巨厚煤层物质组成特征与成因机制——以柴北缘鱼卡地区中侏罗统为例

Composition characteristics and genetic mechanism of ultra thick coal seams:A case study of Middle Jurassic in Yuqia Area,Northern Qaidam Basin

厚煤层蕴含着丰富的地质信息,在全球碳循环中扮演着重要的角色。国内外深时地层中厚煤层广泛分布,近年来其成因机制逐渐成为煤地质学的研究热点。为明晰等时层序地层格架中的巨厚煤层的物质组成特征与形成机理,选取柴北缘鱼卡地区中侏罗统层序S1湖侵体系域发育的巨厚煤层M7为研究对象,等间隔连续采样并进行室内显微组分与矿物鉴定,依据煤层关键界面的识别原理、显微组分与矿物的古环境指示意义进行综合分析。结果显示:显微组分中镜质组体积分数最高(48.5%),惰质组体积分数同样较高(37.22%),类脂组所占比例很低,仅为3.32%。矿物组分体积分数较为稳定,平均值为10.96%。依据煤层与碎屑沉积的交替出现、显微组分与矿物体积分数在垂向上的变化趋势,识别出了多种关键界面,包括陆化面(TeS)、陆地暴露面(ExS)、沼泽化面(PaS)、湖侵淹没面(GUTS)与可容空间转换面(ARS)。沿沉积倾向在向陆与向湖方向分别选择钻孔,与鱼卡煤矿对比分析沼泽环境垂向演化过程。鱼卡煤矿巨厚煤层M7识别出4个沉积旋回,向陆以及向湖方向的YQ6–22与YQ6–12中由陆源碎屑、湖相沉积所分隔的M7–1—M7–3则均识别出3个沉积旋回。发现了巨厚煤层中蕴含的高分辨率层序,经过分析划分为4个四级层序,结合煤层中关键界面所对应的层序界面,揭示了巨厚煤层的成因机制。建立了层序地层格架中的多期泥炭沼泽垂向叠置模式,可进一步应用于国内外更多深时地层中巨厚煤层的研究。

Thick coal seams are widely distributed in the world,which contain rich geological information and play an important role in the global carbon cycle.The genetic mechanism of thick coal seams has recently gained attention in coal geology research.In order to reveal the composition characteristics and genetic mechanism of ultra thick coal seams in the isochronous sequence stratigraphic framework,the ultra thick coal seam M7,developed in lacustrine transgressive system tract of Middle Jurassic sequence S1 in the northern Qaidam Basin,was continuously sampled at equal intervals,and then the macerals and minerals were identified.A comprehensive analysis was conducted based on the recognition principle of key surface in coal seam,and the paleo-environmental indication of maceral and mineral.The results showed that the content of vitrinite is the highest(48.5%),followed by inertinite(37.22%),and liptinite is fairly low(3.32%).The content of minerals is relatively stable,with an average value of 10.96%.Based on the alternated development of coal seams-detrital deposits,and the content variation of macerals and minerals in vertical direction,five key surfaces were identified,including terrestrializa-tion surface,erosional subaerial exposure surface,paludification surface,give-up transgressive surface,and accommodation reversal sur-face.The vertical evolution of swamp was analyzed by selecting boreholes at the landward and basinward locality along the sedimentary trend compared to the Yuqia coal mine.Four sedimentary cycles were identified in the ultra thick coal seam M7 of Yuqia coal mine.In landward YQ6-22 and basinward YQ6-12,three sedimentary cycles were identified due to the separation of M7-1 to M7-3 by terrigen-ous detrital and lacustrine deposits.High resolution sequences were found in ultra thick coal seams,which were divided into four fourth-order sequences.Combined with the sequence boundary corresponding to the key surface in coal seam,the genetic mechanism of ultra thick coal seams was revealed.A multi-peat superposition genetic model in the sequence stratigraphic framework was established,which can be further applied to the research of thick coal seams in more geological times around the world.