延安组煤与页岩沉积古环境对比——以鄂尔多斯盆地南部芦村二号矿为例

Comparison of sedimentary paleoenvironment between coal and shale in Yan’an Formation——A case study of Lucun No.2 Mine in southern Ordos Basin

鄂尔多斯盆地南部延安组底部发育稳定的煤层,其顶部发育厚层页岩。虽然煤和页岩都是富有机质沉积岩,但其沉积期古环境差异特征尚待揭示。在芦村二号矿采集了8件煤样及其顶板发育的6件页岩样品,通过电感耦合等离子体发射光谱仪和电感耦合等离子质谱仪对煤及页岩的主量元素和微量元素进行了测试,明确了其地球化学特征,探究不同岩性沉积期的古环境差异。结果表明:煤和页岩中Al_(2)O_(3)含量最高,Fe_(2)O_(3)的含量次之,相较于上陆壳均值(UCC),煤中P_(2)O_(5)、Fe_(2)O_(3)富集程度较高,而页岩中K_(2)O、Al_(2)O_(3)、Fe_(2)O_(3)富集程度较高;煤中Cr、Ni、Cu、Sr、Mo和U元素明显富集,而页岩中仅有U元素呈富集状态,Ni、Cu、Sr和Mo元素呈明显亏损状态;煤和页岩的CIA值为均指示沉积体系处于温暖-湿润的气候条件,且该气候条件更加有利于煤层的发育;煤沉积期水体盐度变化较大,为半咸水-咸水环境,而页岩沉积期水体盐度较为稳定,为半咸水环境;相较于页岩,成煤时期水体的古生产力更高,且煤形成于覆水较浅的泥炭沼泽环境,后期水体逐渐加深,生产力逐渐减弱;(Fe+Mn)/Ti均值大于25,表明沉积期煤层受热液作用影响,接受了较高温度的烘烤。探究煤和页岩沉积期古环境的差异,为鄂尔多斯盆地延安组煤层的勘探及开发提供理论依据。

The bottom of the Yan’an Formation in the southern part of the Ordos Basin is characterized by the stable development of coal seams,with a thick layer of shale at the top.Although both coal and shale are organic-rich sedimentary rocks,the characteristics of depositional stage differences remain to be revealed.Eight coal samples and six shale samples from the roof of the No.2 Mine in Lucun were collected,and the major elements and trace elements of coal and shale were tested using an Inductively Coupled Plasma Emission Spectrometer and Inductively Coupled Plasma Mass Spectrometer to define their geochemical characteristics and to explore the depositional environments of different lithologies.The results show that:The coal and the shale have the highest Al 2 O 3 content,followed byFe_(2)O_(3).Compared to the Upper Continental Crust(UCC),the coal exhibits relatively high enrichment of P_(2)O_(5)andFe_(2)O_(3),and the high concentration of K 2 O,Al 2 O 3 andFe_(2)O_(3)in shale.In coal,Cr,Ni,Cu,Sr,Mo and U elements are obviously enriched,while in shale,only U elements are enriched and Ni,Cu,Sr and Mo elements are obviously depleted.The CIA values of the coal samples and the shale samples indicate a warm and humid climate in the depositional system and the climate condition is more favorable to the development of coal seam.The salinity of the coal varies greatly during the sedimentary period,and the sedimentary water is a brackish water environment.However,the salinity of water in shale deposition period is relatively stable,and the water in sedimentary period is brackish water environment.Compared to the shale,the coal formed in a shallow water-covered peat swamp environment,with the water body gradually deepening and productivity gradually weakening.The average(Fe+Mn)/Ti value of the coal samples is greater than 25,indicating that the coal seam was influenced by hydrothermal activity during the depositional period and experienced high-temperature baking.This paper provides a theoretical basis for the exploration and development of coal seam in Yan’an Formation of Ordos Basin by exploring the difference of paleoenvironment between coal and shale deposition periods.