松科2井东孔营城组高放射性异常层测井响应特征及成因初探

Abnormally high natural radioactivity zones in the main borehole of the Continental Scientific Drilling Project of Cretaceous Songliao Basin: Geophysical log responses and genesis analysis

松科2井东孔是松辽盆地科学钻探工程的主体钻孔,按照计划在全井段进行地球物理测井资料采集,为深部资源勘探及白垩纪古气候、古环境研究提供准确和全面的地球物理信息.在诸多测井方法中,自然伽马测井和自然伽马能谱测井能够测量地层的天然放射性强度和铀、钍、钾元素含量等参数,被有效地应用于放射性矿床勘查,尤其是铀资源的勘查与研究;电阻率、声波、密度和中子测井等常规测井和核磁共振成像、电阻率成像、元素俘获谱测井等特殊测井能够获取地层岩性和物性参数等信息,为铀矿勘查提供有力支持.本文利用松科2井东孔自然伽马测井资料来识别高放射性异常层;基于常规测井与特殊测井资料,结合前人的地质研究成果,分析放射性异常层的特征和成因.核测井曲线指示营城组存在两段高放射性异常层,深度分别为3096.8～3102.8 m(I号层)、3168.3～3170.9 m(II号层).I号层自然伽马值最高达360 API,铀含量范围20.5～29.3 ppm,综合测井和岩心资料判断该层为砾岩,具有铀成矿潜力;分析表明,构造条件和后生改造作用是影响I号层铀富集的关键因素,推测断裂-火山活动和盆地抬升剥蚀为含铀地下水及油气运移至I号层提供了通道,油气的后生还原作用最终导致了I号异常层的铀富集.II号层自然伽马值最高达250API,钍含量22.4～37.3 ppm,铀含量5.9～11.0 ppm,为集块熔岩及凝灰岩,高放射性异常可能是高钍含量的流纹质成分和粘土矿物对铀的吸附作用导致的.松科2井东孔营城组高放射性异常层具有埋藏深度深、铀含量高等特点,表明松辽盆地深部具有找铀矿前景.

SK-2 East Borehole(SK-2e)is the main borehole of Continental Scientific Drilling Project of Cretaceous Songliao Basin,which is designed to acquire the geophysical log data in the whole borehole,to provide accurate,comprehensive geophysical information for resource exploration and research on paleoclimate and paleoenvironment.Among various well logging methods,natural gamma-ray logging and natural gamma-ray spectral logging can measure natural gamma-ray intensity and uranium(U),thorium(Th),potassium(K)contents,and are effectively applied to radioactive deposit exploration,especially to uranium exploration and research.Conventional logging(including resistivity,acoustic,density and neutron logging)and special logging(including nuclear magnetic resonance,resistivity imaging and elemental capture spectroscopy logging)can obtain lithologic and physical parameters of strata,offering strong support for uranium exploration.In this study,the natural gamma-ray log data was used to identify abnormally high natural radioactivity zones.Based on conventional log data,special log data and previous geological research achievements,the characteristics and genesis of radioactive abnormality were analyzed.The results show that two abnormally high natural radioactivity zones are located in deep-seated Yingcheng Formation.And their depths are 3096.8~3102.8m(Layer I)and 3168.3~3170.9 m(Layer II)respectively.According to the comprehensive analysis of various log data and core data,Layer I is conglomerate,with the maximum gamma of 360 API and high uranium content(20.5~29.3 ppm).Therefore,Layer I has uranium mineralization potential.Analysis shows that tectonic conditions and epigenetic alteration are probably the key factors affecting uranium enrichment which causes abnormally high gamma-ray intensity.It was speculated that fault movements,volcanic activity,and the basin′s uplift and erosion provide uranium-bearing groundwater,oil and gas with the migration channel to I layer.The epigenetic reduction of oil and gas eventually leads to uranium enrichment of Layer I.Layer II consists of agglomerate lava and tuff,with the maximum gamma of 250 API,high thorium content(22.4~37.3 ppm)and high uranium content(5.9~11.0 ppm).The rhyolitic components of high thorium content and uranium adsorption by clay minerals may cause the abnormally high gamma-ray intensity in Layer II.The abnormally high radioactivity zones of Yingcheng Formation in SK-2e are characterized by deep burial depths and high uranium contents,indicating uranium prospecting potential in deep zones of Songliao Basin.