松辽盆地古龙凹陷青山口组页岩油储层中的重力坠落砂脉的发现及其意义

Discovery and significances of gravity fall sand dikes in shale oil reservoirs of Qingshankou Formation in Gulong Sag,Songliao Basin Dong Wanbai1 Zhang

古龙页岩油是松辽盆地重要的接替资源,资源量可达151亿吨之多。从沉积和岩石学特点上看,古龙青山口组页岩最大的特点之一是广泛发育了一种很特殊的砂脉,所以这也是探讨古龙青山口组页岩形成环境的一个重要信息。古龙青山口组页岩油储层中的砂脉总体规模较小,宽度多在1~2 mm,可见长度(或高度)多在1~2 cm,大部分弯曲如肠,少数微曲;多倾斜产出,倾斜方向有一定规律,隔180°对称;一般发育在灰黑色的泥页岩中,少量发育在粉砂岩和白云岩(结核)中。砂脉的上部或顶部一般都有一层粉砂,与砂脉紧密相连,是砂脉的“根”;而砂脉的底部一般都是灰黑色的泥页岩,见不到粉砂层,所以揭示形成砂脉的源物质是来自于顶部的粉砂层,而不是底部。初步研究认为,古龙青山口组页岩油储层中的岩脉有两种成因:第一种是高压液化充注,形成了规模较大的砂脉;第二种由密度倒置引起的重力沉降形成风暴把(粉)砂级颗粒快速搬运到刚沉积的黏土之上,由于风暴浪的振荡,使粉砂液化流动,再加上黏土的密度较小和粉砂层的密度较大,使得粉砂在重力的驱动下沉降到黏土中形成砂脉。研究古龙页岩油储层中的砂脉具有4个意义:1)可以研究砂脉形成时的沉积环境、沉积过程和沉积物状态;2)可以研究泥页岩的成岩压实率;3)辅助确定白云岩(结核)的形成时间;4)可以辅助储层评价。

Gulong shale oil is an important replacement resource in Songliao Basin,with an estimated geological reserve of 15.1 billion tons.From the perspective of sedimentary and petrological characteristics,the most prominent feature of Gulong Qingshankou Formation shale is the extensive development of sand dikes,which is not found in shale oil reservoirs in other basins.Therefore,it is also an important information to explore the formation environment of Gulong Qingshankou Formation shale.The sand veins in the Gulong Qingshankou shale oil reservoir are generally small in scale,with a width of 1~2 mm.Visible length(or height)in 1~2 cm,mostly curved like intestine,a few slightly curved.Multi-tilt output,tilt direction has a certain rule,180°symmetry,generally developed in the gray black mud shale,a small amount of development in siltstone and dolomite(nodules).There is usually a layer of silt on the upper or top of the sand veins,which is closely connected with the sand veins and is the“root”of the sand veins.The bottom of the veins is usually gray black mud shale,and no silt layer can be seen,so it is revealed that the source material forming the veins is from the top silt layer,not the bottom.According to the preliminary study,the dikes in the shale oil reservoir of Qingshankou Formation in Gulong have two kinds of origin:the first is the high pressure liquefaction and filling,forming the large scale sand dikes;the second is caused by density inversion of gravity sedimentation:storm fast handling(powder)sand-size particles to just above the sedimentary clay,as a result of the storm wave oscillation,the silt liquefaction flow,combined with smaller density of clay and silt layer of density is bigger,makes the silt formed in gravity driven sinks down to clay sand vein.The study of sand veins in Gulong shale oil reservoir has four significances:firstly,we can know the sedimentary environment,sedimentary process and sediment state when sand veins were formed;secondly,the diagenetic compaction rate of shale can be known;thirdly,to determine the formation time of dolomite(nodules);fourthly,it can assist reservoir evaluation.