宁东地区延安组煤层元素地球化学特征与古生产力恢复

    Element geochemistry and paleoproductivity reconstruction of coal seams in the Yan'an Formation, Ningdong area

    • 摘要: 基于侏罗系延安组煤系地层发育,但缺乏煤层地球化学特征、沉积环境、成煤条件和煤层赋存空间系统研究的现状,本文系统采集了鄂尔多斯盆地西缘宁东地区侏罗系麦垛山煤矿延安组煤心样品605件,获取了6655个地球化学测试数据;利用数据有效性检验、元素组合、判别图版等方法,剖析了煤层元素地球化学特征和指相意义,重建了延安期古气候、古氧相、古盐度、古水深及古生产力等,揭示了延安期古沉积环境演变、煤层赋存特征和聚煤规律,构建了该区“地球化学示踪—沉积环境反演—聚煤规律分析”的一体化研究框架。研究结果表明:①延安组样品的ICV均值为2.55,反映煤岩未受风化作用影响,样品代表性强;②CIA均值为71.87%,指示物源区经历中等化学风化作用,对应温湿的气候背景,推测当时古温度为17.76 ℃左右,降水量为1011.14 mm/a左右;③镁铝比(m)、钙铁比(n)、w(Sr)/w(Ba)等指标较高,反映延安期古水体盐度较高;④煤灰还原性指数(K)、全硫(Sp,d)、w(Th)/w(U)等指标揭示沼泽以贫氧的弱还原环境为主,w(Fe)+ w(Al)/w(Ca)+ w(Mg)、w(Mn)/w(Ti)均值分别为1.55、0.23,指示为中水位沼泽,覆水程度中等;⑤w(P)/w(Al)、w(P)/w(Ti)、初级古生产力(NPP)均值分别为0.03、0.7、331.53 g/(m2·a),反映延安期湖泊营养程度中等,古生产力水平较高。综上,鄂尔多斯盆地西缘宁东地区侏罗系延安组主要为陆相河流—三角洲平原沉积相,该时期古气候为温暖潮湿类型,成煤环境为弱还原的中水位沼泽,水体盐度较高,具有较高的古生产力水平,为成煤植物的繁衍及其遗体堆积与保存创造了有利条件。该成果对恢复该时期盆地沉积格局、重建古地理及沉积矿产富集规律研究具有重要意义。

       

      Abstract: The Jurassic Yan'an Formation exhibits well-developed coal measure strata. However, there remains a lack of systematic research on the geochemistry, sedimentary environment, coal-forming conditions, and spatial distribution of these coal seams. In this study, 605 coal core samples were systematically collected from the Jurassic Yan'an Formation of the Maiduoshan coal mine in the Ningdong area along the western margin of the Ordos Basin, with 6 655 data points obtained from geochemical tests. The element geochemical characteristics of the coal seams and their implications were analyzed through data validation, element association, and discriminant diagrams. The paleoclimate, paleo-oxygen facies, paleosalinity, paleo-water depth, and paleoproductivity during the deposition of the Yan'an Formation were reconstructed. Accordingly, the paleo-sedimentary environment evolution, coal seam occurrence characteristics, and coal accumulation patterns during this period were revealed. Furthermore, an integrated framework for research on geochemical tracing, sedimentary environment inversion, and analysis of coal accumulation patterns was established for the study area. The results show that the coal samples from the Yan'an Formation exhibited an average index of compositional variability (ICV) of 2.55. This finding indicates that these samples were free from weathering and thus are highly representative. The samples yielded an average chemical index of alteration (CIA) of 71.87%, suggesting that the provenance area underwent moderate chemical weathering, corresponding to a warm and humid climatic setting. Based on these, the paleotemperature and paleo-precipitation of the Yan'an Formation during the deposition of the formation were estimated at approximately 17.76 ℃ and 1 011.14 mm/a, respectively. The high w(Mg)/w(Al) ratios (m), w(Ca)/w(Fe) ratios (n), and w(Sr)/w(Ba) ratios of the coal samples suggest high paleosalinity during the deposition of the Yan'an Formation. The indicators, including the coal ash composition index (K), total sulfur (Sp,d), and w(Th)/w(U) ratio, reveal that the swamp in the Yan'an Formation primarily exhibited a dysoxic, weakly reducing environment. Besides, the swamp showed average w(Fe) + w(Al)/w(Ca) + w(Mg) and w(Mn)/w(Ti) ratios of 1.55 and 0.23, respectively, indicating a medium-water-level swamp with moderate-degree overlying water. The average w(P)/w(Al) and w(P)/w(Ti) ratios and the net primary paleoproductivity (NPP) were 0.03, 0.7, and 331.53 g/(m²·a), respectively, reflecting a moderately nutrient paleolake environment and high paleoproductivity during the deposition of the Yan'an Formation. Overall, the Jurassic Yan'an Formation in the Ningdong area predominantly features the continental fluvial-deltaic plain sedimentary facies. During its deposition, this formation showed a warm and humid paleoclimate, a coal-forming environment of weakly reducing, medium-water-level swamp, high-salinity water bodies, and high paleoproductivity. These conditions are favorable for the proliferation of coal-forming plants, as well as the accumulation and preservation of plant remains. The findings of this study hold great significance for reconstructing the paleosedimentary framework and paleogeography of the basin and understanding the enrichment patterns of minerals deposited in the basin during the deposition of the Yan'an Formation.

       

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