甘蒙北山地区扎格高脑盆地白垩系黑色页岩地球化学特征及沉积古环境

Geochemical Characteristics and Sedimentary Paleoenvironment of Cretaceous Black Shales in Zhagegaonao Basin, Beishan Area, Gansu-Inner Mongolia

摘要

甘蒙北山地区白垩系赤金堡组发育一套有机碳质量分数较高的黑色页岩,显示出良好的油气勘探前景,对其沉积古环境的研究亟待加强.本文基于扎格高脑盆地MGD1井白垩系岩心资料,等间距采集相关样品,运用X射线荧光光谱法、X射线衍射法和电感耦合等离子质谱仪法研究黑色页岩的地球化学特征,恢复其沉积时的古气候、古盐度、古水深和氧化还原条件,并应用孢粉资料对古气候进行验证.研究表明:黑色页岩主要为泥质页岩,其物源主要为长英质岩石,属于近源沉积,未经历沉积再循环,并且后期未受热液作用影响,可以利用其地球化学数据进行古环境分析.综合分析Sr、Ba、Cu、B、V、Ni、Co等环境敏感元素,并在孢粉数据的约束下发现:MGD1井黑色页岩下段沉积于干热、咸水、还原的环境,古水深呈现振荡上升;中段沉积于温暖潮湿、半咸水、还原的环境,古水深保持高位震荡;上段沉积于干热、咸水、还原的环境,古水深略有下降.MGD1井黑色页岩有机质富集受古气候、古盐度、古水深和氧化还原条件等因素共同控制.

Abstract

The Cretaceous Chijinbao Formation has developed a suite of black shales with high organic content, indicating significant potential for oil and gas exploration in the Beishan area of Gansu and Mongolia. Further research into the sedimentary paleoenvironment of these black shales is necessary to enhance our understanding. Based on the MGD1 Cretaceous core data in Zhagegaonao basin, samples were collected at equal intervals. The geochemical characteristics of black shale have been studied by X-ray fluorescence spectrometry, X-ray diffraction, and inductively coupled plasma mass spectrometry. The paleoclimate, paleosalinity, paleo-water depth, and oxidation-reduction conditions of black shale formation were reconstructed with geochemical data and verified by palynological data. The black shales are mainly argillaceous shale. The provenance is predominantly feldspar rock, and they are near-source sediments, which have not undergone sedimentary recycling and have not been affected by later hydrothermal action. The paleoenvironment can be analyzed with its geochemical data. The comprehensive analysis of environmentally sensitive elements, such as Sr, Ba, Cu, B, V, Ni, Co, shows that the lower section of black shale in well MGD1 was deposited in the dry-hot, saline-water and reductive environment, and the paleo-water depth rose with oscillation. The middle section of black shale was deposited in warm, humid, brackish water and reductive environment, and the paleo-water depth kept high oscillation. The upper section of black shale was deposited in a dry-hot, saline-water and reductive environment, and the paleo-water depth decreased slightly. The accumulation of organic matter in the black shale in well MGD1 is the result of the combined action of paleoclimate, paleosalinity, paleo-water depth, and oxidation-reduction conditions.

关键词

北山地区 / 白垩系 / 黑色页岩 / 地球化学 / 古环境 / 扎格高脑盆地

Key words

Beishan area / Cretaceous / black shale / geochemistry / paleoenvironment / Zhagegaonao basin

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基金资助

中国地质调查局项目(DD202402008)

中国地质调查局项目(DD20240200805)

陕西省自然科学基金项目(2024JC-YBMS-204)