准噶尔盆地博格达地区中二叠统芦草沟组热液硅质结核特征及页岩油意义

Characterizing hydrothermal siliceous nodules to guide shale oil exploration in the Middle Permian Lucaogou Formation,Bogda area,Junggar Basin

硅质岩已被广泛应用于重建古环境、示踪热液循环和硅化过程等方面,并且对陆相页岩油勘探开发具有重要意义。以准噶尔盆地博格达地区芦草沟组为研究对象,综合运用X射线衍射、元素面扫、主微量元素、镜质体反射率、岩石热解和总有机碳分析等方法对芦草沟组硅质岩成因机制及油气意义进行了研究。结果表明:芦草沟组硅质结核中软沉积物变形构造非常发育,主要由隐-微晶石英组成,指示结核为同沉积期形成。结核内部发育重晶石、锰菱铁矿、镍铁矿和蓝铜矿等热液矿物,并且稀土元素具有总量低、重稀土元素相对富集和正Eu异常等特征,指示其为热液成因,说明二叠纪研究区存在间歇性热液活动。热液带来的营养元素和金属阳离子提高了古生产力和古盐度,促使底水还原性增强,有利于有机质富集。硅质岩中孔隙和裂缝发育,其中显著富集的石英提高了储层脆性,有利于压裂改造。硅质岩伴生的热液活动参与了有机质生烃演化,使页岩油黏度降低,可动性增强,有利于页岩油开发。

Siliceous rocks are frequently included in reconstructing paleoenvironment,tracing hydrothermal circulation and understanding silicification process,aiming at using the results to guide the exploration and development of continental shale oil.Siliceous rock samples from the Lucaogou Formation in Bogda area,Junggar Basin,were observed through Xray diffraction and scanning electron microscope and analyzed with major and trace elements,vitrinite reflectance,Rock-Eval and total organic carbon data to identify their genetic mechanisms and hydrocarbon significance.The results show that the siliceous nodules in the samples host highly-deformed soft-sediments mainly composed of crypto-microcrystalline silica,indicating a synsedimentary product.Hydrothermal minerals such as barite,mangano-siderite,trevorite and azurite are developed inside the nodules with low total content of rare earth elements and relative high content of heavy rare earth elements as well as positive Eu anomalies,all indicating a hydrothermal origin and intermittent hydrothermal activities in the Bogda area during the Permian.The nutrients and metal cations,brought by the hydrothermal fluid,improved the paleo-productivity and paleo-salinity,and enhanced the reducibility of the bottom water,which was conducive to the enrichment of organic matter.Well-developed pores and fractures with enriched quartz greatly improved the brittleness of the rocks,making the reservoirs in the formation more responsive to fracturing.The hydrothermal activities associated with siliceous rocks participated in the evolution of hydrocarbon generation of organic matters,resulting in reduced crude viscosity and enhanced crude mobility which are conducive to shale oil development.