准噶尔盆地吉木萨尔凹陷芦草沟组细粒湖底扇沉积及其页岩油储层意义

Fine-grained sublacustrine fan deposits and their significance in shale oil reservoirs in the Lucaogou Formation in the Jimsar Sag,Junggar Basin

准噶尔盆地吉木萨尔凹陷是我国页岩油成功勘探的代表性地区之一,但至今对页岩油富集层段二叠系芦草沟组的沉积相认识还存在分歧,存在浅湖与半深湖-深湖2种不同观点,并将其中的砂岩夹层识别为三角洲前缘远砂坝、浅湖滩坝沉积。通过岩心观察和砂岩粒度、测井相、储层物性分析等方法对二叠系芦草沟组沉积相特征进行了研究。研究结果表明,研究区湖盆内部以半深湖-深湖背景下湖底扇发育为特征,以细粒浊流为搬运-沉积机制;以透镜状-波状层理泥质粉砂岩-粉砂岩为主要岩相类型,其次是平行层理粉砂岩、递变层理粉砂岩和块状-递变层理细砂岩;其粒度累计概率曲线呈以悬浮总体为主的平滑上拱式,在C-M图上样品点分布于递变悬浮段。湖底扇包括席状朵叶、重力流水道-天然堤微相,分别表现为反、正沉积旋回。发育于湖底扇(尤其是广泛分布的席状朵叶微相)内部较多的粉-细砂岩夹层,以及作为细粒沉积中外源碎屑颗粒来源的浊流活动,造就了“甜点”储层“低黏土、高斜长石”矿物含量特点和溶蚀孔隙的大量发育,从而提高了细粒储层的储集性。本研究总结了细粒湖底扇的沉积物粒度细、砂层薄和频繁发育等特点,指明了“甜点”储层分布于细粒湖底扇发育的层段和地区,对研究区页岩油勘探具有指导意义。

[Objective]The Jimsar Sag of the Junggar Basin is representative of successful shale oil exploration in China.However,the sedimentary facies in the Permian Lucaogou Formation hosting shale oil has long been disputed.Previous studies proposed two contradictory environments of shallow lakes and semideep to deep lakes in the interior lacustrine basin and identified sandstone interlays as deltaic distal sandbar deposition and beach and bar deposition.[Methods]The study combined core observations with analyses of sandstone granularity,logging facies,and reservoir properties to identify the development of sublacustrine fans in a semi-deep to deep lake background.[Results]Fine-grained turbidity currents played a significant role in the transportation and deposition of sediment in these fans.The lithofacies observed included lenticular-wave bedded muddy siltstone to siltstone(dominant type),parallel bedded siltstone,graded siltstone,and massive-graded bedded siltstone to fine-grained sandstone.The cumulative probability granularity curves exhibited smooth upper arches with a high suspension population,and the cross-spots on the C-M diagram were located in the grade suspension zone.The sublacustrine fans consisted of sedimentary microfacies of sheet-like lobes and gravity flow channel levees,showing fining-and coarsening-upwards successions,respectively.These fans contained numerous interlayers of siltstone and fine-grained sandstone,as well as terrigenous detrital particles that influenced the mineral content and enhanced shale oil reservoir properties by promoting the growth of solution pores.[Conclusion]The study concluded that sublacustrine fan deposits are thin-bedded and fine-grained,and they occur frequently.It also determined that desert reservoirs formed in the areas where sublacustrine fans were deposited,which has significant implications for shale oil exploration in the study area.