Variation of Carbon Isotopic Composition of Alkanes&nbsp;During the Shale Gas Desorption Process and Its Geological Significance: A Case Study of Chang7 Shale ofYanchang Formation in Yishan Slope Southeast of Ordos Basin

MENG Qiang，WANG Xiao-feng，WANG Xiang-zeng，ZHANG Li-xia，JIANG Cheng-fu，LI Xiao-fu，SHI Bao-guang

doi:10.11764/j.issn.1672-1926.2015.02.0333

2015 , Vol. 26 >Issue 2: 333 - 340

DOI: https://doi.org/10.11764/j.issn.1672-1926.2015.02.0333

Variation of Carbon Isotopic Composition of Alkanes During the Shale Gas Desorption Process and Its Geological Significance: A Case Study of Chang7 Shale ofYanchang Formation in Yishan Slope Southeast of Ordos Basin

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1.Key Laboratory of Petroleum Resources,Gansu Province/Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics,Chinese Academy of Sciences,Lanzhou 730000,China； 2.University of Chinese Academy of Sciences,Beijing 100049,China； 3.Shaanxi Yanchang Petroleum (Group) Co.,Ltd.,Xi′an 710075,China

Received date: 2014-04-08

Revised date: 2014-05-23

Online published: 2015-02-10

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Abstract

Taking the Yishan slope southeast Chang7 shale of Yanchang Formation in the Ordos Basin as an example,with desorption experiments of more than 30 shale samples,this paper studied the variation of carbon isotopic composition of alkanes during the shale gas desorption process,also discussed the possible causes of this phenomenon and its geological significance.The study found that carbon isotopic composition become higher by 9.2‰ ( from -50.1‰ to -40.9‰) for methane of desorbed gas (δ¹³C₁),become higher by 2.8‰ ( from -35.5‰ to -32.7‰) for ethane (δ¹³C₂),and nearly no change for propane during the desorption process.At room temperature or constant temperature,carbon isotopic composition of methane and ethane (δ¹³C₁ & δ¹³C₂) continuously increases.When the temperature rises,δ¹³C₁ & & δ¹³C₂values suddenly become low first and then slowly become high during the desorption process of shale gas.The reason for this phenomenon is probably due to the main adsorption/desorption of shale gas and diffusion migration fractionation of isotope.In addition,these variation characteristics of methane and ethane carbon isotope composition of shale gas may be the important reason for the low carbon isotopic composition of methane relative to the thermal evolution degree of source rock,and it can be applied to evaluate the amount of remaining resources of shale gas and the fracturing effect.

Key words： Shale gas; Carbon isotopic composition; Desorption; Chang 7 shale of Yanchang Formation; Geological significance

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MENG Qiang，WANG Xiao-feng，WANG Xiang-zeng，ZHANG Li-xia，JIANG Cheng-fu，LI Xiao-fu，SHI Bao-guang . Variation of Carbon Isotopic Composition of Alkanes During the Shale Gas Desorption Process and Its Geological Significance: A Case Study of Chang7 Shale ofYanchang Formation in Yishan Slope Southeast of Ordos Basin[J]. Natural Gas Geoscience, 2015 , 26(2) : 333 -340 . DOI: 10.11764/j.issn.1672-1926.2015.02.0333

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