不同沉积岩的自然伽玛能谱与磁化率响应特征

Natural Gamma Ray Spectrum and Magnetic Susceptibility Characteristics Analysis of different Sedimentary Rocks

自然伽玛能谱(NGR)与磁化率(MS)均是常用的测井技术手段,它们在古环境研究与寻找韵律层中发挥着日益重要的作用。自然伽玛能谱与磁化率主要在钻井中进行应用,很少应用于露头剖面上,为了验证露头剖面岩性的自然伽玛能谱与磁化率响应特征,本文利用便携式仪器对山东淄博博山地区部分剖面的自然伽玛能谱与磁化率数据进行了测试分析研究,结果表明自然伽玛能谱中的钾(K)、无铀自然伽玛(CGR)值可以有效地区分不同沉积环境下不同沉积岩的粒度,在陆相环境下,细砂岩与粗砂岩的K、CGR值分别以3.3和23为边界,K、CGR值在3.3、23以上为细砂岩,以下为粗砂岩。在海相沉积环境下,灰岩、泥质粉砂岩、中细砂岩与粗砂岩的K、CGR边界值分别约为1、1.8、2.3和10、13.2、16.6。结合前人研究成果可以推测,在沉积近乎相同粒度的沉积物时,陆相沉积因水动力条件的不稳定性而含较多的泥质杂基,从而导致同一粒度的陆相沉积岩的K、CGR值要大于海相沉积岩的K、CGR值。在沉积物供应稳定的条件下,MS值随沉积物的粒度增大有升高的趋势。

Objectives：Natural gamma ray spectrometry （NGS） and magnetic susceptibility （MS） play an increasingly important role in researching paleoenvironmental and searching for rhythm layers, which are common logging techniques, usually used in drilling and rarely on outcrop profiles.  Three short outcrop profiles containing different sedimentary rocks belong to Taiyuan formation and Shanxi formation in Zibo area of Shandong Province were measured by two portable instruments which can test NGS and MS respectively  Methods： The NGS containing Potassium （K）, Uranium （U）, Thorium （Th）, Uranium free gamma ray （CGR） with MS data were measured in single point mode whose sampling distance was 0.12m.The values of Th/K, Th/U, U/CGR were obtained by calculation. NGS and MS characteristics of different sedimentary rocks were researched through data statistics and cluster analysis. Results：The result shows that K and CGR values can distinguish the particle size of different sedimentary rocks in different depositional environment. K and CGR values of fine sandstone are always more than 3.3 and 23 as well as gritstone with less than 3.3 and 23 in continental environment. K and CGR values of limestone are usually less than 1 and 10, and the value of gritstone changes in 1~1.8 and 10~13.2 in marine environment, which were lower comparing the value of gritstone in continental environment. At the same time, K and CGR values of middle—fine sandstone changed in the lower range 1.8~2.3 and 13.2~16.6, as well as argillaceous siltstone with more than 2.3 and 16.6. Conclusions：Combined with the previous research, it was speculated that hydrodynamic force in continental depositing process changed rapidly comparing in marine environment. If the particle size of continental sedimentary rocks was same with that in marine environment, continental sedimentary rocks contained more mud matrix due to the unstable hydrodynamic force condition. The MS would be higher with the increasing of particle size, if the supply of sediments was stable.