陆相坳陷湖盆细粒沉积岩岩相类型及成因——以松辽盆地晚白垩世青山口组为例

Lithofacies Types and Genesis of Fine-Grained Sediments in Terrestrial Depression Lake Basin:Taking Upper Cretaceous Qingshankou Formation in Songliao Basin as an Example

页岩油是目前非常规油气研究的热点与难点。细粒沉积岩岩相类型及成因分析是页岩油“源储”特征研究及“甜点”预测的有效途径。松辽盆地青山口组细粒沉积广泛发育,是页岩油富集的有效层段。本文通过岩性、矿物成分、有机质丰度与沉积构造4个参数,将松辽盆地青山口组细粒沉积岩划分为6种岩相:高有机质泥纹层黏土质页岩(A)、中高有机质含细粉砂纹层长英质页岩(B)、中低有机质含粗粉砂纹层长英质页岩(C)、低有机质层状粉砂岩(D)、低有机质层状介形虫灰岩(E)和低有机质层状白云岩(F)。进一步从水动力学与有机质富集方面探讨了不同岩相类型的成因,并建立了相应的沉积模式。具体为:在风暴浪基面之下的静水、咸水环境、高湖泊生产力背景中,浮游藻类与黏土絮状物均匀悬浮沉降,形成贫富有机质黏土质纹层,沉积A岩相;在正常浪基面之下的相对静水(浪基面附近局部动荡)、半咸水环境、高湖泊生产力背景中,三角洲径流与洪流带来的细粉砂颗粒继续向湖盆中央搬运,经均匀悬浮沉降形成长英质纹层,间歇期悬浮沉降形成黏土质纹层,形成B岩相;正常浪基面之下,洪水携带粉砂级颗粒,顺着水下分流河道搬运至外前缘,以稳定性浊流的形式进入前三角洲,随流速逐渐降低,分异形成粗粉砂—细粉砂纹层,间歇期悬浮沉降形成黏土质纹层,形成C岩相;正常浪基面之下,三角洲前缘早期沉积物在阵发性浊流的作用下,滑塌至较深水区,形成D和E岩相;于正常浪基面与风暴浪基面之间,在相对干燥的气候背景下的咸水环境中,形成F岩相。

Shale oil is currently a hot and difficult topic in unconventional oil and gas research.The analysis of the types and genesis of fine-grained sedimentary lithofacies is an effective way to study the“source and reservoir”characteristics of shale oil and predict its“sweet spot”.The fine-grained sediments of the Qingshankou Formation in the Songliao basin are widely developed and are effective intervals for shale oil enrichment.Based on four parameters,including organic matter abundance,mineral composition,lithology,and sedimentary structures,six kinds of lithofacies of fine-grained sedimentary rocks of the Qingshankou Formation in the Songliao basin are divided in this paper,as clay shale with high content of organic matter and mud-grade lamination(A),felsic shale with medium-high content of organic matter and fine silt-grade lamination(B),felsic shale with medium-low content of organic matter and coarse silt-grade lamination(C),layered siltstone with low content of organic matter(D),layered ostracoid limestone with low content of organic matter(E)and layered dolomite with low content of organic matter(F).Then,the genesis of different lithofacies types were discussed from the perspectives of hydrodynamics and organic matter enrichment,and corresponding sedimentary models were established.In the static and salty water environment below the base of storm waves with high lake productivity,planktonic algae and clay flocs are uniformly suspended and settled,forming organic-rich clay lamination and lithofacies A is deposited.In a relatively static and brackish water environment below the normal wave base(with local turbulence near the wave base)with high lake productivity,the fine silt particles brought by delta runoff and flood flow continue to move towward the center of the lake basin,forming a felsic lamination through uniform suspension and sedimentation,and a clay lamination is formed during the period of intermittent suspension and sedimentation,and lithofacies B is deposited.Below the normal wave base,the flood which carries silt-grade particles are transported to the outer front of delta along the underwater distributary channel,and enter the front delta in the form of stable turbidity current.As the flow velocity gradually decreases,it forms a coarse to fine silt lamination,clay lamination is formed during the period of intermittent suspension and sedimentation,and lithofacies C is deposited.Below the normal wave base,the early sediments of the delta front,under the action of paroxysmal turbidity currents,collapse into deeper water,forming lithofacies D and E.Between the normal wave base and storm wave base,lithofacies F is formed in saline water environment under a relatively dry climate.