对比典型淡水(鄂尔多斯盆地长7段)与咸化碱湖(准噶尔盆地玛湖凹陷风城组)沉积与地球化学特征,揭示了淡水与咸化湖盆水体盐度具有动态变化的特征,淡水湖盆在有机质富集层段具有水体逐渐变咸的趋势,而咸化湖盆也具有水体变淡的特点.全球尺度上古气候的冷暖影响淡水与咸化湖盆的古生产力条件,是陆相湖盆富有机质沉积的主控因素;同时,陆相湖盆有机质多形成于缺氧含铁的水体环境,淡水湖盆咸化层段比咸化碱湖有机质保存效果更好.淡水湖盆长7段有机质水体分层不明显,火山活动等关键地质事件携带的硫酸盐会引起水体盐度发生快速变化,引起湖盆底部水体由长期铁化向硫化快速转变,利于有机质的保存;咸化碱湖玛湖凹陷风城组水体分层显著,火山或热液活动会导致底部水体盐度频繁硫化至甲烷厌氧氧化阶段,对有机质可能具有消耗作用.本研究可加深对陆相页岩油富集规律的认识,对指导页岩油勘探具有一定的理论意义.更多还原

【Abstract】 Organic matter abundance is an important material basis for shale oil enrichment and a key indicator for shale oil sweet spot prediction and resource evaluation. Compared to marine sediments, in continental lake basins the organic matter abundance of shale intervals varies more frequently and is highly heterogeneous, the organic matter formation and preservation are significantly affected by water salinity. Therefore, it is important to investigate water conditions and the factors that influence them to understand the differential enrichment of organic matter in different continental lake facies. In this study,we compared the sedimentary and geochemical characteristics of typical freshwater(Chang 7 Member in the Ordos Basin)and saline alkaline lakes(Fengcheng Formation in the Mahu Sag in the Junggar Basin), and found that the salinity of water bodies in both basins is dynamic: In the freshwater lake basin, there was a trend towards saltier conditions during organic-rich intervals, while in the saline-alkali lake basin, there was a trend towards less salty. By comparing the paleoclimate at the global scale, we propose that the warmth and cold of the climate affects the lacustrine salinity and therefore the paleoproductivity, which is the main factor controlling the deposition of organic-rich shale in continental lake basins.Freshwater lake basins are more developed in the greenhouse climate, and the organic matter in the water is mainly from plankton, benthos and bacteria, the dissolved oxygen content in the surface water of lake basins is higher, and therefore its paleoproductivity is higher than that of saline lake basins. After the decomposition and degradation of organic matter in the shallow water layer, an anoxic ferruginous or euxinic environment is conducive to the preservation of organic matter at the bottom of the water body. In contrast, due to the high salinity in the alkali water in the glacial period, the environment is relatively stable, the dissolved oxygen content is low, and the organic matter may be accumulated and enriched for a relatively long time only in its deep water. Meanwhile, organic matter in lacustrine basins mostly forms in anoxic ferruginous water conditions, the saline intervals in freshwater lake basins are more conducive to the preservation of organic matter than that in alkaline lakes: the stratification in the water in the Chang 7 section is subtle, and the sulfate resulting from key geological events such as volcanic activity can cause rapid changes in water salinity, causing the water body at the bottom of the lake from long-term ferruginous quickly changing to euxinic conditions, which is conducive to the preservation of organic matter; the stratification of the water in the Fengcheng Formation in the Mahu alkaline lake is clear, volcanic or hydrothermal activity can frequently cause the salinity at the bottom of the water body to become euxinic even to the stage of methane anaerobic oxidation, which may have a consuming effect on organic matter, but the mechanism of organic matter enrichment in alkali lake still remains to be explored. Future research is suggested to strengthen the study of organic-inorganic geochemical characteristics in different shale intervals, especially the analysis of stable isotopes of hydrocarbon, oxygen, nitrogen, sulfur and key metal isotopes(Fe, Cu, Hg et al.), to quantitatively reconstruct the spatial and temporal distribution of primary productivity, finely characterize the degree of hypoxia at the bottom of sedimentary water bodies during the formation of organic-rich shale, highlight the role of clay minerals and microorganisms, and establish a quantitative characterization model, to deepen the understanding the enrichment of continental shale oil and to give the theoretical guidance for shale oil exploration.更多还原