Discovery and significances of gravity fall sand dikes in shale oil reservoirs of Qingshankou Formation in Gulong Sag,Songliao Basin
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.
Sand veinLiquefactionGravity fallShale oilQingshankou FormationGulong Sag
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