碳质页岩是一种重要的非常规油气源岩,包含丰富的页岩气储存信息。在地质油气勘探领域,识别物性相近的碳质页岩是地质学的难题之一。为快速准确判别碳质页岩特性,本研究利用激光诱导击穿光谱( LIBS)技术对碳质页岩的光谱进行特征分析。以1064 nm纳秒脉冲激光作为激发光源,采用高分辨中阶梯光栅光谱仪配合ICCD搭建高分辨光学系统,利用LIBS装置分析四川盆地某井2396~3428 m不同深度处的5个典型碳质页岩样品,共获取碳质页岩中主量元素Si,Al,Fe,Ca,Mg,K和微量元素Cu,Cr,Ni,Sr,Mn,Ti,Rb等22种元素的350条发射谱线。基于高分辨LIBS系统采集的丰富谱线信息,采用主成分分析( PCA)法提取光谱贡献率最大的主成分得分二维图,识别不同类别碳质页岩样品,实现定性分析。结果表明,高分辨LIBS实验装置不仅可用于碳质页岩主量元素Si,Al,Fe,Ca,Mg,K等谱线分析,同时对微量元素如Ni,Cr,Mn,Sr等也具有很好的灵敏性。此外,LIBS技术与主成分分析( PCA)法结合可以更好地用于碳质页岩定性分析领域,提供科学的碳质页岩岩性判断数据和快速的分类手段,为页岩气开采和评估提供强有力的工具。%Gas shale is one of the important unconventional hydrocarbon source rocks, whose composition, such as mineral components and trace elements, has been proven as important geochemical proxies playing essential roles in indicating the gas potential and gas productivity in recent geological researches. Fast and accurate measurements of the shale composition will reveal rich information for understanding and evaluation of gas shale reservoirs. In this paper, we demonstrated the potentiality as well as feasibility of laser-induced breakdown spectroscopy ( LIBS ) as an effective technique to perform spectrochemical analysis for shale samples. For this experiment, a Nd:YAG laser at the fundamental wavelength of 1064 nm provided pulses for the shale materials. An echelle spectrometer equipped with an ICCD camera was employed to disperse and record the spectra. Meanwhile, five shale samples were collected at different depth from 2396 m to 3428 m. The LIBS device was used to obtain the spectrum, and combined with the principal component score of each spectrum to draw a two-dimensional diagram. The obtained results revealed that more than 350 lines emitted by 22 different elements were found. Among these species, major elements like Si, Al, Fe, Ca, Mg, K and Na, and redox sensitive trace elements such as Cu, Cr, Ni, Sr, and Ni were detected with high signal-to-noise ratio. In principal component scores diagram, different types of carbonaceous shale were obviously separated, and the results were consistent with the spectra classification. The observed results also show that laser-induced breakdown spectroscopy combined with principal component analysis ( PCA ) method can be used for carbonaceous shale discriminant field in the future, providing scientific data and means to improve classification performance and enhance the exploitation and evaluation of gas shale reservoirs.
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