鲁西地区早二叠世低可容空间浅水三角洲发育演化过程及聚煤机制

    Early Permian low-accommodation shallow-water delta evolution and coal accumulation mechanism in the Luxi area

    • 摘要: 本文研究了鲁西地区早二叠世低可容空间浅水三角洲的发育演化特征及其聚煤机制。通过野外露头和钻孔取芯岩相和垂向序列研究,识别出分流河道、潮汐影响的分流河道、分流间湾、泥炭沼泽等三角洲平原环境,以及河口坝、潮汐影响的河口坝等三角洲前缘环境和前三角洲环境。研究区浅水三角洲沉积序列被划分为C1-C5五个沉积旋回,表现出整体进积背景下的进积-退积-进积次级旋回。综合分析表明,水动力条件分区和三角洲演化控制了聚煤作用。在低可容空间、宽缓地形背景下,三角洲自旋回作用和异旋回作用共同控制了三角洲的发育及聚煤作用。河流作用主导区聚煤作用主要发育在河道间沼泽、分流间湾等区域,煤层相对分散、局限;而潮汐作用主导区聚煤作用主要发育在潮汐影响的泥炭沼泽,煤层分布范围广,厚度大。海侵-海退旋回控制了煤层的整体分布趋势,海退期形成大量的泥炭沼泽,植物繁盛,海侵期迅速淹没埋藏形成煤层。这些发现为华北地区上古生界煤成气勘探提供了借鉴。

       

      Abstract: The development characteristics of low accommodation delta sequences and their coal-forming characteristics and mechanisms have increasingly attracted attention. Previous studies have focused on the coal-forming characteristics and mechanisms from the perspective of accommodation changes, while there has been less research on the relationship between the development and evolution characteristics of shallow water deltas themselves and coal formation. This paper takes the Shanxi Formation of the Lower Permian in the Luxi area as an example to study the development and evolution characteristics of shallow water deltas and their impact on coal formation. Based on detailed outcrop and core logging of lithofacies and vertical sequences, distributary channels, tide-influenced distributary channels, interdistributary bays, peat swamps, and other delta plain environments, as well as delta front environments such as mouth bars and tide-influenced mouth bars, and prodelta environments, have been identified. By comparing single-well and multi-well sedimentary cycles and sedimentary relative analysis, the study area's shallow water delta depositional sequence is divided into five depositional cycles, C1-C5. The delta evolution shows a secondary cycle of progradation-retrogradation-progradation against a background of overall progradation. By integrating the analysis of delta depositional processes and coal-forming characteristics, it is proposed that the water dynamic zoning and delta evolution control the coal-forming action. The results indicate that under the background of low accommodation space and gentle terrain, the self-cycle action and hetero-cycle action of the delta jointly control the development and coal-forming action of the delta. The migration of delta distributary channels causes differences in water dynamic conditions, which control the coal-forming environment and coal seam development characteristics. In the river-dominated area, coal-forming action mainly develops in inter-channel swamps and interdistributary bays, where coal seams are relatively dispersed and limited. In the tide-dominated area, coal-forming action mainly develops in tide-influenced peat swamps, where coal seams have a wide distribution and large thickness; the transgression-regression cycle controls the overall distribution trend of coal seams, with a large amount of peat swamps formed during the regression period, where plants thrive, and coal seams are rapidly buried and formed during the transgression period. The above mechanism of coal formation in shallow water deltas may provide reference for the exploration of coalbed gas in the Upper Paleozoic of the North China region.

       

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