低本底伽马能谱测量在沉积岩分类判别中的应用及其影响因素探讨

Discussion of the Application and Influence Factors of Low-Background Gamma Spectrometry in Distinguishing Sedimentary Rock Types

沉积岩中放射性元素铀、钍、钾的含量主要取决于岩石中泥质的含量,而泥质含量的高低是判别沉积岩类别的主要依据,因此,可以将铀、钍、钾的比活度或含量值作为判别沉积岩岩类的量化指标。利用低本底伽马能谱法测量岩石样品中铀、钍、钾的比活度或含量,可以快速、准确地进行岩类判别。由于放射性测量受样品几何形状、样品质量及其所含水分含量变化的影响,因此岩类判别的准确性会受到影响。通过理论探讨和实验验证,发现各待测核素特征峰计数率值与样品质量成线性关系,且不同的能量区域、不同的岩性,线性关系系数与趋势拟合程度都是不同的;当样品的水分含量不超过10%(取样时,岩屑水分不超过10%)时,对测量结果的影响很小(变化值均在一倍均方差以内),因此,水分含量对低本底伽马能谱法判别沉积岩岩类的准确性不产生显著的影响,一般可以不予考虑。对陕西定边某油气勘查区钻孔岩屑样品进行判别试验,对实测数据进行质量校正后,利用铀、钍、钾道的"归一化"计数率值(即单位质量的计数率)只能粗略地划分不同沉积岩的大类,再以铀、钍、钾特征峰的合峰计数率建立综合判别模式,能够对岩类进行进一步细分,且判别准确度达到75%以上。

The contents of radionuclides uranium,thorium and potassium in the sedimentary rocks mainly depend on the contents of clay in the rocks.And the content of clay is the main basis for distinguishing types of sedimentary rock.Therefore,the value of specific activity or content of uranium,thorium and potassium can be as the quantitative index to distinguish sedimentary rock type.The specific activity or content of radionuclides uranium,thorium and potassium with the method of low-background gamma spectrometry can distinguish the type of rock quickly and accurately.Because of the influence of geometry,mass and moisture content in the sample,the accuracy of distinguishing types of rocks is influenced.This paper makes a theoretical discussion and experimental verification on the influence of mass and moisture content on the results of low-background gamma spectrometry.Results show that there is a linear relationship between（cps）of characteristic peak of all radionuclides and the mass of sample while different energy ranges and lithologies have different linear coefficient and trend fitting degree;The moisture content which is no more than 10%（while collecting samples,the moisture content is no more than 10%）has a little influence on the measurement results（the change values are within the twice standard deviation）,so the moisture content which has no significant influence on the accuracy of distinguishing types of sedimentary rock using the method of low-background gamma spectrometry could not be considered.The distinguishing experiment of drilling cuttings samples collected from one oil and gas exploration area in Shanxi Dingbian is done.By the mass correction of the measured data,normalized（cps）（（cps）of per unit mass）of uranium,thorium and potassium channel can only roughly divide the types of sedimentary rocks.Therefore,synthetic distinguishing mode is established with（cps）of combination peak of characteristic peak of uranium,thorium and potassium.The type of rocks is further subdivided,and the distinguishing accuracy is more than 75%.