Early diagenetic growth of carbonate concretions in the upper Doushantuo Formation in South China and their significance for the assessment of hydrocarbon source rock
Early diagenetic growth of carbonate concretions in the upper Doushantuo Formation in South China and their significance for the assessment of hydrocarbon source rock
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华南陡山沱组上部碳酸盐结核早期成岩发育及其烃源岩评价意义
DOI：
10.1007/s11430-008-0107-3
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发表时间：
2008-09-01
期刊：
影响因子：
--
通讯作者：
Liu JunLai
中科院分区：
文献类型：
--
作者：
Dong Jin;Zhang ShiHong;Liu JunLai
陡山沱组上部碳酸盐结核的矿物学、结构特征及有机碳组成。551 Ma），表明它们在成岩早期（浅部）生长于富含有机质的页岩中。高有机碳含量（高达10%）和丰富的framboidal黄铁矿在托管页岩表明缺氧或缺氧的沉积环境。结核中保存完好的硬粘土组构表明其形成于0-3米的埋藏深度，可能与有机物的微生物分解和甲烷的厌氧氧化有关。通过有机物分解和/或产甲烷产生的气体产生气泡和空腔，并且在硫酸盐还原区的厌氧甲烷氧化导致碳酸盐沉淀，填充气泡和空腔以形成结核的球形结构。岩石热解分析表明，碳酸盐结核的总有机碳（TOC）含量较低，但有效碳含量较高。这可能是由于有机质被包裹在结核孔隙中，使有机质在深埋过程中不被进一步改造，即使在过成熟的烃源岩中也能保持较高的生烃潜力。结核作为一种微生物，具有特殊的生长条件，可以提供沉积和埋藏早期环境中微生物活动的重要信息。
Mineralogical and textural characteristics and organic carbon composition of the carbonate concretions from the upper Doushantuo Formation (ca. 551 Ma) in the eastern Yangtze Gorge area reveal their early diagenetic (shallow) growth in organic-rich shale. High organic carbon content (up to 10%) and abundance of framboidal pyrites in the hosting shale suggest an anoxic or euxinic depositional environment. Well-preserved cardhouse clay fabrics in the concretions suggest their formation at 0–3 m burial depth, likely associated with microbial decomposition of organic matter and anaerobic oxidation of methane. Gases through decomposition of organic matter and/or from methanogenesis created bubbles and cavities, and anaerobic methane oxidation at the sulfate reduction zone resulted in carbonate precipitation, filling in bubbles and cavities to form spherical structures of the concretions. Rock pyrolysis analyses show that the carbonate concretions have lower total organic carbon (TOC) content but higher effective carbon than those in the host rocks. This may be caused by enclosed organic matter in pores of the concretions so that organic matter was protected from further modification during deep burial and maintained high hydrocarbon generating potential even in over-matured source rock. As a microbialitesensu latu, concretions have special growth conditions and may provide important information on the microbial activities in depositional and early burial environments.