准东吉南凹陷二叠系芦草沟组烃源岩有机质富集机制

Mechanism of organic matter enrichment in the Permian Lucaogou Formation,Ji’nan Sag,eastern Junggar Basin

准噶尔盆地东部(准东)吉南凹陷发育大套二叠系芦草沟组烃源岩,但其芦草沟组烃源岩沉积古环境与有机质富集机理尚不明确。通过总有机碳、岩石热解、主微量元素和饱和烃色谱/质谱等分析手段,对研究区芦草沟组烃源岩沉积古环境、古生产力进行了研究,以此探讨芦草沟组烃源岩有机质富集机理。结果表明:①通过总有机碳和岩石热解参数分析认为吉南凹陷二叠系芦草沟组有机质丰度普遍达到极好,有机质类型以Ⅰ型、Ⅱ1型为主,整体处于低熟—成熟阶段,生烃潜力高。②生物标志物参数分析表明,芦草沟组有机质主要为水生生物和陆源高等植物混合贡献,且芦二段具有更高的陆源有机质输入。③根据主微量元素结合生物标志物相关参数,认为研究区芦一段烃源岩沉积于干旱、半咸水、弱还原的浅水—半深水环境,古生产力较低;芦二段烃源岩沉积于湿润—半干旱、淡水、弱还原—弱氧化的浅水沉积环境,古生产力中等。④根据研究区芦草沟组烃源岩有机质丰度与不同古沉积环境条件的综合研究分析认为,芦一段有机质的富集主要受水生生物的繁盛和保存条件控制;芦二段初始生产力主导了有机质的富集。

The Lucaogou Formation of the Permian is developed in the Ji’nan Sag,located in the eastern Junggar Basin.However,there remains a lack of clarity regarding the sedimentary paleoenvironment and organic matter enrichment mechanism within this formation.Therefore,this study aims to investigate the sedimentary paleoenvironment and paleoproductivity of the Lucaogou Formation in our study area using various analytical techniques such as total organic carbon analysis,rock pyrolysis,major and trace element analysis,as well as saturated hydrocarbon chromatography/mass spectrometry.This will enable us to gain insights into the mechanisms responsible for organic matter enrichment within the Lucaogou Formation.Our findings reveal that:(1)Based on total organic carbon and rock pyrolysis parameters analysis,it can be concluded that there is generally abundant organic matter within the Lucaogou Formation in Ji’nan Sag with predominantly type Ⅰ and type Ⅱ1 kerogen indicating low-mature to mature stages with high hydrocarbon generation potential.(2)Biomarker parameter analysis suggests that aquatic organisms and terrigenous higher plants are major contributors to organic matter composition within the Lucaogou Formation;additionally,higher input of terrigenous organic matter is observed in its second member.(3)Analysis of major and trace elements along with biomarkers-related parameters indicates that source rocks from our study area were deposited under arid or brackish water conditions characterized by weak reduction levels ranging from shallow water to semi-deep water environments with low paleoproductivity;however,wet-semi-arid conditions prevailed during deposition of source rocks from second member accompanied by fresh water settings exhibiting weak reduction-weak oxidation characteristics at shallow depths resulting in moderate paleoproductivity levels.(4)According to the comprehensive study and analysis of the abundance of organic matter in the source rocks of Lucaogou Formation and different paleosedimentary environment conditions in the study area,the enrichment of organic matter in Lu1 Member is mainly controlled by the abundance and preservation conditions of aquatic organisms.The initial productivity of Lu2 Member dominated the enrichment of organic matter.