Evidence of magmatic hydrothermal action in the shale of Qingshankou Formation of Gulong sag,Songliao Basin
To explain the formation mechanism of light oil in Qingshankou Formation of Gulong sag under the condition of shallow burial depth(2000m to 2600m),based on the analyses of back scattered images,energy spectra,rare earth elements,carbon and oxygen isotopes,fluid inclusions,as well as the observation of cores and thin sections,the evidence that the shale oil of Qingshankou Formation in Gulong sag was subjected to magmatic hydrothermal action was found and analyzed.The evidences of magmatic hydro-thermal action mainly include as follows.(1)The minerals related to magmatic hydrothermal solution were discovered in shale oil res-ervoirs,including chalcopyrite,copper,pyrite,sphalerite and zinc disulfide,argentite and silver sulfide,Fe-Cr-Ni-Ti and Ni-Fe-Mo alloys,barite,fluorapatite,and titanium oxide.(2)High temperature magmatic hydrothermal solution elements such as Nb,Ta,W,and Hg were detected in the oil seeped out of the sample.(3)In the core subjected to magmatic hydrothermal alteration,the homogenization temperature for the high temperature section of the saline inclusions of calcite vein ranges from 130.0℃ to 166.8℃,which is higher than the normal temperature of calcite veins in non-hydrothermal altered cores(30℃ to 58℃).(4)The analysis re-sults of carbon-oxygen isotopes and rare earth elements reflect that the Qingshankou Formation shale of Gulong sag has undergone magmatic hydrothermal action.(5)A large number of magmatic hydrothermal alteration phenomena were found in the cores of Well Jin X in the north and Well Yingye Y in the middle of Qijia-Gulong sag,including"glass",spherulite,hornfel,pore,almond struc-tures.From a regional perspective,there is a magnetic anomaly,gravity anomaly and electrical anomaly zone in the deep part of Gu-long sag.The seismic section reveals that there is a large area of continuous magmatic hydrothermal zone in the basement of Gulong sag,with a horst underneath.The normal faults on both sides of the horst create conditions for magmatic hydrothermal intrusion in-to the Qingshankou Formation shale.A dike-like intrusion was encountered during drilling the Well Jin X,and strong magmatic hy-drothermal alteration was also found in the core samples.The discovery of the evidence of magmatic hydrothermal action is of great significance for studying the shale reservoir spaces and shale oil distribution in Qingshankou Formation of Gulong sag.
magmatic hydrothermal actionhydrothermal mineralshale oilQingshankou FormationGulong sag
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